Centriole assembly and the role of Mps1: defensible or dispensable?
Cell Divisionvolume 6, Article number: 9 (2011)
The Mps1 protein kinase is an intriguing and controversial player in centriole assembly. Originally shown to control duplication of the budding yeast spindle pole body, Mps1 is present in eukaryotes from yeast to humans, the nematode C. elegans being a notable exception, and has also been shown to regulate the spindle checkpoint and an increasing number of cellular functions relating to genomic stability. While its function in the spindle checkpoint appears to be both universally conserved and essential in most organisms, conservation of its originally described function in spindle pole duplication has proven controversial, and it is less clear whether Mps1 is essential for centrosome duplication outside of budding yeast. Recent studies of Mps1 have identified at least two distinct functions for Mps1 in centriole assembly, while simultaneously supporting the notion that Mps1 is dispensable for the process. However, the fact that at least one centrosomal substrate of Mps1 is conserved from yeast to humans down to the phosphorylation site, combined with evidence demonstrating the exquisite control exerted over centrosomal Mps1 levels suggest that the notion of being essential may not be the most important of distinctions.
I. The Centrosome
In proliferating cells, organization of a bipolar mitotic spindle is facilitated by the presence of two mature centrosomes, each of which contains a pair of centrioles. Accordingly, the single centrosome that proliferating cells inherit must be precisely duplicated exactly once prior to mitosis . The proper structure and function of centrosomes is dependent upon the strict doubling of existing centrioles , making the molecular mechanisms underlying the centriole assembly cycle of particular importance in the maintenance of genomic integrity.
The canonical centrosome assembly pathway results in the construction of a single procentriole at a site adjacent to each existing centriole (Figure 1). Much of what is known about centrosome duplication is derived from studies in model organisms [3, 4]. A powerful proteomic and comparative genomic analysis in green algae led to the characterization of 18 core proteins that form the Proteome of Centrioles, called the Poc proteins , and genome-wide RNAi screens in C. elegans identified five essential centriole biogenesis proteins, SPD-2, ZYG-1, SAS-4, SAS-5, and SAS-6 [6, 7]. Live cell imaging of worm embryos placed these proteins into an ordered assembly pathway: pro-centriole formation is initiated by the recruitment of SPD-2 to an existing centriole, SPD-2 leads to recruitment of the ZYG-1 protein kinase, which in turn recruits a complex containing SAS-5 and SAS-6 that promotes the formation of a central tube that determines basic centriole structure, followed by SAS-4 that facilitates the assembly of microtubules and mediates pro-centriole elongation [6–11]. The procentriole formation pathway delineated in C. elegans represents a core centriole assembly program that is conserved in organisms as distinct as T. thermophila , D. melanogaster  and H. sapiens [14–16]. The past year has seen an explosion in our understanding of the canonical centriole assembly pathway in humans. The human SPD-2 orthologue Cep192 is required for both centriole biogenesis and centrosome maturation, binds to Plk4, and is required for Plk4-dependent centriole overproduction , suggesting that it might function analogously to SPD-2. However, Plk4 recruitment also requires Cep152, the human orthologue of D. melanogaster asterless [18, 19]. Recruitment of Plk4, the distant relative and presumptive functional counterpart to ZYG-1 [15, 16, 20], is then followed by that of hSas6 [14, 15] and the human orthologue of Sas-4, CPAP/CENP-J [21–23]. Several additional proteins are recruited during S and G2 to promote procentriole assembly and elongation, including Cep135 [15, 24], γ-tubulin, and CP110 [15, 25]. However, centriole assembly in human cells involves many additional proteins not found in worms, including δ- and ε- tubulins [26, 27], Mps1 [28–32], Centrin 2 (Cetn2) [28, 33], hPoc5 , and Cep76 , among others.
It was previously thought that mother centrioles serve as a template for the assembly of procentrioles, in analogy to DNA replication. This has largely been disproven , and perhaps a more accurate description is that a centriolar template is assembled at a site adjacent to an existing centriole. In worms this template is a central tube formed by the SAS5/SAS6 complex , while in vertebrates and many other organisms it is a structure called the cartwheel ; both structures serve as a platform around which procentrioles are assembled, and Sas-6 forms the central hub responsible for nine-fold symmetry . Once assembled, procentrioles undergo elongation through S and G2, but remain attached to the proximal end of their mother until mitosis, when they become physically disengaged . Centriole assembly would thus appear to be a modular process, wherein centrioles are built from the bottom up (Figure 1); existing centrioles provide a surface for the assembly of a cartwheel, which serves as platform onto which successive modules are assembled in a proximal to distal fashion. In support of this modular nature of centriole assembly, both hPoc5 and Cetn2 appear to be dispensable for the initiation of centriole assembly, but depletion of hPoc5 leads to production of centrioles that lack distal elements , and overexpression of Cetn2 can organize a subset of distal centriole proteins at sites other than cartwheels . Two of the key players in this core centriole assembly pathway, Plk4 [40–42] and hSas6  are regulated by degradation. The result of this regulation is that hSas6 can only be assembled into procentrioles once per cell cycle, and centriolar hSas6 is degraded during mitosis. As a consequence, while cells have either two (G1) or four (S, G2, M) centrioles, they have zero (G1 or M) or two (S and G2) hSas6 foci.
The presence of extra centrosomes leads to the formation of extra spindle poles and multipolar spindles . These extra centrosomes perturb the proper connection of chromosomes to opposite spindle poles, resulting in chromosome mis-segregation and aneuploidy, even when the extra centrosomes cluster to produce pseudo-bipolar spindles [43, 44]. Indeed, excess centrosomes have recently been shown to generate aneuploidy, and genetic instability driven by the presence of extra centrosomes is thought to be an early event in prostate and breast tumor progression [43–46]. The exact mechanisms responsible for restricting centrosome duplication remain to be elucidated, but overexpression of Cep76 can suppress centrosome re-duplication , while overexpression of several centriole assembly factors such Mps1 [31, 47], Plk4 , hSas6 , or Cetn2  can generate extra centrioles, indicating the importance of the strict regulation of centriole assembly factors throughout the cell cycle. This review aims to address recent advances in our understanding of the role of Mps1 in centriole assembly, as well as the controversy surrounding the function of centrosomal Mps1, and the future challenges for its understanding.
II. An Introduction to Mps1
Mps1, the p53 of the cytoskeleton?
Mps1 is a dual specificity protein kinase conserved from yeast to mammals . Originally identified for its role in spindle pole body (SPB) duplication in budding yeast, Mps1 was named for the phenotype observed in mutant cells, which fail in SPB duplication and form M onop olar s pindles upon entering mitosis . Mps1 was subsequently shown to regulate the mitotic spindle assembly checkpoint (SAC) [50, 51], and has since been found to function in a wide range of cellular processes in diverse organisms including hypoxia-induced mitotic arrest  and the meiotic assembly checkpoint in flies , and tissue regeneration in fish . In human cells reported hMps1 functions include centrosome duplication [28–32], the SAC , SMAD signaling , the p53-dependent post-mitotic checkpoint , the Chk2-dependent DNA damage response , and cytokinesis . The most recently documented function for Mps1 ties it to the actin cytoskeleton through a newly identified binding partner Mip1, a novel component of the actin cytoskeleton that regulates the interaction of the mitotic spindle with the cell cortex . While each of its described functions is relevant to the control of genomic integrity, it is not clear to what extent each function of Mps1 is conserved. Regarding its originally defined functions from yeast, there is wide agreement that the SAC function of Mps1 is universally conserved among eukaryotes  but reports differ regarding a role for Mps1 at centrosomes [30, 32, 60, 61]. Though the list of Mps1 functions continues to expand, this review will concentrate on its controversial role in centrosome duplication.
