Cdk/Cyclin activity helps set mitotic centrosome size by influencing the centrosome growth rate and growth period

Mitotic centrosomes assemble when centrioles recruit large amounts of pericentriolar material (PCM) around themselves in preparation for cell division. How the mitotic PCM grows to the correct size is unclear. In Drosophila syncytial embryos, thousands of mitotic centrosomes assemble in a common cytoplasm as the embryo proceeds through 13 rounds of near-synchronous nuclear division. During nuclear cycles (NCs) 11-13 these divisions gradually slow, and we find that mitotic centrosomes respond by reciprocally slowing their growth rate and increasing their growth period so that they grow to a consistent size at each cycle. This size homeostasis is enforced, at least in part, by the Cdk/Cyclin cell cycle oscillator (CCO). Moderate levels of CCO activity appear to initially promote centrosome growth by stimulating Polo/PLK1 recruitment to centrosomes, while higher levels of activity subsequently inhibit centrosome growth by phosphorylating centrosome proteins to decrease their centrosomal recruitment and/or maintenance as the embryos enter mitosis. Thus, the CCO initially promotes, and subsequently restricts, mitotic centrosome growth to help ensure that centrosomes grow to a consistent size.