Marked amplification and diversification of products of ras genes from rat brain, Rab GTPases, in the ciliates Tetrahymena thermophila and Paramecium tetraurelia.

Small GTPase Rab (products of ras genes from rat brain) is a widely conserved molecular switch among eukaryotes and regulates membrane trafficking pathways. It is generally considered that the number of Rab encoded in the genome correlates with multicellularity; however, we found that unicellular ciliates Tetrahymena thermophila (Tt) and Paramecium tetraurelia (Pt) possess many more Rab genes in their genome than the 64 HsRab genes in the human genome. We succeeded in isolating 86 cDNA clones of 88 TtRab genes in the Tetrahymena genome. By comparing the amino acid sequence of Rab in humans and the budding yeast Saccharomyces cerevisiae, 42 TtRab belonged to subfamilies functionally characterized and designated as conventional Rab, while the remaining 44 TtRab were considered to be species-specific. To examine the diversity of Rab in ciliates, we searched for Rab genes in the genome database of P. tetraurelia. Overall, 229 PtRab genes were found and categorized as 157 conventional and 72 species-specific PtRab, respectively. Among them, nine PtRab genes showed high homology to seven TtRab, suggesting the conservation of ciliate-specific Rab. These data suggested that the range of Rab is markedly amplified and diversified in ciliates, which may support the elaborate cellular structures and vigorous phagocytosis of those organisms.