The cdc2+ gene product p34cdc2 is located immunocytochemically in both the nucleus and cytoplasm of human cells. It is uniformly distributed throughout the cytoplasm and is irregularly distributed in the nucleus. Part of p34cdc2 is associated with the centrosome and centrosomal staining increases late in the cell cycle and at the onset of mitosis. This distribution is corroborated by cell fractionation which also indicates that slower migrating forms of p34cdc2 are found in isolated centrosomes and in Triton-insoluble fractions. We propose that one role of the p34Cdc2 protein kinase is to modify the centrosome bringing about formation of the mitotic spindle. At anaphase p34cdc2 becomes associated with vesicles in the middle of the cell between the reforming nuclei. A similar location is found for pl3SUCl and we suggest that the vesicular localization plays a role in p34cdc2 kinase inactivation at the end of mitosis.
Entry of a cell into mitosis induces a series of structural and functional changes including arrest of intracellular transport. Knowledge of how the mitotic cycle is driven progressed substantially with the identification of the p34cdc2 protein kinase as a subunit of maturation-promoting factor, the universal regulating component of the mitotic cycle. Activation of the kinase at the onset of mitosis is thought to trigger the important mitotic events by phosphorylating key proteins. Small guanine nucleotide-binding proteins have been implicated in regulating transport pathways. For instance, two small Ras-related GTP-binding proteins, Sec4p and Ypt1p, control distinct stages of the secretory pathway in budding yeast. The GTP-binding proteins of the Rab family in rats and humans display strong homologies with Sec4p and Ypt1p, and might therefore also be involved in regulating intracellular transport. Indeed, distinct Rab proteins are located in the exocytotic and endocytotic compartments. Interruption of vesicular transport during mitosis might involve modification of these proteins. We now present biochemical evidence for a mitosis-specific p34cdc2 phosphorylation of Rab1Ap and Rab4p. By contrast, Rab2p and Rab6p are not phosphorylated. We also show that the distribution of Rab1Ap and Rab4p between cytosolic and membrane-bound forms is different in interphase and mitotic cells. This may provide a clue to the mechanism by which phosphorylation could affect membrane traffic during mitosis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.