Ric1p and Rgp1p form a complex that catalyses nucleotide exchange on Ypt6p

Cells lacking the GTPase Ypt6p have defects in intracellular traffic and ar e temperature sensitive. Their growth is severely impaired by additional mu tation of IMH1, which encodes a non-essential Golgi-associated coiled-coil protein. A screen for mutants that, like ypt6, specifically impair the grow th of imh1 cells led to the identification of RIC1, Ric1p forms a tight com plex with a previously uncharacterized protein, Rgp1p, The Ric1p-Rgp1p comp lex binds Ypt6p in a nucleotide-dependent manner, and purified Ric1p-Rgp1 s timulates guanine nucleotide exchange on Ypt6p in vitro. Deletion of RIC1 o r RGP1, like that of YPT6, blocks the recycling of the exocytic SNARE Snc1p from early endosomes to the Golgi and causes temperature-sensitive growth, but this defect can be relieved by overexpression of YPT6, Ric1p largely c olocalizes with the late Golgi marker Sec7p. Ypt6p shows a similar distribu tion, but this is altered when RIC1 or RGP1 is mutated, We infer that the R ic1p-Rgp1p complex serves to activate Ypt6p on Golgi membranes by nucleotid e exchange, and that this is required for efficient fusion of endosome-deri ved vesicles with the Golgi.