AMINO-TERMINAL AND CARBOXY-TERMINAL DOMAINS OF THE YEAST RAB ESCORT PROTEIN ARE BOTH REQUIRED FOR BINDING OF YPT SMALL G-PROTEINS

The Rab escort protein (REP) is an essential component of the heterotr imeric enzyme Rab geranylgeranyl transferase that modifies the carboxy -terminal cysteines of the Ras-like small G proteins belonging to the Rab/Ypt family. Deletions in the human CHM locus, encoding one of the two REPs known in humans, result in a retinal degenerative syndrome ca lled choroideremia. The only known yeast homologue of the choroideremi a gene product is encoded by an essential gene called MRS6. Besides th ree structurally conserved regions (SCRs) previously detected in the a mino-terminal half of REPs and RabGDIs, three other regions in the car boxy-terminal domain (RCR 1-3) are here identified as being characteri stic of REPs alone. We have performed the first mutational analysis of a REP protein to experimentally define the regions functionally impor tant for Rab/Ypt protein binding, making use of the genetic system of the yeast Saccharomyces cerevisiae. This analysis has shown that the S CRs are necessary but not sufficient for Ypt1p binding by the yeast RE P, the carboxy-terminal region also being required.