RAN1 FUNCTIONS TO CONTROL THE CDC10 SCT1 COMPLEX THROUGH PUC1/

We have undertaken a biochemical analysis of the regulation of the G(1 )/S-phase transition and commitment to the cell cycle in the fission y east Schizosaccharomyces pombe. The execution of Start requires the ac tivity of the Cdc2 protein kinase and the Sct1/Cdc10 transcription com plex. Progression through G(1) also requires the Ran1 protein kinase w hose inactivation leads to activation of the meiotic pathway under con ditions normally inhibitory to this process. We have found that in add ition to Cdc2, Sct1/Cdc10 complex formation requires Ran1. We demonstr ate that the Puc1 cyclin associates with Ran1 and Cdc10 in vivo and th at the Ran1 protein kinase functions to control the association betwee n Puc1 and Cdc10. In addition, we present evidence that the phosphoryl ation state of Cdc10 is altered upon inactivation of Ran1. These resul ts provide biochemical evidence that demonstrate one mechanism by whic h the Ran1 protein kinase serves to control cell fate through Cdc10 an d Puc1.