Rab GDI: a Solubilizing and Recycling Factor for Rab9 Protein

Rab proteins are thought to function in the processes by which transport vesicles identify and/or fuse with their respective target membranes. The bulk of these proteins are membrane associated, but a measurable fraction can be found in the cytosol. The cytosolic forms of rab3A, rab11, and Sec4 occur as equimolar complexes with a class of proteins termed "GDIs," or "GDP dissociation inhibitors." We show here that the cytosolic form of rab9, a protein required for transport between late endosomes and the trans Golgi network, also occurs as a complex with a GDI-like protein, with an apparent mass of approximately 80 kD. Complex formation could be reconstituted in vitro using recombinant rab9 protein, cytosol, ATP, and geranylgeranyl diphosphate, and was shown to require an intact rab9 carboxy terminus, as well as rab9 geranylgeranylation. Monoprenylation was sufficient for complex formation because a mutant rab9 protein bearing the carboxy terminal sequence, CLLL, was prenylated in vitro by geranylgeranyl transferase I and was efficiently incorporated into 80-kD complexes. Purified, prenylated rab9 could also assemble into 80-kD complexes by addition of purified, rab3A GDI. Finally, rab3A-GDI had the capacity to solubilize rab9GDP, but not rab9GTP, from cytoplasmic membranes. These findings support the proposal that GDI proteins serve to recycle rab proteins from their target membranes after completion of a rab protein-mediated, catalytic cycle. Thus GDI proteins have the potential to regulate the availability of specific intracellular transport factors.