From yeast to house apes
SPB duplication is well recognized as a model for centrosome duplication , and once vertebrate orthologues of Mps1 (originally cloned as Esk in mice  and TTK [63, 64] or PYT  in humans, referred to hereafter generally as Mps1 or explicitly as mMps1 or hMps1, respectively) were recognized as such it was anticipated that some aspect of Mps1 function in SPB duplication might be conserved in vertebrates. mMps1 and hMps1 can autophosphorylate on tyrosines, and both were identified through screens for receptor tyrosine kinases. However, initial studies demonstrated them to be cell cycle regulated dual-specificity kinases, and suggested that both translational and post-translational inputs controlled their cell cycle profile [62, 63, 65]. Notably, while the levels of hMps1 message, protein, and kinase activity peaked during mitosis, there is a sharp peak of hMps1 kinase activity at the G1/S boundary that is not accompanied by a significant change in hMps1 protein levels  (Figure 2). Thus, hMps1 shows a sharp peak of modest total activity but high specific activity at G1/S that is coincident with centrosome duplication, and a second much larger peak of activity at G2/M coincident with the SAC.
Mps1, of mice and men
The first functional studies of vertebrate Mps1 examined the mouse  and frog  enzymes. Both endogenous mMps1 and GFP-mMps1 localized to centrosomes and kinetochores in NIH 3T3 cells . Overexpression of mMps1 caused centrosome re-duplication, while the catalytically inactive kinase dead (KD) version mMps1-KD blocked centrosome duplication, suggesting that mMps1 regulated centrosome duplication . Concurrently, the X. laevis Mps1 was shown to be present at kinetochores where it is required for SAC function . Accordingly, these initial studies supported the idea that the functions of Mps1 described in yeast were conserved in vertebrates. However, the first functional study of hMps1 cast doubt on this suggestion, as it failed to find hMps1 at centrosomes, or to reveal any defect in centrosome duplication after the overexpression of hMps1 (hMps1 or hMps1KD) or its siRNA-mediated depletion . Rather, the only phenotype reported in that study was the failure of hMps1-depleted cells to arrest in mitosis in response to the microtubule poison nocodazole .
There were important experimental differences between the original mMps1 and hMps1 studies, and when used with timing and expression levels similar to the mMps1 study, hMps1KD and wild type hMps1 were subsequently shown to attenuate centrosome duplication and accelerate centrosome re-duplication, respectively, in human cells [30, 32]. In addition, other groups have subsequently observed transgenic hMps1 at centrosomes [30, 68], and at least four hMps1 antibodies (three different rabbit polyclonal, and one mouse monoclonal) have subsequently been shown to stain centrosomes [29, 32, 68, 69] (although the vast majority of hMps1 antibodies do not). Notably, a recent study that identified activating phosphorylation sites within hMps1 not only found active Mps1 at centrosomes, but also showed that it interacts with γ-tubulin . Moreover, the siRNA-mediated depletion of hMps1 can attenuate centrosome duplication . Titration of hMps1-specific siRNAs suggested that centrosome duplication requires significantly less Mps1 than the SAC, and that not even mitotic catastrophe was indicative of complete hMps1 depletion . Indeed, this finding is consistent with the very low levels of hMps1 at G1/S observed by Hogg et al  (Figure 2). Interestingly, these initial functional studies of hMps1 highlighted a difference between mouse and human cells; while moderate overexpression of wild type Mps1 is sufficient to drive centrosome re-duplication in mouse cells , even gross overexpression of wild type Mps1 is not sufficient do so in human cells [32, 61], a difference that is due to cell type and not the source of enzyme (unpublished observations). Our subsequent explorations have suggested that this difference is due to the regulation of Cdk2 activity between mice and humans, although this has not been explicitly tested.
III. Exquisite control over centrosomal Mps1 levels
The G1 to S transition in vertebrate cells marks "the point of no return" in the cell cycle, beyond which the cell is committed to division, and Hogg et al. showed a dramatic peak of hMps1 kinase activity at G1/S with little associated change in total protein levels  (Figure 2). This suggests that the function of Mps1 in centrosome duplication is controlled by a posttranslational mechanism, and the initial mMps1 study showed that Cdk2 suppresses the proteasome-mediated degradation of mMps1 . Overexpression of the Cdk2 partner cyclin A causes centrosome reduplication in human cells , and further studies suggested that cyclin A-associated Cdk2 (Cdk2/A) specifically regulates the degradation of a centrosomal pool of hMps1; while cyclin A overexpression increased the level of hMps1 at centrosomes by ~2.5-fold, modulation of Cdk2 or the proteasome had little effect on the whole cell levels of Mps1 . Amino acids 420-507 that are encoded by hMPS1 exons 12 and 13 contain an M ps1 D egradation S ignal (MDS, see Figure 2). The MDS is responsible for the proteasome-dependent removal of hMps1 from centrosomes in the absence of Cdk2 activity, and a single site within the MDS, T468, is phosphorylated by Cdk2/A . The non-phosphorylatable GFP-hMps1T468A can accumulate in the cytoplasm but is constitutively removed from centrosomes in a proteasome-dependent manner . GFP-hMps1T468A cannot substitute for the function of endogenous hMps1 in centrosome duplication, demonstrating that it is the centrosomal pool of hMps1 that is relevant to its function in centrosome duplication . In contrast, the GFP-hMps1T468D and GFP-hMps1T468E mutants that mimic T468 phosphorylation, and GFP-hMps1Δ12/13 that lacks the MDS, can accumulate at centrosomes in the absence of Cdk2 activity, and cause centrosome re-duplication  in all human cells thus far tested , even at expression levels that are roughly two-fold lower than that of endogenous hMps1 . Accordingly, in human cells the failure of wild type hMps1 to cause centrosome re-duplication appears to be the consequence of its efficient proteasome-dependent removal from centrosomes, but preventing this removal is sufficient to cause centrosome re-duplication [29, 31, 47]. Moreover, while cyclin A-dependent centrosome re-duplication requires hMps1, that caused by hMps1Δ12/13, hMps1T468D, or hMps1T468E no longer requires cyclin A , suggesting that cyclin A promotes centrosome re-duplication by suppressing the degradation of hMps1 at centrosomes.
The observation that very modest changes in centrosomal hMps1 can cause centrosome re-duplication suggests that defects in the control of hMps1 degradation might contribute to centrosome abnormalities in human tumors. In fact, hMps1 is not appropriately degraded in response to the Cdk2 inhibitor roscovitine in a variety of tumor-derived cells . Transgenic hMps1 can be degraded appropriately in one such cell type, the U2OS osteosarcoma cell line well known to undergo centrosome re-duplication, suggesting that at least in these cells the defect is intrinsic to endogenous hMps1. While no hMPS1 coding mutations were found in U2OS cells, the hMPS1Δ12/13 mRNA lacking exons 12 and 13, which generates the internally truncated hMps1Δ12/13 protein lacking amino acids 420-507 discussed above, was cloned by RT-PCR amplification of hMPS1 mRNAs from U2OS cells . However, the 21NT mammary carcinoma cell line fails to degrade both endogenous and transgenic hMps1, and GFP-hMps1T468A can accelerate centrosome re-duplication in 21NT cells, suggesting that defects in hMps1 degradation can also be caused by deficiencies in the factors that regulate its degradation . In fact, we have recently shown that hMps1 degradation is controlled by a potential tumor suppressor, Ornithine Decarboxylase Antizyme (OAZ, or Antizyme) . Together, these studies demonstrate that cells normally exercise exquisite control over centrosomal Mps1, and that even modest changes in the centrosomal pool of Mps1 are sufficient to break the control of centrosome duplication to generate excess centrioles.
