YEAST BETA-COAT AND BETA'-COAT PROTEINS (COP) - 2 COATOMER SUBUNITS ESSENTIAL FOR ENDOPLASMIC RETICULUM-TO-GOLGI PROTEIN TRAFFIC

To understand better the role of non-clathrin coat proteins in membran e traffic, we have cloned and characterized two essential genes encodi ng subunits of the yeast coatomer, SEC26 and SEC27. Sec26p is a 109-kD a protein that shares 43% sequence identity with mammalian beta-coat p rotein (beta-COP). Sec26p depleted cells accumulate endoplasmic reticu lum (ER) forms of secretory precursor proteins, and growth ceases afte r a dramatic accumulation of ER membranes. Sec26p overproduction parti ally suppresses sec27-1, a new mutant that shows a temperature-sensiti ve 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 ma mmalian beta'-COP. Our sequence data support a two-domain model for th e SEC27 protein: a conserved amino-terminal domain, composed of five W D-40 repeats similar to those found in beta-subunits of trimeric G pro teins, and a less conserved carboxyl-terminal domain. Genetic interact ions connect sec27-1 and sec21-1 (coatomer gamma subunit) with the ARF 1 and ARF2 genes and with the SEC22, BET1, and BOS1 genes, which encod e membrane proteins involved in ER-to-Golgi transport.