Cell Cycle-Dependent Regulation of Centriole Duplication

Abstract

Centrosomes are organelles that promote microtubule growth. Normally, a single centrosome duplicates once each cell cycle to guide assembly of a bipolar mitotic spindle, ensuring that each daughter cell inherits an equal complement of the genome and a single centrosome. Centrosomes are composed of a pair of ‘mother-daughter’ centrioles and, during duplication, each mother centriole assembles one daughter at a single site. However, mother centrioles can inappropriately assemble multiple daughters, thereby generating centriole amplification (or overduplication), resulting in multipolar spindle assembly and, consequently, chromosome missegration - a driving force for chromosomal instability/aneuploidy which induces birth defects, miscarriage, and tumorigenesis. We have elucidated how the cell cycle control program regulates the centriole duplication machinery to limit centriole duplication to one event per cell cycle via the cell cycle-dependent regulation of Ana2/STIL and PLK4 degradation. In the case of the centrosome licensing factor Plk4, we found that autophosphorylation promotes its own destruction during interphase, which is then counteracted by the Protein Phosphatase 2A (PP2A) in complex with its Twins (tws) regulatory subunit during mitosis. This promotes stabilization of Plk4 and thus allows for the licensing of the mother centriole, making it competent to duplicate during the proceeding S-phase. While PP2Atws plays a positive role in regulating Plk4 to promote centriole duplication, we found that PP2A complexed with the Well-rounded (wrd) and Widerborst (wdb) regulatory subunits negatively regulates Ana2 by promoting its degradation to limit centriole duplication. PP2Awrd/wdb dephosphorylates numerous serine/threonine residues residing in Ana2, including several CDK phosphorylation consensus motifs. We found that CDK1/cycA and CDK2/cycE phosphorylate these residues to promote Ana2 stabilization from S-phase, the start of centriole duplication, to M-phase, the start of centriole duplication licensing. Interestingly, we found that the tumorigenic SV40 virus protein Small Tumor Antigen (ST) amplifies centrioles by targeting the PP2A complex to stabilize Plk4 as well as Ana2, underscoring the oncogenic importance of these newly discovered centriole duplication pathways. Finally, we shed insight into the mechanism for centriole amplification upon Ana2 stabilization by showing that Ana2 associates with Plk4 to promote Plk4 kinase activity as well as Plk4 stabilization.