| Summary: | Mitotic centrosomes assemble when centrioles recruit large amounts of pericentriolar material (PCM) around themselves. The PCM comprises hundreds of proteins, and there is much debate about its physical nature. Here, we show that Drosophila Spd-2 (human CEP192) fluxes out from centrioles, recruiting Polo and Aurora A kinases to catalyze the assembly of two distinct mitotic-PCM scaffolds: a Polo-dependent Cnn scaffold, and an Aurora A–dependent TACC scaffold, which exhibit solid- and liquid-like behaviors, respectively. Both scaffolds can independently recruit PCM proteins, but both are required for proper centrosome assembly, with the Cnn scaffold providing mechanical strength, and the TACC scaffold concentrating centriole and centrosome proteins. Recruiting Spd-2 to synthetic beads injected into early embryos reconstitutes key aspects of mitotic centrosome assembly on the bead surface, and this depends on Spd-2’s ability to recruit Polo and Aurora A. Thus, Spd-2 orchestrates the assembly of two scaffolds, with distinct biophysical properties, that cooperate to build mitotic centrosomes in flies.
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