TY - JOUR T1 - Polo kinase phosphorylation determines <em>C. elegans</em> centrosome size and density by biasing SPD-5 toward an assembly-competent conformation JF - bioRxiv DO - 10.1101/067223 SP - 067223 AU - Oliver Wueseke AU - David Zwicker AU - Anne Schwager AU - Yao Liang Wong AU - Karen Oegema AU - Frank Jülicher AU - Anthony A. Hyman AU - Jeffrey B. Woodruff Y1 - 2016/01/01 UR - http://biorxiv.org/content/early/2016/08/01/067223.abstract N2 - 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. ER -