p25(rum1) promotes proteolysis of the mitotic B-cyclin p56(cdc13) during G(1) of the fission yeast cell cycle

The fission yeast Schizosaccharomyces pombe CDK inhibitor p25(rum1) plays a major role in regulating cell cycle progression during G(1), Here we show that p25(rum1) associates with the CDK p34(cdc2)/p56(cdc13) during G(1) in normally cycling cells and is required for the rapid proteolysis of p56(cdc13). In vitro binding data indicate that p25(rum1) has specificity for the B-cyclin p56(cdc13) component of the CDK and can bind the cyclin even in the absence of the cyclin destruction box, At the G(1)-S-phase transition, p25(rum1) levels decrease and p56(cdc13) levels increase, We also show that on release from a G(1) block, the rapid disappearance of p25(rum1) requires the activity of the CDK p34(cdc2)/cig1p and that this same CDK phosphorylates p25(rum1) in vitro, We propose that the binding of p25(rum1) to p56(cdc13) promotes cyclin proteolysis during G(1), with p25(rum1) possibly acting as an adaptor protein, promoting transfer of p56(cdc13) to the proteolytic machinery, At the G(1)-S-phase transition, p25(rum1) becomes targeted for proteolysis by a mechanism which may involve p34(cdc2)/cig1p phosphorylation. As a consequence, at this point in the cell cycle p56(cdc13) proteolysis is inhibited, leading to a rise of p56(cdc13) levels in preparation for mitosis.