Molecular characterization of rabE, a developmentally regulated Dictyostelium homolog of mammalian rab GTPases

The superfamily of small GTPases includes a subgroup, rab proteins, thought to function in the regulation of discrete steps of membrane traffic. Using a screen based on the polymerase chain reaction, we identified six partial gene sequences of novel rab genes from the soil slime mold amoeba, Dictyostelium. Stretches of conserved sequence for these genes identified them clearly as rab GTPases; unique sequences showed these were novel rab genes. A full-length clone for one gene, which we named rabE, was characterized in detail. The coding sequence of rabE was 1.1 kb in length and contained three introns. RNAse protection analysis revealed rabE expression to be under developmental regulation, with an onset of message expression after 8 h of development. Comparison of the rabE amino acid sequence with the database showed that its unique domains were most similar to the products of four mammalian rab genes. Interestingly, only the rabE protein and its four mammalian homologs contained the sequence WDIAGQE, a variation of a conserved GTPase domain. The WDIAGQE motif thus defines a subgroup of rab proteins. Identification of a Dictyostelium homolog of this group of proteins opens an experimental system to explore the structure and function of this group of WDIAGQE-containing rab proteins.