A vesicular intermediate in protein transport from the endopiasmic reticulum is detected in a subset of temperature-sensitive mutants blocked early in the yeast secretory pathway. By electron microscopy three of the mutants, set%, sec77, and sec22, accumulate 50 nm vesicles at the nonpermissive temperature. Vesicle accumulation is blocked by the mutations secl2, secf3, sec76, and sec23 as shown by analysis of double-mutant strains. Thus the early SEC genes can be divided into vesicle forming and vesicle fusion functions. Synthetic lethal interactions between set mutations define two groups of SEC genes, corresponding to the groups involved in vesicle formation or fusion. Mutations in two of the genes involved in vesicle fusion, SEC17 and SEC18, are lethal in combination, and five of six possible pairwise combinations of mutations in genes required for vesicle formation, SECIP, SEC13, SEC16, and SEC23, are lethal. These interactions suggest cooperation between different SEC gene products in vesicle budding and vesicle fusion processes. introduction

In eukaryotic cells, secreted proteins pass through a series of membrane-bounded compartments en route to the cell surface (Palade, 1975). At each step in the transport pathway, secreted proteins are thought to be carried by transport vesicles that bud from one compartment and then fuse with the appropriate acceptor compartment. In the case of transport between the endoplasmic reticulum (El?) and the Golgi apparatus, several lines of evidence indicate the existence of specific vesicle carriers. An intermediate vesicle stage between the ER and Golgi apparatus was first suggested by the appearance in the electron microscope of vesicles budding from the transitional region of the ER, located between the rough ER and the Golgi complex (Saraste and Kuismanen, 1984). Membrane preparations with properties suggestive of vesicles specific for transport between the ER and Golgi have been identified by fractionation of tissue culture cells (Lodish et al., 1987) and have been produced in vitro by incubation of isolated ER membranes with ATP and cytosol (Paulik et al., 1988). Recently, NSF (N-ethylmaleimidesensitive fusion protein), a protein factor required for transport within the Golgi apparatus in vitro (Orci et al., 1989) was found also to act in ER to Golgi transport in vivo (Wilson et al., 1989) and in vitro (Beckers et al., 1989).