What is the MDS?
The MDS contains no known targeting motifs for either the Skp/Cullin/F-box (SCF) or Anaphase Promoting Complex/Cyclosome (APC/C) E3-ubiquitin ligases, and its only obvious similarity is to other vertebrate Mps1 proteins , suggesting that the MDS controls centrosomal Mps1 through a mechanism independent of typical SCF or APC/C degradation pathways. OAZ is a potential tumor suppressor that targets a handful of known substrates for ubiquitin-independent proteasome-mediated degradation . For example, OAZ regulates polyamine biosynthesis by binding to and targeting Ornithine decarboxylase to the proteasome for degradation in a ubiquitin-independent manner . Mangold et al. showed that OAZ and its inhibitor (Antizyme Inhibitor, or AZI) are found at centrosomes, and that OAZ activity suppresses centrosome re-duplication; increased OAZ activity and/or levels suppressed centrosome re-duplication, while reducing OAZ led to centrosome amplification . Based on this data Mangold et al. hypothesized that OAZ promoted the degradation of a centrosomal substrate whose continued presence at centrosomes promoted centrosome re-duplication . The phenotypes described by Mangold et al. could be easily explained if that substrate were hMps1, making OAZ a very attractive candidate for mediating the MDS-dependent removal of hMps1 from centrosomes. Indeed, OAZ binds to hMps1 through the MDS (Figure 2), regulating centrosomal hMps1 levels in a proteasome-dependent manner, and antagonizing hMps1-dependent centrosome re-duplication in a manner that is attenuated by T468 phosphorylation .
Is the MDS the only control over centrosomal Mps1?
Given the multiple functions of Mps1, it seems likely that it might be controlled by multiple mechanisms. Indeed, yeast Mps1 has been shown to be a substrate of the APC/C, and mutual antagonism between Mps1 and the APC/C is of central importance to proper activation and silencing of the SAC in yeast . Moreover, a recent study by Cui et al. has shown that hMps1 is a ubiquitinated protein, and that both Cdc20 and Cdh1 promote the APC/C-dependent degradation of hMps1 during mitosis and G1, respectively . Interestingly, Cui et al. found that mutation of the single D-box present in the N-terminus of hMps1 (amino acids 256-263, see Figure 2) not only led to increased hMps1 levels, but also caused centrosome amplification . While they did not characterize the mechanism of centrosome amplification, it is tempting to speculate that increasing the cytoplasmic pool of hMps1 indirectly increased the centrosomal pool as well, leading to centrosome re-duplication. Together, this suggests that two mechanisms cooperate to control the function of hMps1 in centrosome duplication (Figure 2); first, APC/C-mediated proteolysis acts on hMps1 through its D-box as cells exit mitosis and enter G1 in order to reduce the cytoplasmic pool of hMps1  (which is roughly 5-10 fold higher during the SAC ), and second, OAZ-mediated proteolysis acts on hMps1 through its MDS to keep centrosomal hMps1 levels low until Cdk2 activity is sufficiently high to launch the early events of centrosome duplication. Once centrosomes have duplicated, OAZ again acts through the MDS to reduce centrosomal levels and suppress centrosome re-duplication [29, 73].
IV. Recent studies identify multiple centrosomal functions for hMps1
OAZ control of Mps1 reveals a role for Mps1 in procentriole assembly
The existence of such exquisite control over centrosomal hMps1 and the consequences of subtle changes in this control make it tempting to speculate that hMps1 must play some important role at centrosomes, and indeed two recent studies have revealed at least two functions for hMps1 in centrosome duplication. First, our study documenting the role of OAZ in removing hMps1 from centrosomes revealed a role for Mps1 early in centriole assembly . The majority of experiments in that study explored the ability of OAZ to suppress the function of hMps1 in centrosome re-duplication; the binding of OAZ to hMps1 promotes the MDS- and proteasome-dependent removal of hMps1 from centrosomes, preventing it from accelerating centrosome re-duplication . However, we also found that OAZ antagonized the canonical centrosome duplication cycle. In a G1 enrichment and release assay, overexpression of OAZ led to a five-fold increase in the number of S-phase HeLa cells with just two centrioles , and the majority of these had a single focus of hSas6 staining . As discussed above, cells normally have zero or two hSas6 foci. Accordingly, the observation that OAZ-overexpressing cells with two centrioles have one hSas6 focus supports a role for hMps1 early in procentriole assembly. Both the increase of cells with two centrioles and the hSas6 assembly phenotype appear to be due to attenuation of hMps1, because both were complemented by overexpression of wild type hMps1, and both were phenocopied by overexpression of hMps1KD . OAZ overexpression likely causes a delay in assembly rather than a block, because the number of S-phase cells overexpressing OAZ that have two centrioles was markedly lower in asynchronous cultures that had not been previously enriched in G1 . Thus, our examination of the control of centrosomal hMps1 levels led to the demonstration that hMps1 functions very early in centriole assembly, and suggests that hMps1 phosphorylation promotes the remodeling of an hSas6-containing intermediate into procentrioles (Figure 3). This proposed function for hMps1 is similar to the function of budding yeast Mps1p in SPB assembly. Mps1 phosphorylation of Spc42p remodels overexpressed Spc42p from a spherical structure adjacent to the SPB into an planar extension of the SPB central plaque . Similarly, Mps1p phosphorylation was found to modulate complexes formed by Cdc31p (the yeast centrin orthologue) and Spc29p, suppressing the interaction between Cdc31p and Kar1p to promote satellite assembly, and enhancing the interaction between Spc29p and Bbp1p to promote the subsequent insertion of the nascent SPB into the nuclear envelope .
Analysis of the Mps1 substrate Cetn2 reveals multiple Mps1 functions
The observation that non-degradable hMps1 proteins cause centriole overproduction  even at very modest expression levels  suggests that sustained phosphorylation of some set of centriole proteins by hMps1 leads to excess centriole assembly. A second study identifying the centriolar protein Cetn2 as one such hMps1 substrate provided additional evidence for an early function for hMps1, and also suggested at least one additional function for hMps1 in centriole assembly . Three in vitro sites of hMps1 phosphorylation were identified in Cetn2, and each was shown to be important for the control of centriole number (although in vivo phosphorylation at these sites was not demonstrated) . Interestingly, a role for Cetn2 in centriole assembly has also been controversial. The first siRNA-mediated depletion of Cetn2 suggested that it was essential for centriole assembly in HeLa cells, and while there was a dearth of centriolar markers available for analysis at the time, electron microscopy showed an increase of cells with fewer than expected centrioles . In contrast, subsequent studies found that co-depletion of Cetn2 and Cetn3 had no affect on Plk4-mediated centriole overproduction , and that depletion of Cetn2 alone had no affect on recruitment of hSas6 to centrioles . Our experiments support the suggestion that while hMps1 phosphorylation sites in Cetn2 enhance the rate of centriole assembly, at least in HeLa cells Cetn2 is dispensable for the canonical centriole assembly pathway . However, the presence of Cetn2 increases the rate of centriole assembly (as judged by CP110 recruitment), and this function of Cetn2 requires the major site of in vitro hMps1 phosphorylation, T118 . Moreover, Cetn2 is required for the maturation of excess centrioles produced in cells expressing hMps1Δ12/13 . Interestingly, we found that even in the absence of Cetn2 expression of hMps1Δ12/13 led to the assembly of excess hSas6 containing structures, but that these structures could not recruit pericentriolar material in the absence of Cetn2 . The observation that attenuation of hMps1 retards hSas6 assembly in the canonical centriole assembly cycle , while hMps1Δ12/13 promotes the assembly of excess hSas6 containing structures  further supports the suggested role for hMps1 in procentriole assembly. Moreover, this data also suggests that Cetn2 cannot be the only Mps1 substrate whose phosphorylation is critical for centriole overproduction. Regardless, taken together the OAZ and Cetn2 studies suggest that Mps1 has at least two functions in centriole assembly (Figure 3). In one function, mediated by as yet unidentified substrate(s), hMps1 promotes the remodeling of a single hSas6-containing precursor into cartwheels adjacent to each existing centriole. In a second function, hMps1 promotes the Cetn2-dependent maturation of the resulting procentrioles.
