A NOVEL RAB GTPASE, RAB33B, IS UBIQUITOUSLY EXPRESSED AND LOCALIZED TO THE MEDIAL GOLGI CISTERNAE

Small GTP-binding proteins of the Rab family play important roles at d efined steps of vesicular transport in protein secretion and the endoc ytosis pathway. In mammals, more than 30 proteins belonging to the Rab family have been reported to date. We report here the molecular cloni ng and characterization of a novel Rab protein, Rab33B. The amino acid sequence of Rab33B shows 55.3% identity to the Rab33A protein (previo usly called S10), and these two proteins share unique amino acid seque nces at the effector domain. The genomic organization of rab33B was th e same as rab33A: it consists of two exons. Thus, these two proteins m ake a subclass within the Rab family Northern blot analysis showed tha t rab33B is expressed ubiquitously in mouse tissues, in contrast to ra b33A whose expression is restricted to the brain and the immune system . A 26 kDa protein was detected by western blotting using a Rab33B-spe cific monoclonal antibody. Using immunofluorescence studies, Rab33B wa s shown to co-localize with alpha-mannosidase II, a Golgi-specific mar ker. Immunoelectron microscopy analysis further defined the localizati on of Rab33B to the medial Golgi cisternae. These results suggest Rab3 3B plays a role in intra-Golgi transport.