Accumulation of rab4GTP in the cytoplasm and association with the peptidyl-prolyl Isomerase Pin1 during mitosis

Transport through the endocytic pathway is inhibited during mitosis. The me chanism responsible for this inhibition is not understood. Rab4 might be on e of the proteins involved as it regulates transport through early endosome s, is phosphorylated by p34(cdc2) kinase, and is translocated from early en dosomes to the cytoplasm during mitosis. We investigated the perturbation o f the rab4 GTPase cycle during mitosis. Newly synthesized rab4 was less eff iciently targeted to membranes during mitosis. By subcellular fractionation of mito tic cells, we found a large increase of cytosolic rab4 in the acti ve GTP-form, an increase not associated with the cytosolic rabGDP chaperone GDI. Instead, phosphorylated rab4 is in a complex with the peptidyl-prolyl isomerase Pin1 during mitosis, but not during interphase. Our results show that less efficient recruitment of rab4 to membranes and a bypass of the n ormal GDI-mediated retrieval of rab4GDP from early endosomes reduce the amo unt of rab4GTP on membranes during mitosis. We propose that phosphorylation of rab4 inhibits both the recruitment of rab4 effector proteins to early e ndosomes and the docking of rab4-containing transport vesicles. This mechan ism might contribute to the inhibition of endocytic membrane transport duri ng mitosis.