Dsl1p, an essential protein required for membrane traffic at the endoplasmic reticulum/Golgi interface in yeast

To identify novel factors required for ER to Golgi transport in yeast we pe rformed a screen for genes that, when mutated, confer a dependence on a dom inant mutant form of the ER to Golgi vesicle docking factor Sly1p, termed S ly1-20p. DSL1, a novel gene isolated in the screen, encodes an essential pr otein with a predicted molecular mass of 88 kDa. DSL1 is required for trans port between the ER and the Golgi because strains bearing mutant alleles of this gene accumulate the pre-Golgi form of transported proteins at the res trictive temperature. Two strains bearing temperature-sensitive alleles of DSL1 display distinct phenotypes as observed by electron microscopy at the restrictive temperature; although both strains accumulate ER membrane, one also accumulates vesicles. Interestingly, the inviability of strains bearin g several mutant alleles of DSL1 can be suppressed by expression of either Erv14p (a protein required for the movement of a specific protein from the ER to the Golgi), Sec21p (the gamma -subunit of the COPI coat protein compl ex coatomer), or Sly1-20p. Because the strongest suppressor is SEC21, we pr oposed that Dsl1p functions primarily in retrograde Golgi to ER traffic, al though it is possible that Dsl1p functions in anterograde traffic as well, perhaps at the docking stage, as suggested by the suppression by SLY1-20.