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© The Rockefeller University Press, 0021-9525/1998//135 $5.00
The Journal of Cell Biology, Volume 143, Number 1, , 1998 135-145

Regular Articles

Clb5-associated Kinase Activity is Required Early in the Spindle Pathway for Correct Preanaphase Nuclear Positioning in Saccharomyces cerevisiae



Marisa Segal, Duncan J. Clarke, and Steven I. Reed

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.

Key Words: cell cycle • nuclear migration • spindle dynamics

Abbreviations used in this paper: cdk, cyclin-dependent kinase; DIC, differential interference contrast; SPB, spindle pole body.

M. Segal acknowledges fellowships from the European Molecular Biology Organization and the Human Frontiers in Science Program. D.J. Clarke was supported by a fellowship from the European Molecular Biology Organization. This work was supported by United States Public Health Service grant GM38328 to S.I. Reed.

Address all correspondence to Steven I. Reed, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, CA 92037. Tel.: (619) 784-9836. Fax: (619) 784-2781. E-mail: sreed{at}scripps.edu


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