Identification of a very large Rab GTPase family in the parasitic protozoan Trichomonas vaginalis.

Rab proteins are pivotal components of the membrane trafficking machinery in all eukaryotes. Distinct Rab proteins locate to specific endomembrane compartments and genomic studies suggest that Rab gene diversity correlates with endomembrane system complexity; for example unicellular organisms generally possess 5-20 Rab family members and the size of the repertoire increases to 25-60 in multicellular systems. Here we report 65 open reading frames from the unicellular protozoan Trichomonas vaginalis that encode distinct Rab proteins (TvRabs), indicating a family with complexity that rivals Homo sapiens in number. The detection of gene transcripts for the majority of these genes and conservation of functional motifs strongly suggests that TvRabs retain functionality and likely roles in membrane trafficking. The T. vaginalis Rab family includes orthologues of the conserved subfamilies, Rab1, Rab5, Rab6, Rab7 and Rab11, but the majority of TvRabs are not represented by orthologues in other systems and includes six novel T. vaginalis specific Rab subfamilies (A-F). The extreme size of the T. vaginalis Rab family, the presence of novel subfamilies plus the divergent nature of many TvRab sequences suggest both the presence of a highly complex endomembrane system within Trichomonas and potentially novel Rab functionality. A family of more than 65 Rab genes in a unicellular genome is unexpected, but may be a requirement for progression though an amoeboid life-cycle phase as both Dictyostelium discoideum and Entamoeba histolytica share with T. vaginalis both an amoeboid life cycle stage and very large Rab gene families.