Interestingly, our study of Cetn2 also suggests that Cetn2 has multiple roles in centriole assembly. Mutations that mimic phosphorylation at any of the three hMps1 phosphorylation sites, e.g. Cetn2T118D, led to a robust overproduction of bona fide centrioles . The ability of phosphomimetic Cetn2 mutants to promote centriole overproduction required hSas6 , suggesting that hMps1 phosphorylation sites within Cetn2 stimulate centriole overproduction through cartwheel templates. Centriole overproduction in cells expressing phosphomimetic Cetn2 mutants also required hMps1 , further supporting the suggestion that additional centriolar hMps1 substrates must exist. Interestingly, we found that overexpression of wild type Cetn2 led to the overproduction of both bona fide centrioles and aberrant centriole-like structures  that required hMps1 but not hSas6, suggesting that wild type Cetn2 did not lead to centriole overproduction through a cartwheel template. These observations suggest that Mps1 and Cetn2 cooperate to promote centriole overproduction via multiple mechanisms (Figure 4). First, expression of the non-degradable hMps1Δ12/13 induces the typical "Bottom-up" production of excess centrioles via hSas6-containing precursors. The initiation step does not require Cetn2, but is followed by the Cetn2-dependent maturation of these structures into centrioles. Although it remains to be tested, this pathway is presumably similar to that induced by overexpression of Plk4 or hSas6. Second, hMps1 is also required for the production of excess centriole-like structures induced by overexpression of wild type Cetn2. Because it does not require hSas6, the assembly of these structures is presumably initiated from Cetn2-containing precursors that first organize distal centriole elements, and then recruit proximal elements such as Cep135 and hSas6 in a "Top-down" fashion. Because these Cetn2-induced structures can recruit hSas6, it seems possible that they may contain cartwheels, although their initiation is not cartwheel dependent. While the initiation step in Cetn2-induced centriole overproduction does not require hSas6, some aspects of the maturation of these structures is hSas6-dependent; in the absence of hSas6 these structures can no longer recruit proximal centriole elements like Cep135, although they can still recruit pericentriolar material and function as mitotic spindle poles . Together, these studies on OAZ and Cetn2 support the suggestion that hMps1 has both an early role in procentriole assembly, and a later Cetn2-dependent role in centriole maturation. Consistent with findings from yeast [76–78], our studies suggest that hMps1 phosphorylation of centriolar protein(s) remodels protein complexes to promote initiation and maturation of procentrioles, and that this is relevant to both the canonical centriole assembly pathway and centriole overproduction.
V. But is Mps1 essential?
Despite observations with Cetn2 and OAZ that suggest centrosomal functions for hMps1, several recent studies using chemical genetics [79–81] or small molecule inhibitors [60, 82–84] have lent support to the suggestion that hMps1 is not essential for centrosome duplication. In two of the chemical genetics studies, cell lines that depend solely on an analog-sensitive (as) version of hMps1 (hMps1-as, engineered to be sensitive to inhibition by bulky ATP analogs) were constructed by expressing an RNAi-resistant version of hMps1-as together with an hMps1-specific shRNA [80, 81]. In a third chemical genetics study hMps1-as was expressed using retroviral vectors in cells harboring a deletion at the endogenous hMps1 locus constructed using Adeno-Associated Viral vectors . Although two of these studies did not mention centrosomes, Tighe et al. comment that they observed no centrosomal defects . Several small molecule inhibitors of hMps1 have also been reported, and while they all perturb mitosis, those studies that examined centrosomes similarly reported that hMps1 inhibition had no consequence for centrosome duplication [60, 83, 84].
There are of course caveats with both chemical genetics and kinase inhibitors. First, as discussed above, partial depletion of hMps1 causes mitotic catastrophe despite the presence of residual hMps1 , and it seems likely that a partial inhibition of hMps1 would produce the same effects. Indeed, the zebrafish nightcap mutation was shown to be a hypomorphic allele of Mps1 that caused mitotic defects despite the presence of residual Mps1 activity  (although a null allele ultimately suggested that Mps1 was not required for centrosome duplication in zebrafish ). Given that partial depletion  or inhibition  of Mps1 can cause such severe mitotic defects, it becomes difficult to know when depletion/inhibition is complete. Second, as we demonstrated for hMps1 siRNA, cells with reduced hMps1 undergo mitotic and cytokinetic failures that produce G1 cells with two centrosomes, potentially masking subsequent defects in centrosome duplication ; we suspect that this would be a greater issue for the acute inhibition of hMps1 than for the gradual rundown of hMps1 levels achieved using RNAi. Third, unlike RNAi experiments, the presence of the inhibited kinase might allow hMps1 to participate in non-enzymatic functions; that is, accepting the conclusions from chemical genetics and small molecule studies, they tell us that hMps1 kinase activity is dispensable, but do not demonstrate that the hMps1 protein is dispensable. Small molecule inhibitors have the additional caveat that they may not be equally effective against all substrates, and this appears to be the case for the IN-1 inhibitor, which we obtained as a very gracious gift from Dr. Nathaniel Gray (Dana-Farber Cancer Institute, Boston MA). At 10 μM IN-1 abolishes the in vitro phosphorylation of MBP by hMps1 (Figure 5) and causes mitotic defects in vivo , but it requires significantly higher concentrations of IN-1 to completely block in vitro Cetn2 phosphorylation (Figure 5). While this in vitro result was obtained under different conditions than used by Kwiatkowski et al.  and cannot be directly compared to the use of IN-1 in vivo, it supports the possibility that despite causing mitotic failures, hMps1 inhibitors might leave sufficient residual hMps1 activity to support centrosome duplication.
Regardless, there is now a preponderance of evidence to suggest that hMps1 is dispensable for centriole assembly. Even some aspects of our own data on Cetn2 support this suggestion. Although hMps1 has Cetn2-independent functions in centriole assembly , the identification of a non-essential substrate for hMps1 suggests that at least one of its functions might be dispensable. Moreover, the data we used to conclude that Cetn2 is dispensable for centriole assembly  was very similar to that used to reach the opposite conclusion for hMps1 . In our first study of hMps1, we assessed centriole number as HeLa cells that had been enriched in G1 by serum starvation entered S-phase after re-addition of serum (HeLa cells do not arrest in response to serum starvation, but rather grow slowly and accumulate in G1). Using a single time point eight hours after the addition of serum and 5-bromo-2'-deoxyuridine (BrdU), we observed that hMps1 depletion led to a large increase in the percentage of cells that had entered S-phase during the release period but had just two centrioles, and concluded that hMps1 was required for centrosome duplication . In our Cetn2 study we measured centriole number in BrdU positive cells form unstarved HeLa cultures after a 4 hr BrdU pulse, and found that Cetn2-depletion caused a two-fold increase in the number of S-phase cells that had just two centrioles . However, we also included a chase period in order to determine the fate of the centrioles that had not duplicated during the initial BrdU pulse. We found that after a four hour chase period the percentage of Cetn2-depleted cells with two centrioles dropped to that seen in controls , suggesting that centriole assembly is initially delayed in the absence of Cetn2, but is ultimately completed. While there are important differences between these two approaches (starved vs unstarved cells, long labeling vs pulse chase), they are similar enough to prompt us to consider whether our original data with hMps1 might also reflect a delay. Accordingly, it still remains unclear whether hMps1 is essential for centriole assembly, and it will ultimately require a detailed comparison of the depletion/inhibition of hMps1 with the depletion of other essential (e.g. Plk4) and non-essential factors (e.g. Cetn2) under identical experimental conditions to resolve the issue.
