B-type cyclins regulate G1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor.

The onset of S phase in fission yeast is regulated at Start, the point of commitment to the mitotic cell cycle. The p34cdc2 kinase is essential for G1 progression past Start, but until now its regulation has been poorly understood. Here we show that the cig2/cyc17 B-type cyclin has an important role in G1 progression, and demonstrate that p34cdc2 kinase activity is periodically associated with cig2 in G1. Cells lacking cig2 are defective in G1 progression, and this is particularly clear in small cells that must regulate Start with respect to cell size. We also find that the cig1 B-type cyclin can promote G1 progression. Whilst p25rum1 can inhibit cig2/cdc2 activity in vitro, and may transiently inhibit this complex in vivo, cig1 is regulated independently of p25rum1. Since cig1/cdc2 kinase activity peaks in mitotic cells, and decreases after mitosis with similar kinetics to cdc13-associated kinase activity, we suggest that cig2 is likely to be the principal fission yeast G1 cyclin. cig2 protein levels accumulate in G1 cells, and we propose that p25rum1 may transiently inhibit cig2-associated p34cdc2 activity until the critical cell size required for Start is reached.