The derlin Dfm1 promotes retrotranslocation of folded protein domains from the endoplasmic reticulum

Abstract

Endoplasmic reticulum (ER) proteins are degraded by proteasomes in the cytosol through ER-associated degradation (ERAD). This process involves retrotranslocation of substrates across the ER membrane, their ubiquitination and subsequent membrane extraction by the Cdc48/Npl4/Ufd1 ATPase complex prior delivery to proteasomes for degradation. Recently a mechanism for the retrotranslocation of misfolded substrates by the Hrd1 ubiquitin ligase complex was described. However, how substrates with folded luminal domains are retrotranslocated remains unknown. Here, we identify Dfm1 as an essential membrane component for the retrotranslocation of proteins with folded luminal domains. Both Dfm1 intramembrane rhomboid-like and the cytosolic Cdc48-binding domains are essential for substrate retrotranslocation. Substrate processing by Dfm1 and Cdc48 complex requires the ubiquitin shuttle factors Rad23/Dsk2 and the multi-ubiquitination enzyme Ufd2. Our findings suggest a pathway in which a series of ubiquitin modifying factors recruit Dfm1 to resolve a stalled retrotranslocation intermediate due to the presence of a folded luminal domain.