In metazoans, Plk4 kinases control daughter centriole assembly. Plk4 homologs have an N-terminal kinase domain, a C-terminal polo box, and a central domain termed the 'cryptic polo box' (CPB) that has been shown to dimerize, to be sufficient for centriole localization and to be required for Plk4 to promote centriole assembly. Probable serine/threonine-protein kinase zyg-1 (EC:2.7.11.1) (ZYG-1) is a Plk4 homlog found in C. elegans. Crystal structure for the CPB of C. elegans ZYG-1, reveals that it forms a Z-shaped dimer containing an intermolecular beta-sheet with an extended basic surface patch. Electrostatic interactions between the basic patch on the ZYG-1 CPB dimer and the SPD-2 acidic region dock ZYG-1 onto centrioles to promote new centriole assembly. ZYG-1 CPB contains two tandem polo boxes (PB1 and PB2), each containing a six-stranded beta-sheet with an alpha-helix packed against one side [1].

1: Structure of the C. elegans ZYG-1 cryptic polo box suggests a conserved mechanism for centriolar docking of Plk4 kinases. Shimanovskaya E, Viscardi V, Lesigang J, Lettman MM, Qiao R, Svergun DI, Round A, Oegema K, Dong G; Structure. 2014;22:1090-1104. PMID:24980795