RT Journal Article SR Electronic T1 Polo kinase phosphorylation determines C. elegans centrosome size and density by biasing SPD-5 toward an assembly-competent conformation JF bioRxiv FD Cold Spring Harbor Laboratory SP 067223 DO 10.1101/067223 A1 Oliver Wueseke A1 David Zwicker A1 Anne Schwager A1 Yao Liang Wong A1 Karen Oegema A1 Frank Jülicher A1 Anthony A. Hyman A1 Jeffrey B. Woodruff YR 2016 UL http://biorxiv.org/content/early/2016/08/01/067223.abstract AB Centrosomes are major microtubule-organizing centers composed of centrioles surrounded by an extensive proteinacious layer called the pericentriolar material (PCM). In C. elegans embryos, the mitotic PCM expands by Polo-kinase (PLK-1) phosphorylation-accelerated assembly of SPD-5 molecules into supramolecular scaffolds. However, how PLK-1 phosphorylation regulates SPD-5 assembly is not known. We found that a mutant version of SPD-5 that is insensitive to PLK-1 phosphorylation (SPD-54A) could localize to PCM but was unable to rescue the reduction in PCM size and density when wild-type SPD-5 levels were decreased. In vitro, purified SPD-54A self-assembled into functional supramolecular scaffolds over long time scales, suggesting that phosphorylation only controls the rate of SPD-5 scaffold assembly. Furthermore, the SPD-5 scaffold, once assembled, remained intact and supported microtubule nucleation in the absence of PLK-1 activity in vivo. We conclude that Polo Kinase is required for rapid assembly of the PCM scaffold but not for scaffold maintenance or function. Based on this idea, we developed a theoretical model that adequately predicted PCM growth rates in different mutant conditions in vivo. We propose that PLK-1 phosphorylation-dependent conversion of SPD-5 into an assembly-competent form underlies PCM formation in vivo and that the rate of this conversion determines final PCM size and density.