FIG. 3.

Proteins involved at the S. pombe G₂/M cell cycle boundary. As in higher eukaryotes, the ultimate determinant of entry into mitosis in S. pombe is the kinase activity of the Cdc2-cyclin B complex. Levels of this complex increase throughout G₂, but premature activity is prevented by phosphorylation on Y15, catalyzed by Wee1 and Mik1. In late G₂, the inhibitory phosphorylation is removed by Cdc25, which creates an active complex that can promote entry into mitosis. Cdc25 can itself become phosphorylated, and this targets it for cytoplasmic sequestration by the Rad24/25 proteins, thereby preventing access to Cdc2-cyclin B in the nucleus.