An Altered Nuclear Migration into the Daughter Bud is Induced by the Cyclin Al-mediated Cdc28 Kinase through an Aberrant Spindle Movement in Saccharomyces cerevisiae

A strain of Saccharomyces cerevisiae that contains an integrated copy of a Xenopus cyclin Al gene under the control of the GAL1 promoter has been constructed. On inducing expression of cyclin Al, 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 Al unable to activate Cdc28 kinase. The nuclear abnormality induced by cyclin Al was overcome by cdc28 mutations that abolish its ability to bind cyclin Al. Both yeast cyclin Clb3 and Xenopus mitotic cyclin B produced the same phenotype, whereas Gl 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.