Cloning of two genes encoding Rab7 in Paramecium

Rab7 is a small GTPase that plays a crucial role in the regulation of transport from early to late endosomes and lysosomes, phagosome maturation and in lysosomal biogenesis in mammalian cells. It contains conserved and unique sequence elements that mediate its function. Two Rab7 genes, Rab7a (703 bp) and Rab7b (707 bp) were identified in the unicellular eukaryote Paramecium by PCR amplification. They contain three short introns of different lengths (28-32 bp) and sequence located at identical positions in both genes. The presence of two Rab7 genes in the Paramecium genome was confirmed by Southern hybridization analysis performed with six different restriction enzymes. Expression of both genes was assessed by Northern blot and RT-PCR. Two transcripts of 1.8 and 2.2 kb were identified by hybridization analysis. The cloned complementary DNAs, both of 618 nucleotides in length, encode polypeptides of 206 amino acids that are 97.6% identical and differ in their C-termini. The predicted protein sequences of Rab7a and Rab7b contain all characteristic domains essential for Rab function: the effector domain (YRATVGADF) and four GTP-binding consensus sequences (GDSGVGKT, WDTAGQ, NKLD, SAK) as well as the prenylation motif (-CC) at the C-terminus indispensable for Rab binding to the membrane. Similarity searches revealed 81.6-82.1% homology of Paramecium Rab7 isoforms to human Rab7 and a lack of an insert typical for the Kinetoplastida - the species that appeared earlier in evolution. Paramecium is the first free-living lower eukaryote in which homologues of Rab7 have been identified that exhibit features similar to those of mammalian Rab7.