Do we even care if hMps1 is essential?
If hMps1 has no role in centrosome duplication, its conservation is somewhat hard to explain, particularly given that phosphorylation of centrins is conserved down the actual phosphorylation site; yeast Mps1p phosphorylates Cdc31p at T110 , which is analogous to the T118 residue found to be the major hMps1 phosphorylation site within Cetn2 . If hMps1 has no role in centrosome duplication it is also hard to understand why cells would exert so much control over the centrosomal levels of hMps1, and more importantly why subtle changes in centrosomal hMps1 levels would lead to centrosome re-duplication [28, 29, 31, 47]. Therefore, it seems clear that hMps1 plays some role in centriole assembly, regardless of whether it is essential for the process. Perhaps hMps1 takes on different importance in different cellular contexts. Not only did our recent work show that Centrins 1, 2, and 3 are not functionally equivalent, but it also showed that Cetn2 has cell type specific effects . Perhaps hMps1 has similar cell type specificity, and is not essential for centriole assembly in the commonly used cell lines. Cep76 also exhibits cell type specific function , suggesting that the requirements for centriole assembly may vary considerably according to cell type. It is also possible that hMps1 plays a more important role at some point in development, but that tumor cells re-activate multiple pathways for centriole assembly such that it becomes dispensable.
Another possibility is that Mps1 may regulate the fidelity of centriole assembly or transmission. Depletion of the T. thermophila TtCEN1 leads to the progressive loss of basal bodies (structures analogous to centrioles), and those that remain are deteriorated, suggesting that TtCEN1 is required not just for assembly but also for the maintenance of basal bodies . A similar phenotype was seen in PtCEN2-depleted P. tetraurelia, but PtCEN2 was demonstrated to have no overt role in either the initiation or assembly of new basal bodies . Rather, PtCEN2 was shown to regulate the selection of the site and orientation of basal body assembly as well the stability of basal bodies, both of which are critical for proper basal body retention and function [87, 88]. Our data suggest that Cetn2 increases the rate of centriole assembly, and given the requirement for hMps1 in Cetn2 function , perhaps hMps1 is also required for aspects of site selection and maintenance that are dispensable for the actual assembly of centrioles.
An Interesting Twist
Inhibition of hMps1 via chemical genetics and small molecules suggests that it is essential in vertebrates [60, 79–84], and consensus is that its essential function is in the SAC . It is interesting to note that in yeast the SAC itself is not essential, and so represents the dispensable function of yeast Mps1p. In contrast, in vertebrates the SAC is essential and centrosomes themselves are dispensable (at least for the progression of mitosis) . If Mps1 is dispensable for centrosome duplication in vertebrates, perhaps this is because vertebrates evolved to rely on it for the spindle checkpoint, which presumably becomes increasingly important with increasing genomic complexity, and thus evolved mechanisms to replace its function in centrosome duplication.
We may ultimately find that hMps1 is dispensable for centriole assembly. However, that would not dampen our interest in Mps1, and would still leave it in good company. Cdk2 was long assumed to be essential not just for centrosome duplication but for DNA replication as well, yet mice null for Cdk2 are viable  and Cdk2 is dispensable for centrosome duplication . This doesn't mean that Cdk2 is unimportant, and indeed Cdk2 is required for centrosome re-duplication . Likewise, Cetn2, perhaps the most well known centriole marker, is dispensable for centriole assembly [14, 15, 28]. While it is clearly necessary to understand the core evolutionarily conserved centriole assembly pathway, it is also critical to understand how centriole assembly occurs within a human cell when all the components are present. It is also critical to understand the types of defects that might contribute to the appearance of extra centrosomes in tumors, even if those defects involve players that are not essential for the canonical assembly cycle such as Cdk2  and Cetn2 . Indeed, centriole overproduction has proven a useful model system for understanding centriole assembly in general . Regardless of whether centrioles can be assembled in the absence of hMps1, our recent data support the idea that when present it plays multiple roles in centriole assembly. Indeed, even this seems to be conserved from yeast, where Mps1 acts in at least two steps in SPB duplication . Moreover, subtle changes in hMps1 or its regulators can lead to centriole overproduction [28, 29, 31, 47], and although it remains to be demonstrated we presume that this reflects the failure to properly control its functions in the canonical centriole assembly cycle, even if those functions are dispensable.
Mps1 is an intriguing multifunctional enzyme with multiple roles in both the centriole and nuclear cycles (Figure 6). In the centriole cycle these functions include its roles in procentriole assembly and centriole maturation, discussed above, critical for bipolar mitotic spindle assembly and accurate segregation of the duplicated genome. In the nuclear cycle these functions include the SAC that ensures proper segregation of the duplicated genome, cytokinesis that ensures the final partitioning of the duplicated genome, and at least two different cellular responses that halt cell cycle progression in the presence of missegregated or damaged DNA. Accordingly, each of the functions thus far described for Mps1 in human cells is related to the maintenance of genomic integrity. While these Mps1 functions may be of differing importance in different contexts, it will be critical to understand how each of these functions is regulated given that hMps1 has been proposed as a target for cancer therapy (see  and references therein). Clearly, there are many challenges ahead for understanding the role of hMps1 at centrosomes. First, we must determine one way or the other whether it is essential for centriole assembly. However, regardless of the answer, we must take a more systematic approach to identifying the centrosomal substrates of hMps1, in order to characterize the role of hMps1 phosphorylation, dispensable or otherwise, in centriole assembly at a molecular level. In addition, we need to place hMps1 in the great puzzle that is centriole assembly. While Plk4 is the major driver of centriole assembly, it is not functionally interchangeable with Zyg-1 , nor is it apparently a true orthologue of Zyg-1, which according to a recent study is more closely related to Mps1 [20, 92]. Moreover, while in C. elegans the recruitment of Sas-6 to the procentriole requires ZYG-1 [7, 8], Plk4 is not required for recruitment of hSas6 to the centriole, although it is required for the maintenance of hSas6 at the centriole and for daughter centriole assembly . This suggests that Plk4 does not assume all of the functions ascribed to ZYG-1, and that other players must be involved in human cells. We assume that hMps1 is one of these players, and a future challenge will be to determine the relationships between hMps1, Plk4, and hSas6. Moreover, Cetn2 has as yet unidentified functions in centriole assembly that are independent of hSas6 but require hMps1, suggesting that hMps1 might also have centrosomal functions outside of the canonical cartwheel driven centriole assembly pathway.
