Binding of Rab3A to synaptic vesicles

Prenylated Rab GTPases cycle between membrane-bound and soluble forms. Memb rane-bound GrDP-Rabs interact with GDP dissociation inhibitor (GDI), result ing in the dissociation of a Rab GDI complex, which in turn serves as a pre cursor for the membrane re-association of Rabs, We have now characterized t he binding of Rab3A to synaptic vesicles in vitro using either purified com pletes or rat brain cytosol as source for GDI.Rab3A Binding of Rab3A result s in the immediate release of GDI from the membrane. Furthermore, binding d oes not require the presence of additional guanine nucleotides (GDP or GTP) or of cytosolic factors. Although nucleotide exchange follows binding, bin ding is initially reversible, suggesting that binding of GDP-Rab3A and nucl eotide exchange are separate and independent events. Comparison with the bi nding of Rab1B revealed that both Rab proteins bind preferentially to their respective resident membranes although some promiscuity was observable, Bi nding is saturable and involves a protease-sensitive binding site that is t ightly associated with the vesicle membrane.