Making a single centrosome

Separase cuts sister chromosomes apart at the end of mitosis. The same enzyme also, say Meng-Fu Bryan Tsou and Tim Stearns (Stanford University, Stanford, CA), releases a block to centriole and thus centrosome duplication. “It's so simple to have separase involved in both processes, because it is so critical to not do either one prematurely,” says Stearns. “It does make perfect sense that it is arranged this way.”

Microtubules can focus to form an organizing center in several ways, but “in dividing cells, the centrosome is the main player,” says Stearns. “And if you control centriole number you've controlled centrosome number.”

His group found recently that there is a block to reduplication that is intrinsic to centrosomes rather than being determined by the cytoplasm surrounding them. This block is now found to be released not by mitotic exit or by G1 kinase activity but by separase activity.

The separase disengages each tightly apposed pair of centrioles—a process that is subtle in cultured cells but more obvious in frog extracts where there is no G1 phase. During the subsequent cell cycle, a new centriole then forms orthogonal to each of the two disengaged centrioles. The visible fibers that connect daughter centrioles may contain a separase substrate, but there are no obvious candidates as yet.

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