CLB5-ASSOCIATED KINASE-ACTIVITY IS REQUIRED EARLY IN THE SPINDLE PATHWAY FOR CORRECT PREANAPHASE NUCLEAR POSITIONING IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, a single cyclin-dependent kinase, Cdc28, regulates both G1/S and G2/M phase transitions by associating with sta ge-specific cyclins. During progression through S phase and G2/M, Cdc2 8 is activated by the B-type cyclins Clb1-6. Because of functional red undancy, specific roles for individual Clbs have been difficult to ass ign. To help genetically define such roles, strains carrying a cdc28(t s) allele, combined with single CLB deletions were studied. We assumed that by limiting the activity of the kinase, these strains would be r endered more sensitive to loss of individual Clbs. By this approach, a novel phenotype associated with CLB5 mutation was observed. Homozygou s cdc28-4(ts) clb5 diploids were inviable at room temperature. Cells w ere defective in spindle positioning, leading to migration of undivide d nuclei into the bud. Occasionally, misplaced spindles were observed in cdc28-4 clb5 haploids; additional deletion of CLB6 caused full pene trance. Thus, CLB5 effects proper preanaphase spindle positioning, yet the requirement differs in haploids and diploids. The execution point for the defect corresponded to the time of Clb5-dependent kinase acti vation. Nevertheless, lethality of cdc28-4 clb5 diploids was not rescu ed by CLB2 or CLB4 overexpression, indicating a specificity of Clb5 fu nction beyond temporality of expression.