Abstract
The ERD2 gene of Saccharomyces cerevisiae encodes the HDEL receptor that sorts ER proteins; it is essential for growth. In the absence of Erd2p the Golgi apparatus is both functionally and morphologically perturbed. Here we describe the isolation of four SED genes (suppressors of the erd2-deletion) which, when present in multiple copies, allow cells to grow in the absence of ERD2. The suppressed strains secrete the ER protein BiP and their internal membranes show a variety of morphological abnormalities. Sequence analysis indicates that all these SED genes encode membrane proteins: SED1 encodes a probable cell surface glycoprotein; SED2 is identical to SEC12, a gene required for the formation of ER-derived transport vesicles; SED4 encodes a protein whose cytoplasmic domain is 45% identical to that of Sec12p; SED3 is DPM1, the structural gene for dolichol-P-mannose synthase. We suggest that the absence of ERD2 causes an imbalance between membrane flow into and out of the Golgi apparatus, and that the SED gene products can compensate for this either by slowing transport from the ER or by stimulating vesicle budding from Golgi membranes.