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 P RA1 is localized both in the cytosol and tightly associated with the membra ne compartment. The membrane bound form can be partially extracted with phy siological buffer and urea, suggesting that PRA1 is an extrinsic membrane p rotein. 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 bi nd weakly to GDP dissociation inhibitor (GDI), a protein involved in the so lubilization of membrane-bound Rab GTPases. Addition of PRA1 inhibited the extraction of membrane-bound Rab3A by GDI, suggesting that membrane localiz ation of Rab GTPases is dependent on the opposing action of PRA1 and GDI. T he binding of Rab and VAMPS to PRA1 is mutually exclusive such that Rab3A c an displace VAMPS in a preformed VAMP2-PRA1 complex.