Genetic interactions of Hrd3p and Der3p/Hrd1p with Sec61p suggest a retro-translocation complex mediating protein transport for ER degradation

The endoplasmic reticulum contains a quality control system that subjects m isfolded or unassembled secretory proteins to rapid degradation via the cyt osolic ubiquitin proteasome system. This requires retrograde protein transp ort from the endoplasmic reticulum back to the cytosol. The Sec61 pore, the central component of the protein import channel into the endoplasmic retic ulum, was identified as the core subunit of the retro-translocon as well. A s import of mutated proteins into the endoplasmic reticulum lumen is succes sfully terminated, a new targeting mechanism must exist that mediates re-en tering of misfolded proteins into the Sec61 pore from the lumenal side de n ovo, The previously identified proteins Der3p/Hrd1p and, as we show here, H rd3p of the yeast Saccharomyces cerevisiae, are localised in the endoplasmi c reticulum membrane and are essential for the degradation of several subst rates of the endoplasmic reticulum degradation machinery. Based on genetic studies we demonstrate that they functionally interact with each other and with Sec61p, probably establishing the central part of the retro-translocon . In the absence of Hrd3p, the otherwise stable protein Der3p/Hrd1p becomes rapidly degraded. This depends on a functional ubiquitin proteasome system and the presence of substrate molecules of the endoplasmic reticulum degra dation system. When overexpressed, Der3p/Hrd1p accelerates CPY* degradation in Delta hrd3 cells. Our data suggest a recycling process of Der3p/Hrd1p t hrough Hrd3p, The retro-translocon seems to be build up at least by the Sec 61 pore, Der3p/Hrd1p and Hrd3p and mediates both retrograde transport and u biquitination of substrate molecules.