BET3 ENCODES A NOVEL HYDROPHILIC PROTEIN THAT ACTS IN CONJUNCTION WITH YEAST SNARES

Here we report the identification of BET3, a new member of a group of interacting genes whose products have been implicated in the targeting and fusion of endoplasmic reticulum (ER) to Golgi transport vesicles with their acceptor compartment. A temperature-sensitive mutant in bet 3-1 was isolated in a synthetic lethal screen designed to identify new genes whose products may interact with BET1, a type II integral membr ane protein that is required for ER to Golgi transport. At 37 degrees C, bet3-1 fails to transport invertase, Ir-factor, and carboxypeptidas e Y from the ER to the Golgi complex. As a consequence, this mutant ac cumulates dilated ER and small vesicles. The SNARE complex, a docking/ fusion complex, fails to form in this mutant. Furthermore, BET3 encode s an essential 22-kDa hydrophilic protein that is conserved in evoluti on, which is not a component of this complex. These findings support t he hypothesis that Bet3p may act before the assembly of the SNARE comp lex.