Project Summary Centrioles form the core of the centrosome or microtubule-organizing center (MTOC), and serve as the basal body for ciliogenesis. To faithfully execute these functions in cycling cells, both the architecture and copy number of the centriole need to be correctly specified. The structural integrity of the centriole is established through stepwise assemblies of a series of structural components, many of which serve as the building block, scaffold, or stabilizing factor. While self-assembly is a feature of centriole formation, one or more steps of the assembly pathway are guarded by enzymatic regulators to allow quality and quantity controls. Plk4 kinase and PP2A phosphatase are two such enzymes known to closely involve in centriole assembly. However, the specific assembly step(s) controlled by Plk4 or PP2A, the relevant substrates, how the two (biochemically) opposing enzymes coordinate, and whether/how their enzymatic activities are regulated during centriole assembly remain outstanding questions. Equally unclear is the regulatory module we have recently shown to promote centriole to centrosome conversion at the late mitosis, a key process through which centriole homeostasis is maintained in proliferating cells. Despite its importance, the centriolar factors involved in centriole-to- centrosome conversion, and how these factors or activities interact with mitotic kinases are completely unknown. The aims of this proposal are to characterize (i) the regulation and action of Plk4 & PP2A in centriole assembly, and (ii) the molecular basis and functional implications of centriole-to-centrosome conversion.