DESTRUCTION OF THE CDC28 CLB MITOTIC KINASE IS NOT REQUIRED FOR THE METAPHASE TO ANAPHASE TRANSITION IN BUDDING YEAST

It is widely assumed that degradation of mitotic cyclins causes a decr ease in mitotic cdc2/CDC28 kinase activity and thereby triggers the me taphase to anaphase transition. Two observations made on the budding y east Saccharomcyces cerevisiae are inconsistent with this scenario: (i ) anaphase occurs in the presence of high levels of kinase in cdc15 mu tants and (ii) overproduction of a B-type mitotic cyclin causes arrest not in metaphase as previously reported but in telophase. Kinase dest ruction is therefore implicated in the exit from mitosis rather than t he entry into anaphase. The behaviour of esp1 mutants shows in additio n that kinase destruction can occur in the absence of anaphase complet ion. The execution of anaphase and the destruction of CDC28 kinase act ivity therefore appear to take place independently of one another.