The controls acting at mitosis in Schizosaccharomyces pombe

The control over entry into mitosis in many different organisms involves the activity of a key enzyme, p34cdc2 which is a serine/threonine protein kinase. In fission yeast p34cdc2 activity is regulated by phosphorylation on tyrosine 15 (Y15). Dephosphorylation of Y15 activates the p34cdc2 kinase and this initiates mitosis. Regulation of p34cdc2 activity requires the function of cdc13, cdc25, wee1 and mik1 gene products. The cdc13 gene encodes a B type cyclin which forms a complex with p34cdc2 and is required for activation of the kinase. The cdc25, wee1 and mik1 genes function on two separate pathways that either activate or inhibit p34cdc2 activity. Inactivation of p34cdc2 and exit from mitosis involves the destruction of cyclin B and probably also requires the activity of several other gene products including p13sucl and phosphatases. The checkpoint control that ensures that cells do not enter mitosis until DNA replication is completed also acts through p34cdc2.