Sec34p, a protein required for vesicle tethering to the yeast Golgi apparatus, is in a complex with Sec35p

A screen for mutants of Saccharomyces cerevisiae secretory pathway componen ts previously yielded sec34, a mutant that accumulates numerous vesicles an d fails to transport proteins from the ER to the Golgi complex at the restr ictive temperature (Wuestehube, L.J., R. Duden, A. fun, S. Hamamoto, P. Ker n, R, Ram, and R, Schekman, 1996, Genetics, 142:393-406), We find that SEC3 4 encodes a novel protein of 93-kD, peripherally associated with membranes. The temperature-sensitive phenotype of sec34-2 is suppressed by the rab GT Pase Ypt1p that functions ear ly in the secretory pathway, or by the domina nt form of the ER to Golgi complex target-SNARE (soluble N-ethylmaleimide s ensitive fusion protein attachment protein receptor)-associated protein Sly 1p, Sly1-20p, Weaker suppression is evident upon overexpression of genes en coding the vesicle tethering factor Uso1p or the vesicle-SNAREs Sec22p, Bet 1p, or Ykt6p, This genetic suppression profile is similar to that of sec35- 1, a mutant allele of a gene encoding an ER to Golgi vesicle tethering fact or and, like Sec35p, Sec34p is required in vitro for vesicle tethering. sec 34-2 and sec35-1 display a synthetic lethal interaction, a genetic result e xplained by the finding that Sec34p and Sec35p can interact by two-hybrid a nalysis. Fractionation of yeast cytosol indicates that Sec34p and Sec35p ex ist in an similar to 750-kD protein complex, Finally, we describe RUD3? a n ovel gene identified through a genetic screen for multicopy suppressors of a mutation in USO1,which suppresses the sec34-2 mutation as well.