Figure 3

The chromosomal passenger complex generates an alternative APC/C inhibitory signal that amplifies the 'wait anaphase' signal from unattached kinetochores. Based on different sets of data (see text for details), we propose that in response to inappropriately attached kinetochores, the chromosomal passenger complex not only destabilises these attachments but also elicits an additional signal that inhibits the APC/C [35]. This could be via direct phosphorylation of APC/C subunits, or via direct control of the spindle assembly checkpoint. Ways be which the chromosomal passenger complex could exert direct control over the spindle checkpoint are through regulation of BubR1/Bub1 kinetochore levels, modulation of the mitotic checkpoint complex or via an as yet unknown pathway. Regardless the mechanism, this additional signal is thought to amplify the unattached kinetochore-derived signal, resulting in a robust checkpoint response when the number of unattached kinetochores is low.