Yeast β- and β'-coat proteins (COP). Two coatomer subunits essential for endoplasmic reticulum-to-Golgi protein traffic

To understand better the role of non-clathrin coat proteins in membrane traffic, we have cloned and characterized two essential genes encoding subunits of the yeast coatomer, SEC26 and SEC27. Sec26p is a 109-kDa protein that shares 43% sequence identity with mammalian β-coat protein (β-COP). Sec26p-depleted cells accumulate endoplasmic reticulum (ER) forms of secretory precursor proteins, and growth ceases after a dramatic accumulation of ER membranes. Sec26p overproduction partially suppresses sec27-1, a new mutant that shows a temperature-sensitive defect in ER-to-Golgi transport. The SEC27 gene was cloned, and the sequence predicts a 99.4-kDa protein with 45% sequence identity to mammalian β'-COP. Our sequence data support a two- domain model for the SEC27 protein: a conserved amino-terminal domain, composed of five WD-40 repeats similar to those found in β-subunits of trimeric G proteins, and a less conserved carboxyl-terminal domain. Genetic interactions connect sec27-1 and sec21-1 (coatomer γ subunit) with the ARF1 and ARF2 genes and with the SEC22, BET1, and BOS1 genes, which encode membrane proteins involved in ER-to-Golgi transport.