The structure of PLK4 and zyg-1. PLK4 differs from the other members of the PLK family in that it only has a single Polo-box, as opposed to two, and possesses a crypto Polo-box instead. These domains are involved in targeting the kinase to the centrosome and both are independently able to localize to the centrosome when fused to GFP. In Drosophila PLK4, a PEST sequence located after the catalytic domain is an important element controlling the stability of the kinase. This PEST sequence is also present and functional in mouse and human PLK4 along with two PEST sequences at the carboxy terminus of the kinase. The C. elegans kinase, zyg1 is also shown because it fulfills a similar role as PLK4 in the nematode although it is not related to it.