spindle pole body
Monopolar spindles 1
Spindle Assembly Checkpoint
Cyclin A-associated cyclin-dependent kinase 2
Mps1 degradation signal
Ornithine Decarboxylase Antizyme
anaphase promoting complex/cyclosome
Nigg EA: Centrosome duplication: of rules and licenses. Trends Cell Biol 2007, 17: 215–221. 10.1016/j.tcb.2007.03.003
Tsou MF, Stearns T: Controlling centrosome number: licenses and blocks. Curr Opin Cell Biol 2006, 18: 74–78. 10.1016/j.ceb.2005.12.008
Adams IR, Kilmartin JV: Spindle pole body duplication: a model for centrosome duplication? Trends Cell Biol 2000, 10: 329–335. 10.1016/S0962-8924(00)01798-0
Fisk HA, Mattison CP, Winey M: Centrosomes and tumour suppressors. Curr Opin Cell Biol 2002, 14: 700–705. 10.1016/S0955-0674(02)00385-X
Keller LC, Romijn EP, Zamora I, Yates JR, Marshall WF: Proteomic analysis of isolated chlamydomonas centrioles reveals orthologs of ciliary-disease genes. Curr Biol 2005, 15: 1090–1098. 10.1016/j.cub.2005.05.024
Delattre M, Canard C, Gonczy P: Sequential protein recruitment in C. elegans centriole formation. Curr Biol 2006, 16: 1844–1849. 10.1016/j.cub.2006.07.059
Pelletier L, O'Toole E, Schwager A, Hyman AA, Muller-Reichert T: Centriole assembly in Caenorhabditis elegans. Nature 2006, 444: 619–623. 10.1038/nature05318
Dammermann A, Muller-Reichert T, Pelletier L, Habermann B, Desai A, Oegema K: Centriole assembly requires both centriolar and pericentriolar material proteins. Dev Cell 2004, 7: 815–829. 10.1016/j.devcel.2004.10.015
Leidel S, Gonczy P: Centrosome duplication and nematodes: recent insights from an old relationship. Dev Cell 2005, 9: 317–325. 10.1016/j.devcel.2005.08.004
O'Connell KF: The ZYG-1 kinase, a mitotic and meiotic regulator of centriole replication. Oncogene 2002, 21: 6201–6208.
Kemp CA, Kopish KR, Zipperlen P, Ahringer J, O'Connell KF: Centrosome maturation and duplication in C. elegans require the coiled-coil protein SPD-2. Dev Cell 2004, 6: 511–523. 10.1016/S1534-5807(04)00066-8
Pearson CG, Winey M: Basal Body Assembly in Ciliates: The Power of Numbers. Traffic 2009,10(5):461–71. 10.1111/j.1600-0854.2009.00885.x
Stevens NR, Dobbelaere J, Brunk K, Franz A, Raff JW: Drosophila Ana2 is a conserved centriole duplication factor. J Cell Biol 2010, 188: 313–323. 10.1083/jcb.200910016
Strnad P, Leidel S, Vinogradova T, Euteneuer U, Khodjakov A, Gonczy P: Regulated HsSAS-6 levels ensure formation of a single procentriole per centriole during the centrosome duplication cycle. Dev Cell 2007, 13: 203–213. 10.1016/j.devcel.2007.07.004
Kleylein-Sohn J, Westendorf J, Le Clech M, Habedanck R, Stierhof YD, Nigg EA: Plk4-induced centriole biogenesis in human cells. Dev Cell 2007, 13: 190–202. 10.1016/j.devcel.2007.07.002
Habedanck R, Stierhof YD, Wilkinson CJ, Nigg EA: The Polo kinase Plk4 functions in centriole duplication. Nat Cell Biol 2005, 7: 1140–1146. 10.1038/ncb1320
Franck N, Montembault E, Rome P, Pascal A, Cremet JY, Giet R: CDK11 Is Required for Centriole Duplication and Plk4 Recruitment to Mitotic Centrosomes. PLoS One 2010, 6: e14600. 10.1371/journal.pone.0014600
Cizmecioglu O, Arnold M, Bahtz R, Settele F, Ehret L, Haselmann-Weiss U, Antony C, Hoffmann I: Cep152 acts as a scaffold for recruitment of Plk4 and CPAP to the centrosome. J Cell Biol 2010, 191: 731–739. 10.1083/jcb.201007107
Hatch EM, Kulukian A, Holland AJ, Cleveland DW, Stearns T: Cep152 interacts with Plk4 and is required for centriole duplication. J Cell Biol 2010, 191: 721–729. 10.1083/jcb.201006049
Sillibourne JE, Bornens M: Polo-like kinase 4: the odd one out of the family. Cell Div 2010, 5: 25. 10.1186/1747-1028-5-25
Tang CJ, Fu RH, Wu KS, Hsu WB, Tang TK: CPAP is a cell-cycle regulated protein that controls centriole length. Nat Cell Biol 2009, 11: 825–831. 10.1038/ncb1889
Schmidt TI, Kleylein-Sohn J, Westendorf J, Le Clech M, Lavoie SB, Stierhof YD, Nigg EA: Control of centriole length by CPAP and CP110. Curr Biol 2009, 19: 1005–1011. 10.1016/j.cub.2009.05.016
Kohlmaier G, Loncarek J, Meng X, McEwen BF, Mogensen MM, Spektor A, Dynlacht BD, Khodjakov A, Gonczy P: Overly long centrioles and defective cell division upon excess of the SAS-4-related protein CPAP. Curr Biol 2009, 19: 1012–1018. 10.1016/j.cub.2009.05.018
Kim K, Lee S, Chang J, Rhee K: A novel function of CEP135 as a platform protein of C-NAP1 for its centriolar localization. Exp Cell Res 2008, 314: 3692–3700. 10.1016/j.yexcr.2008.09.016
Spektor A, Tsang WY, Khoo D, Dynlacht BD: Cep97 and CP110 suppress a cilia assembly program. Cell 2007, 130: 678–690. 10.1016/j.cell.2007.06.027
Chang P, Stearns T: δ-Tubulin and ε-tubulin: two new human centrosomal tubulins reveal new aspects of centrosome structure and function. Nature: Cell Biology 2000, 2: 30–35. 10.1038/71350
Chang P, Giddings TH Jr, Winey M, Stearns T: Epsilon-tubulin is required for centriole duplication and microtubule organization. Nat Cell Biol 2003, 5: 71–76. 10.1038/ncb900
Yang CH, Kasbek C, Majumder S, Mohd Yusof A, Fisk HA: Mps1 phosphorylation sites regulate the function of Centrin 2 in centriole assembly. Molecular Biology of the Cell 2010, 21: 4361–4372. 10.1091/mbc.E10-04-0298
Kasbek C, Yang CH, Fisk HA: Antizyme Restrains Centrosome Amplification by Regulating the Accumulation of Mps1 at Centrosomes. Molecular Biology of the Cell 2010, 21: 3879–3889. 10.1091/mbc.E10-04-0281
Kanai M, Ma Z, Izumi H, Kim SH, Mattison CP, Winey M, Fukasawa K: Physical and functional interaction between mortalin and Mps1 kinase. Genes Cells 2007, 12: 797–810.
