Sgf1p, a new component of the Sec34p/Sec35p complex

Here we report the identification of SGF1 as a high-copy suppressor of the sec35-1 mutant. SGF1 encodes an essential hydrophilic protein of similar to 100 kDa. Using the yeast two-hybrid system and coprecipitation studies, we demonstrate that Sgf1p is a new subunit of the multiprotein Sec34p/Sec35p complex. Reduced levels of Sgf1p lead to the accumulation of a variety of m embranes as well as a kinetic block in endoplasmic reticulum to Golgi traff ic. Immunofluorescence studies demonstrate that Sec34p is found throughout the Golgi, with a high concentration on early Golgi. Although an earlier st udy suggested that Sec34p (Grd20p) is not required for protein secretion, w e show here that the sec34-2 and sec35-1 mutations lead to a pleiotropic bl ock in the secretion of all proteins into the growth medium.