Prenylation of a Rab1B mutant with altered GTPase activity is impaired in cell-free systems but not in intact mammalian cells

Previous studies have reached differing conclusions as to whether or not guanine-nucleotide-dependent conformational changes affect the ability of Rab proteins to undergo post-translational modification by Rab: geranylgeranyltransferase (Rab-GGTase). We now show that the ability of a Rab1B mutant [Q67L (Gln-67 --> Leu)] with reduced intrinsic GTPase activity to undergo geranylgeranylation in cell-free assays depends on the guanine nucleotide composition of the system. When GTP is the predominant nucleotide in the assay, Rab1B(Q67L) is a poor substrate. However, when GDP is present and GTP is omitted, prenylation of the Q67L mutant is comparable with that of the wild-type ((WT)) protein. These studies, coupled with the poor prenylation of Rab1B(WT) in the presence of the non-hydrolysable GTP analogue guanosine 5'-[gamma-thio]triphosphate, support the notion that Rab-GGTase prefers substrates in the GDP conformation. When the abilities of Rab1B(Q67L) and Rab1B(WT) to undergo prenylation were compared by metabolic labelling of transiently expressed proteins in cultured human 293 cells, we did not observe a decline in prenylation of the mutant protein as predicted on the basis of the cell-free assays. Moreover, the Q67L mutant was comparable with the wild-type Rab1B in its ability to associate with coexpressed Rab GDP dissociation inhibitors in 293 cells. These findings raise the possibility that unidentified proteins present in intact cells may compensate for the reduced intrinsic GTPase activity of the Q67L mutant, allowing a significant proportion of the nascent Rab1B(Q67L) to assume a GDP conformation. The differential prenylation of Rab1B(Q67L) in cell-free systems versus intact cells underscores the importance of evaluating the posttranslational modification of specific Rab mutants in vivo, where poorly characterized regulatory proteins may have a significant effect on GTPase activity or nucleotide exchange rates.