Recruitment of the p97 ATPase and ubiquitin ligases to the site of retrotranslocation at the endoplasmic reticulum membrane

Misfolded proteins are eliminated from the endoplasmic reticulum (ER) by retrotranslocation into the cytosol, a pathway hijacked by certain viruses to destroy MHC class I heavy chains. The translocation of polypeptides across the ER membrane requires their polyubiquitination and subsequent extraction from the membrane by the p97 ATPase [also called valosin-containing protein (VCP) or, in yeast, Cdc48]. In higher eukaryotes, p97 is bound to the ER membrane by a membrane protein complex containing Derlin-1 and VCP-interacting membrane protein (VIMP). How the ubiquitination machinery is recruited to the p97/Derlin/VIMP complex is unclear. Here, we report that p97 interacts directly with several ubiquitin ligases and facilitates their recruitment to Derlin-1. During retrotranslocation, a substrate first interacts with Derlin-1 before p97 and other factors join the complex. These data, together with the fact that Derlin-1 is a multispanning membrane protein forming homo-oligomers, support the idea that Derlin-1 is part of a retrotranslocation channel that is associated with both the polyubiquitination and p97-ATPase machineries.