Characterization of GAPCenA, a GTPase activating protein for Rab6, part ofwhich associates with the centrosome

The Rab6 GTPase regulates intracellular transport at the level of the Golgi apparatus, probably in a retrograde direction. Here, we report the identif ication and characterization of a novel human Rab6-interacting protein name d human GAPCenA (for 'GAP and centrosome-associated'). Primary sequence ana lysis indicates that GAPCenA displays similarities, within a central 200 am ino acids domain, to both the yeast Rab GTPase activating proteins (GAPs) a nd to the spindle checkpoint proteins Saccharomyces cerevisiae Bub2p and Sc hizosaccharomyces pombe Cdc16p, We demonstrate that GAPCenA is indeed a CAP , specifically active in vitro On Rab6 and, to a lesser extent, on Rab4 and Rab2, proteins. Immunofluorescence and cell fractionation experiments show ed that GAPCenA is mainly cytosolic but that a minor pool is associated wit h the centrosome, Moreover, GAPCenA was found to form complexes with cytoso lic gamma-tubulin and to play a role in microtubule nucleation. Therefore, GAPCenA may be involved in the coordination of microtubule and Colgi dynami cs during the cell cycle.