Kasbek C, Yang CH, Yusof AM, Chapman HM, Winey M, Fisk HA: Preventing the degradation of mps1 at centrosomes is sufficient to cause centrosome reduplication in human cells. Mol Biol Cell 2007, 18: 4457–4469. 10.1091/mbc.E07-03-0283
Fisk HA, Mattison CP, Winey M: Human Mps1 protein kinase is required for centrosome duplication and normal mitotic progression. Proc Natl Acad Sci USA 2003, 100: 14875–14880. 10.1073/pnas.2434156100
Salisbury J, Suino K, Busby R, Springett M: Centrin-2 is required for centriole duplication in Mammalian cells. Curr Biol 2002, 12: 1287. 10.1016/S0960-9822(02)01019-9
Azimzadeh J, Hergert P, Delouvee A, Euteneuer U, Formstecher E, Khodjakov A, Bornens M: hPOC5 is a centrin-binding protein required for assembly of full-length centrioles. J Cell Biol 2009, 185: 101–114. 10.1083/jcb.200808082
Tsang WY, Spektor A, Vijayakumar S, Bista BR, Li J, Sanchez I, Duensing S, Dynlacht BD: Cep76, a centrosomal protein that specifically restrains centriole reduplication. Dev Cell 2009, 16: 649–660. 10.1016/j.devcel.2009.03.004
Loncarek J, Hergert P, Magidson V, Khodjakov A: Control of daughter centriole formation by the pericentriolar material. Nat Cell Biol 2008, 10: 322–328. 10.1038/ncb1694
Azimzadeh J, Marshall WF: Building the centriole. Curr Biol 2010, 20: R816–825. 10.1016/j.cub.2010.08.010
Kitagawa D, Vakonakis I, Olieric N, Hilbert M, Keller D, Olieric V, Bortfeld M, Erat MC, Fluckiger I, Gonczy P, Steinmetz MO: Structural basis of the 9-fold symmetry of centrioles. Cell 2011, 144: 364–375. 10.1016/j.cell.2011.01.008
Tsou MF, Stearns T: Mechanism limiting centrosome duplication to once per cell cycle. Nature 2006, 442: 947–951. 10.1038/nature04985
Guderian G, Westendorf J, Uldschmid A, Nigg EA: Plk4 trans-autophosphorylation regulates centriole number by controlling betaTrCP-mediated degradation. J Cell Sci 2010, 123: 2163–2169. 10.1242/jcs.068502
Holland AJ, Lan W, Niessen S, Hoover H, Cleveland DW: Polo-like kinase 4 kinase activity limits centrosome overduplication by autoregulating its own stability. J Cell Biol 2010, 188: 191–198. 10.1083/jcb.200911102
Korzeniewski N, Zheng L, Cuevas R, Parry J, Chatterjee P, Anderton B, Duensing A, Munger K, Duensing S: Cullin 1 functions as a centrosomal suppressor of centriole multiplication by regulating polo-like kinase 4 protein levels. Cancer Res 2009, 69: 6668–6675. 10.1158/0008-5472.CAN-09-1284
Ganem NJ, Godinho SA, Pellman D: A mechanism linking extra centrosomes to chromosomal instability. Nature 2009, 460: 278–282. 10.1038/nature08136
McDermott KM, Zhang J, Holst CR, Kozakiewicz BK, Singla V, Tlsty TD: p16(INK4a) prevents centrosome dysfunction and genomic instability in primary cells. PLoS Biology 2006, 4: e51. 10.1371/journal.pbio.0040051
Lingle WL, Barrett SL, Negron VC, D'Assoro AB, Boeneman K, Liu W, Whitehead CM, Reynolds C, Salisbury JL: Centrosome amplification drives chromosomal instability in breast tumor development. Proc Natl Acad Sci USA 2002, 99: 1978–1983. 10.1073/pnas.032479999
Pihan GA, Wallace J, Zhou Y, Doxsey SJ: Centrosome abnormalities and chromosome instability occur together in pre-invasive carcinomas. Cancer Res 2003, 63: 1398–1404.
Kasbek C, Yang CH, Fisk HA: Mps1 as a link between centrosomes and genetic instability. Environmental and Molecular Mutagenesis 2009, 50: 654–665. 10.1002/em.20476
Fisk HA, Mattison CP, Winey M: A Field Guide to the Mps1 Family of Protein Kinases. Cell Cycle 2004, 3: 439–442. 10.4161/cc.3.4.784
Winey M, Goetsch L, Baum P, Byers B: MPS1 and MPS2 : Novel yeast genes defining distinct steps of spindle pole body duplication. J of Cell Biol 1991, 114: 745–754. 10.1083/jcb.114.4.745
Weiss E, Winey M: The Saccharomyces cerevisiae spindle pole body duplication gene MPS1 is part of a mitotic checkpoint. J Cell Biol 1996, 132: 111–123. 10.1083/jcb.132.1.111
Hardwick K, Weiss E, Luca FC, Winey M, Murray A: Activation of the budding yeast spindle assembly checkpoint without mitotic spindle disruption. Science 1996, 273: 953–956. 10.1126/science.273.5277.953
Fischer MG, Heeger S, Hacker U, Lehner CF: The mitotic arrest in response to hypoxia and of polar bodies during early embryogenesis requires Drosophila Mps1. Curr Biol 2004.
Gilliland WD, Wayson SM, Hawley RS: The meiotic defects of mutants in the Drosophila mps1 gene reveal a critical role of Mps1 in the segregation of achiasmate homologs. Curr Biol 2005, 15: 672–677. 10.1016/j.cub.2005.02.062
Poss KD, Nechiporuk A, Hillam AM, Johnson SL, Keating MT: Mps1 defines a proximal blastemal proliferative compartment essential for zebrafish fin regeneration. Development 2002, 129: 5141–5149.
Lan W, Cleveland DW: A chemical tool box defines mitotic and interphase roles for Mps1 kinase. J Cell Biol 2010, 190: 21–24. 10.1083/jcb.201006080
Zhu S, Wang W, Clarke DC, Liu X: Activation of Mps1 promotes transforming growth factor-beta-independent Smad signaling. J Biol Chem 2007, 282: 18327–18338. 10.1074/jbc.M700636200
Huang YF, Chang MD, Shieh SY: TTK/hMps1 mediates the p53-dependent postmitotic checkpoint by phosphorylating p53 at Thr18. Mol Cell Biol 2009, 29: 2935–2944. 10.1128/MCB.01837-08
Yeh YH, Huang YF, Lin TY, Shieh SY: The cell cycle checkpoint kinase CHK2 mediates DNA damage-induced stabilization of TTK/hMps1. Oncogene 2009, 28: 1366–1378. 10.1038/onc.2008.477
Mattison CP, Stumpff J, Wordeman L, Winey M: Mip1 associates with both the Mps1 kinase and actin, and is required for cell cortex stability and anaphase spindle positioning. Cell Cycle 2011., 10: 10.4161/cc.10.5.14955
Kwiatkowski N, Jelluma N, Filippakopoulos P, Soundararajan M, Manak MS, Kwon M, Choi HG, Sim T, Deveraux QL, Rottmann S, et al.: Small-molecule kinase inhibitors provide insight into Mps1 cell cycle function. Nat Chem Biol 2010, 6: 359–368. 10.1038/nchembio.345
Stucke VM, Sillje HH, Arnaud L, Nigg EA: Human Mps1 kinase is required for the spindle assembly checkpoint but not for centrosome duplication. Embo J 2002, 21: 1723–1732. 10.1093/emboj/21.7.1723
Douville EM, Afar DEH, Howell BW, Letwin K, Tannock L, Ben-David Y, Pawson T, Bell JC: Multiple cDNAs encoding the esk kinase predict transmembrane and intracellular enzyme isoforms. Mol Cell Biol 1992, 12: 2681–2689.
Mills GB, Schmandt R, McGill M, Amendola A, Hill M, Jacobs K, May C, Rodricks A, Campbell S, Hogg D: Expression of TTK, a novel human protein kinase, is associated with cell proliferation. J Biol Chem 1992, 267: 16000–16006.
Hogg D, Guidos C, Bailey D, Amendola A, Groves T, Davidson J, Schmandt R, Mills G: Cell cycle dependent regulation of the protein kinase TTK. Oncogene 1994, 9: 89–96.
Lindberg RA, Fischer WH, Hunter T: Characterization of a human protein threonine kinase isolated by screening an expression library with antibodies to phosphotyrosine. Oncogene 1993, 8: 351–359.
