An altered nuclear migration into the daughter bud is induced by the cyclin A1-mediated Cdc28 kinase through an aberrant spindle movement in Saccharomyces cerevisiae

Abstract

A strain of Saccharomyces cerevisiae that contains an integrated copy of a Xenopus cyclin A1 gene under the control of the GAL1 promoter has been constructed. On inducing expression of cyclin A1, the nuclear migration that occurs prior to division becomes aberrant. Instead of migrating to the neck between the mother cell and daughter bud, the nucleus, the short mitotic spindle and its associated two spindle pole bodies entered the daughter bud. This phenotype was induced by expression of an indestructible cyclin mutant, but not by a mutated cyclin A1 unable to activate Cdc28 kinase. The nuclear abnormality induced by cyclin A1 was overcome by cdc28 mutations that abolish its ability to bind cyclin A1. Both yeast cyclin Clb3 and Xenopus mitotic cyclin B produced the same phenotype, whereas G1 cyclin Cln2 did not. The results suggest that the proper movement of the nucleus through the spindle function during mitosis requires the appropriate activity of Cdc28 kinase mediated by specific cyclins.