THE CDC2 PROTEIN-KINASE CONTROLS CDC10 SCT1 COMPLEX-FORMATION/

In the fission yeast Schizosaccharomyces pombe, the execution of Start requires the activity of the Cdc2 protein kinase and the Cdc10/Sct1 t ranscription complex. The loss of any of these genes leads to G(1) arr est and activation of the mating pathway under appropriate conditions. We have undertaken a genetic and biochemical analysis of these genes and their protein products to elucidate the molecular mechanism that g overns the regulation of Start. We demonstrate that serine-196 of Cdc1 0 is phosphorylated in vivo and provide evidence that suggests that ph osphorylation of this residue is required for Cdc10 function. Substitu tion of serine-196 of Cdc10 with alanine (Cdc10 S196A) leads to inacti vation of Cdc10. We show that Cdc10 S196A is incapable of associating with Sct1 to form a heteromeric complex, whereas substitution of this serine with aspartic acid (S196D) restores DNA-binding activity by all owing Cdc10 to associate with Sct1. Furthermore, we demonstrate that C dc2 activity is required for the formation of the heteromeric Sct1/Cdc 10 transcription complex and that the Cdc10 S196D mutation alleviates this requirement. We thus provide biochemical evidence to demonstrate one mechanism by which the Cdc2 protein kinase may regulate Start in t he fission yeast cell cycle.