Rab24 is an atypical member of the Rab GTPase family - Deficient GTPase activity, GDP dissociation inhibitor interaction, and prenylation of Rab24 expressed in cultured cells

The function of Rab24 is currently unknown, but other members of the Rab GT Pase family are known to participate in various protein trafficking pathway s. Rab proteins are thought to cycle on and off vesicle membranes in conjun ction with changes in their guanine nucleotide state. The present studies i ndicate that Rab24 possesses several unusual characteristics that distingui sh it from other Rab proteins. 1) Based on [P-32]orthophosphate labeling of protein-bound nucleotide, Rab24 exists predominantly in the GTP state when expressed in cultured cells. The low GTPase activity is related to the pre sence of serine instead of glutamine at the position cognate to Ras Gln-61. 2) Posttranslational geranylgeranylation of Rab24, determined by metabolic labeling or detergent partitioning assays, is inefficient when compared wi th other Rabs ending with the common CXC and CC carboxyl-terminal motifs. T his is partly due to the presence of two histidines distal to the target cy steines, but also involves other unidentified features. 3) Most of the Rab2 4 in the cytoplasmic compartment of cultured cells is not associated with R ab GDP dissociation inhibitors. These findings indicate that, if Rab24 func tions in vesicular transport processes, it may operate through a novel mech anism that does not depend on GTP hydrolysis or GDP dissociation inhibitor- mediated recycling.