CHARACTERIZATION OF THE INTERACTION OF THE MONOMERIC GTP-BINDING PROTEIN RAB3A WITH GERANYLGERANYL TRANSFERASE-II

The monomeric GTP-binding protein Rab3a controls exocytosis in neuroen docrine and neuronal cells. Like other members of the Rab family, Rab3 a is posttranslationally modified by the addition of hydrophobic geran ylgeranyl groups to its C-terminus. The geranylgeranylation reaction i s catalysed by the heterotrimeric geranylgeranyl transferase II. We de scribe the cDNA cloning of the beta-subunit of human geranylgeranyl tr ansferase II by means of the yeast two-hybrid system. The human enzyme , which is 49% and 96% similar to yeast and rat isoforms, respectively , can complement the beta-subunit deficiency in the yeast strain ANY11 9. Furthermore, by means of the two-hybrid system and in vitro geranyl geranylation reactions with purified recombinant rat geranylgeranyl tr ansferase II, we have characterized Rab3a domains implicated in the in teraction with geranylgeranyl transferase II. We find that the N-termi nus, the effector loop, the hypervariable region of the C-terminus, an d the geranylgeranyl-acceptor cysteines have roles in this interaction . The GDP-bound form of Rab3a is the preferred substrate of geranylger anyl transferase II.