GDP DISSOCIATION INHIBITOR SERVES AS A CYTOSOLIC ACCEPTOR FOR NEWLY SYNTHESIZED AND PRENYLATED RAB5

In vitro synthesis and post-translational prenylation of Rab5 is accom plished using reticulocyte lysate supplemented with prenyl precursors (Sanford, J. C., Pan, Y., and Wessling-Resnick, M. (1993) J. Biol. Che m, 268, 23773-23776), When Rab5 is translated in the presence of bioti n-lysine-tRNA, it incorporates biotin-lysine into its peptide backbone and is efficiently prenylated; since this modification is dependent o n guanine nucleotide binding, biotin-Rab5's functional integrity must be maintained, Prenylated biotin-Rab5 associates with a 45-kDa reticul ocyte GDP dissociation inhibitor (GDI), sedimenting as a similar to 70 -kDa particle on 5-20% sucrose density gradients, The GDI-Rab5 complex can be captured using streptavidin-linked agarose beads, Only Rab5 pe ptides that are substrates for prenylation are found to cosediment wit h the lysate GDI on sucrose gradients, Post-translational association of Rab5 and GDI is a novel finding, since previous reports suggested R ab5 remains associated with Rab escort protein (REP) after prenylation (Alexandrov, K., Horiuchi, H., Steele-Mortimer, O., Seabra, M. C., an d Zerial, M. (1994) EMBO J. 13, 5262-5273), Since post-translational p renylation is catalytically mediated by REP, our study suggests that a complex between Rab5 and this factor is transient in nature, Thus, ne wly synthesized and prenylated Rab5 is most likely escorted to its tar get membrane by a GDI acceptor molecule. Biotin-Rab5 provides a novel tool for future efforts to capture and characterize additional accesso ry factors required for Rab protein function in vesicle transport.