Molecular Mechanisms of Centriole Assembly

Abstract

Chromosomal Instability (CIN) occurs in over 90% of all sporadic tumors and manifests as whole chromosome loss or gain, gene deletions, amplifications, inversion, and translocations. CIN is not only a hallmark of cancer but promotes tumorigenesis. CIN is caused by errors during mitosis and one major CIN-promoting mechanism is centrosome over-duplication (amplification); another cancer hallmark. Centrosome amplification causes abnormal mitotic spindle assembly, directly promoting chromosome mis-segration with consequent aneuploidy and other forms of CIN. Central to controlling centrosome numbers and function are the Polo kinases, including Polo-like kinase 4 (Plk4). Plk4 is a component of centrosomes and recognized as the master-regulator of centrosome function and duplication. Plk4 is a mitotic kinase whose levels increase throughout S-phase and G2 to peak in mitosis. During late mitosis, Plk4 localizes to a spot on parent centrioles, licensing this single site for future daughter centriole assembly. Plk4 activity initiates the hierarchial recruiment of two conserved essential centriole proteins: Ana2, followed by the cartwheel protein Sas6. By analysis in a yeast-2-hybrid screen, we identified several novel interactions of centriole proteins, including the interaction of Ana2 and Plk4. Plk4 phosphorylates Ana2 to both positively and negatively regulate centriole duplication. Our preliminary data suggests that Plk4 recruits Ana2 by phosphorylating a protein on the outer centriole surface, generating a phospho-landing platform, and that this Plk4 target is Sas4 (CPAP in humans). Notably, the Sas4 pattern on centrioles is complex, forming both a ring and an asymmetric spot during mitotic progression. Like Sas4, Ana2 is a Plk4 substrate, and when mixed with purified Ana2, Sas4 stimulates Ana2 hyperphosphorylation in vitro. Thus, Plk4 influences centriole assembly on multiple platforms.