A yeast model for ion-channel ER export: How Yvc1p, a yeast TRP channel homolog, is exported out of the ER

The budding yeast, Saccharomyces cerevisiae has been used as a model system to help us to understand fundamental biological process in a cell. By using a combination of genomic and electrophysiologic approaches, we discovered that a yeast TRP-like gene ( YVC1) encodes the non-selective cation channel in the vacuole. To identify the region of this channel, which is important for gating, we searched for mutants that are toxic to the yeast. Instead of channels with gating defects, we found that mutant channels are retained in the ER and interfere with ER export machinery, which consists of a guanine-nucleotide exchange factor Sec12p on the ER membrane, and cytosolic COPII coat proteins, one of which is a G-protein Sar1p. We showed that wild type Yvc1p can co-IP with Sec12p and Sar1p respectively. Truncation mutant of Yvc1p co-IPs with Sec12p very poorly and cannot be exported out of the ER. Using the toxicity of yvc1 mutants, we screened the entire yeast genome for intergenic suppressors that restore normal growth. All three suppressor mutations isolated were found to be in SEC12 . Based on the genetic and biochemical data, we propose that Sec12p probably function as a “guard” molecule, which “inspects” cargoes before their loading into the vesicle for export. During the “inspection” process, Yvc1p interacts with one side of the Sec12p β propeller to enhance its nucleotide exchange activity on the opposite side, which binds to Sar1p.