Fisk HA, Winey M: The mouse mps1p-like kinase regulates centrosome duplication. Cell 2001, 106: 95–104. 10.1016/S0092-8674(01)00411-1
Abrieu A, Magnaghi-Jaulin L, Kahana JA, Peter M, Castro A, Vigneron S, Lorca T, Cleveland DW, Labbe JC: Mps1 is a kinetochore-associated kinase essential for the vertebrate mitotic checkpoint. Cell 2001, 106: 83–93. 10.1016/S0092-8674(01)00410-X
Liu ST, Chan GK, Hittle JC, Fujii G, Lees E, Yen TJ: Human MPS1 Kinase Is Required for Mitotic Arrest Induced by the Loss of CENP-E from Kinetochores. Mol Biol Cell 2003, 14: 1638–1651. 10.1091/mbc.02-05-0074
Tyler RK, Chu ML, Johnson H, McKenzie EA, Gaskell SJ, Eyers PA: Phosphoregulation of human Mps1 kinase. Biochem J 2009, 417: 173–181. 10.1042/BJ20081310
Balczon RC: Overexpression of cyclin A in human HeLa cells induces detachment of kinetochores and spindle pole/centrosome overproduction. Chromosoma 2001, 110: 381–392. 10.1007/s004120100157
Kahana C, Asher G, Shaul Y: Mechanisms of protein degradation: an odyssey with ODC. Cell Cycle 2005, 4: 1461–1464. 10.4161/cc.4.11.2115
Zhang M, Pickart CM, Coffino P: Determinants of proteasome recognition of ornithine decarboxylase, a ubiquitin-independent substrate. Embo J 2003, 22: 1488–1496. 10.1093/emboj/cdg158
Mangold U, Hayakawa H, Coughlin M, Munger K, Zetter BR: Antizyme, a mediator of ubiquitin-independent proteasomal degradation and its inhibitor localize to centrosomes and modulate centriole amplification. Oncogene 2008, 27: 604–613. 10.1038/sj.onc.1210685
Palframan WJ, Meehl JB, Jaspersen SL, Winey M, Murray AW: Anaphase Inactivation of the Spindle Checkpoint. Science 2006,313(5787):680–684. 10.1126/science.1127205
Cui Y, Cheng X, Zhang C, Zhang Y, Li S, Wang C, Guadagno TM: Degradation of the human mitotic checkpoint kinase Mps1 is cell cycle-regulated by APC-cCdc20 and APC-cCdh1 ubiquitin ligases. J Biol Chem 2010, 285: 32988–32998. 10.1074/jbc.M110.140905
Castillo AR, Meehl JB, Morgan G, Schutz-Geschwender A, Winey M: The yeast protein kinase Mps1p is required for assembly of the integral spindle pole body component Spc42p. J Cell Biol 2002, 156: 453–465. 10.1083/jcb.200111025
Araki Y, Gombos L, Migueleti SP, Sivashanmugam L, Antony C, Schiebel E: N-terminal regions of Mps1 kinase determine functional bifurcation. J Cell Biol 2010, 189: 41–56. 10.1083/jcb.200910027
Schutz AR, Winey M: New alleles of the yeast MPS1 gene reveal multiple requirements in spindle pole body duplication. Mol Biol Cell 1998, 9: 759–774.
Maciejowski J, George KA, Terret ME, Zhang C, Shokat KM, Jallepalli PV: Mps1 directs the assembly of Cdc20 inhibitory complexes during interphase and mitosis to control M phase timing and spindle checkpoint signaling. J Cell Biol 2010, 190: 89–100. 10.1083/jcb.201001050
Sliedrecht T, Zhang C, Shokat KM, Kops GJ: Chemical genetic inhibition of Mps1 in stable human cell lines reveals novel aspects of Mps1 function in mitosis. PLoS One 2010, 5: e10251. 10.1371/journal.pone.0010251
Tighe A, Staples O, Taylor S: Mps1 kinase activity restrains anaphase during an unperturbed mitosis and targets Mad2 to kinetochores. J Cell Biol 2008, 181: 893–901. 10.1083/jcb.200712028
Hewitt L, Tighe A, Santaguida S, White AM, Jones CD, Musacchio A, Green S, Taylor SS: Sustained Mps1 activity is required in mitosis to recruit O-Mad2 to the Mad1-C-Mad2 core complex. J Cell Biol 2010, 190: 25–34. 10.1083/jcb.201002133
Santaguida S, Tighe A, D'Alise AM, Taylor SS, Musacchio A: Dissecting the role of MPS1 in chromosome biorientation and the spindle checkpoint through the small molecule inhibitor reversine. J Cell Biol 2010, 190: 73–87. 10.1083/jcb.201001036
Colombo R, Caldarelli M, Mennecozzi M, Giorgini ML, Sola F, Cappella P, Perrera C, Depaolini SR, Rusconi L, Cucchi U, et al.: Targeting the mitotic checkpoint for cancer therapy with NMS-P715, an inhibitor of MPS1 kinase. Cancer Res 2010, 70: 10255–10264. 10.1158/0008-5472.CAN-10-2101
Poss KD, Nechiporuk A, Stringer KF, Lee C, Keating MT: Germ cell aneuploidy in zebrafish with mutations in the mitotic checkpoint gene mps1. Genes Dev 2004, 18: 1527–1532. 10.1101/gad.1182604
Stemm-Wolf AJ, Morgan G, Giddings TH Jr, White EA, Marchione R, McDonald HB, Winey M: Basal body duplication and maintenance require one member of the Tetrahymena thermophila centrin gene family. Mol Biol Cell 2005, 16: 3606–3619. 10.1091/mbc.E04-10-0919
Ruiz F, Garreau de Loubresse N, Klotz C, Beisson J, Koll F: Centrin deficiency in Paramecium affects the geometry of basal-body duplication. Curr Biol 2005, 15: 2097–2106. 10.1016/j.cub.2005.11.038
Gogendeau D, Klotz C, Arnaiz O, Malinowska A, Dadlez M, de Loubresse NG, Ruiz F, Koll F, Beisson J: Functional diversification of centrins and cell morphological complexity. J Cell Sci 2008, 121: 65–74. 10.1242/jcs.019414
Khodjakov A, Cole RW, Oakley BR, Rieder CL: Centrosome-independent mitotic spindle formation in vertebrates. Current Biology 2000, 10: 59–67. 10.1016/S0960-9822(99)00276-6
Berthet C, Aleem E, Coppola V, Tessarollo L, Kaldis P: Cdk2 knockout mice are viable. Curr Biol 2003, 13: 1775–1785. 10.1016/j.cub.2003.09.024
Duensing A, Liu Y, Tseng M, Malumbres M, Barbacid M, Duensing S: Cyclin-dependent kinase 2 is dispensable for normal centrosome duplication but required for oncogene-induced centrosome overduplication. Oncogene 2006, 25: 2943–2949. 10.1038/sj.onc.1209310
Carvalho-Santos Z, Machado P, Branco P, Tavares-Cadete F, Rodrigues-Martins A, Pereira-Leal JB, Bettencourt-Dias M: Stepwise evolution of the centriole-assembly pathway. J Cell Sci 123: 1414–1426. 10.1242/jcs.064931
Acknowledgements and Funding
The work on centrosomal Mps1, OAZ, and Cetn2 summarized in this review was supported by a seed grant from the Ohio Cancer Research Associates, and NIH R01 GM077311, both to HAF.
The authors declare that they have no competing interests.
ANP and HAF made equal contributions to construction of the manuscript. ANP performed the kinase assays in Figure 5, and HAF constructed the figures. Both authors read and approved the final manuscript