The centrosome is a selective phase that nucleates microtubules by concentrating tubulin

Centrosomes are non-membrane-bound compartments that nucleate microtubule arrays. They consist of nanometer-scale centrioles surrounded by a micron-scale, dynamic assembly of protein called the pericentriolar material (PCM). To study how PCM forms a spherical compartment that nucleates microtubules, we reconstituted PCM-dependent microtubule nucleationin vitrousing recombinantC.elegansproteins. We found that macromolecular crowding drives phase separation of the key PCM scaffold protein SPD-5 into spherical droplets that morphologically and dynamically resemblein vivoPCM. These SPD-5 droplets recruited the microtubule polymerase ZYG-9 (XMAP215 homologue) and the microtubule-stabilizing protein TPXL-1 (TPX2 homologue). Together, these three proteins concentrated tubulin ~4- fold over background, which was sufficient to reconstitute nucleation of microtubule astersin vitro.Our results suggest thatin vivoPCM is a selective phase that organizes microtubule arrays through localized concentration of tubulin by microtubule effector proteins.One Sentence SummaryPhase separation ofC. eleganscentrosome proteins drive the formation of micron-sized microtubule organizing centers.