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J. Cell Biol.,
Volume 143, Number 1, October 5, 1998 135-145
Department of Molecular Biology, MB7, The Scripps Research Institute, La Jolla, California 92037
In Saccharomyces cerevisiae, a single cyclin-dependent kinase, Cdc28, regulates both G1/S and
G2/M phase transitions by associating with stage-specific cyclins. During progression through S phase and
G2/M, Cdc28 is activated by the B-type cyclins Clb1-6.
Because of functional redundancy, specific roles for individual Clbs have been difficult to assign. To help genetically define such roles, strains carrying a cdc28ts allele, combined with single CLB deletions were studied.
We assumed that by limiting the activity of the kinase,
these strains would be rendered more sensitive to loss
of individual Clbs.
By this approach, a novel phenotype associated with
CLB5 mutation was observed. Homozygous cdc28-4ts
clb5 diploids were inviable at room temperature. Cells
were defective in spindle positioning, leading to migration of undivided nuclei into the bud. Occasionally,
misplaced spindles were observed in cdc28-4 clb5 haploids; additional deletion of CLB6 caused full penetrance. 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 activation. Nevertheless, lethality of cdc28-4 clb5 diploids was
not rescued by CLB2 or CLB4 overexpression, indicating a specificity of Clb5 function beyond temporality of
expression.
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