Pecani K, Cross FR

B-type cyclins promote mitotic entry and inhibit mitotic exit. In Saccharomyces cerevisiae, four B-type cyclins, Clb1-4, carry out essential mitotic roles, with substantial but incomplete overlap of function amongst them. Previous work in many organisms has indicated that B-type cyclin-dependent inhibition of mitotic exit imposes a requirement for mitotic destruction ...

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of B-type cyclins. For instance, precise genomic removal of the Clb2 destruction box prevents mitotic proteolysis of Clb2 and blocks mitotic exit. Here we show that despite significant functional overlap between Clb2 and Clb3, destruction-box-dependent Clb3 proteolysis is completely dispensable for mitotic exit. Removal of the Clb3 destruction box results in abundant Clb3 protein and associated kinase throughout the cell cycle, but mitotic exit occurs with close to normal timing. Clb3 degradation is required for pre-Start G1 control in the succeeding cell cycle. Deleting the CLB3 destruction box essentially eliminates all time delay before cell cycle Start following division, even in very small newborn cells. CLB3∆db cells show no cell cycle arrest response to mating pheromone, and CLB3∆db completely bypasses the requirement for CLN G1 cyclins, even in the absence of the early-expressed B-type cyclins CLB5,6 Thus, regulated mitotic proteolysis of Clb3 is specifically required to make passage of Start in the succeeding cell cycle 'memoryless': dependent on conditions within that cycle, and independent of events such as B-type cyclin accumulation that occurred in the preceding cycle.

Date: Oct. 28, 2016

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