CYCLIN-B (P56(CDC13)) LOCALIZATION IN THE YEAST SCHIZOSACCHAROMYCES-POMBE - AN ULTRASTRUCTURAL AND IMMUNOCYTOCHEMICAL STUDY

The eucaryote cell cycle is driven by a set of cyclin dependent kinase s (CDKs) associated to cyclins, which confer not only the activity but also the substrate specificity and the proper localization of the kin ase activity. In the fission yeast Schizosaccharomyces pombe, only one cyclin, the product of the cdcl3 gene (p56(cdc13)), is required to be associated with p34(cdc2), to control the complete cell cycle. Earlie r studies have localized this complex mainly in the nucleus and its pe riphery. Using new improved electron microscopy (EM) technologies, bas ed on high pressure freezing fixation, we refined previous studies, ev idencing cytoplasmic localization of p56(cdc13), in addition to the nu clear localization previously observed. Further immunofluorescence stu dies, performed on aldehydically fixed cells, confirmed our EM results , emphasizing the major cytoplasmic localization of p56(cdc13) in inte rphase cells and the relocalization towards the nucleus in mitotic cel ls, suggesting that the S pombe cyclin B localization is cell cycle-re gulated.