Mrs6p, the yeast homologue of the mammalian choroideraemia protein: immunological evidence for its function as the Ypt1p Rab escort protein.

The Saccharomyces cerevisiae MRS6 gene belongs to the same gene family as that responsible for the mammalian Rab escort protein (REP) and the Rab GDP dissociation inhibitor protein (GDI). Both REP and GDI are regulators of the Ras-related small G-proteins Rab/YPT1 which are involved in intracellular vesicular trafficking in yeast and in mammals. Here we characterize an antiserum directed against Mrs6p and show that it specifically inhibits the geranylation of the YPT1 protein in an in vitro assay. These findings provide direct evidence for the role of Mrs6p as the REP component of the yeast Rab geranylgeranyl transferase enzyme.