PRA1 inhibits the extraction of membrane-bound rab GTPase by GDI1

Rab is a family of small Ras-like GTPases regulating intracellular vesicle transport. We have previously reported that prenylated Rab acceptor or PRA1 interacts with Rab GTPases and vesicle-associated membrane protein (VAMP2). Structural prediction programs suggest that PRA1, with its two extensive hydrophobic domains, is likely to be an integral membrane protein. However, subcellular fractionation and immunocytochemical analyses indicated that PRA1 is localized both in the cytosol and tightly associated with the membrane compartment. The membrane-bound form can be partially extracted with physiological buffer and urea, suggesting that PRA1 is an extrinsic membrane protein. Deletion of the carboxyl-terminal domain resulted in a protein that behaved as an integral membrane protein, indicating that this domain plays an essential role in maintaining PRA1 in a soluble state. PRA1 can also bind weakly to GDP dissociation inhibitor (GDI), a protein involved in the solubilization of membrane-bound Rab GTPases. Addition of PRA1 inhibited the extraction of membrane-bound Rab3A by GDI, suggesting that membrane localization of Rab GTPases is dependent on the opposing action of PRA1 and GDI. The binding of Rab and VAMP2 to PRA1 is mutually exclusive such that Rab3A can displace VAMP2 in a preformed VAMP2-PRA1 complex.