Phosphorylation of two small GTP-binding proteins of the Rab family by p34^cdc2

ENTRY of a cell into mitosis induces a series of structural and functional changes including arrest of intracellular transport^1-4. Knowledge of how the mitotic cycle is driven progressed substantially with the identification of the p34^cdc2 protein kinase as a subunit of maturation-promoting factor^5-7, the universal regulating component of the mitotic cycle⁸. Activation of the kinase at the onset of mitosis⁹ is thought to trigger the important mitotic events by phosphorylating key proteins¹⁰. Small guanine nucleo-tide-binding proteins have been implicated in regulating transport pathways. For instance, two small Ras-related GTP-binding proteins, Sec4p and Yptlp, control distinct stages of the secretory pathway in budding yeast^11-15. The GTP-binding proteins of the Rab family in rats and humans^16,17 display strong homologies with Sec4p and Yptlp, and might therefore also be involved in regulating intracellular transport. Indeed, distinct Rab proteins are located in the exocytotic and endocytotic compartments^18-21. Interruption of vesicular transport during mitosis might involve modification of these proteins. We now present biochemical evidence for a mitosis-specific p34^cdc2 phosphorylation of RablAp and Rab4p. By contrast, Rab2p and Rab6p are not phosphorylated. We also show that the distribution of RablAp 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.