Membrane-bound Ubx2 recruits Cdc48 to ubiquitin ligases and their substrates to ensure efficient ER-associated protein degradation

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Abstract
Endoplasmic reticulum (ER)-associated protein degradation (ERAD) is a quality control system that removes misfolded proteins from the ER1,2. ERAD substrates are channelled from the ER via a proteinacious pore to the cytosolic ubiquitin–proteasome system — a process involving dedicated ubiquitin ligases3,4,5 and the chaperone-like AAA ATPase Cdc48 (also known as p97)6,7,8,9,10. How the activities of these proteins are coupled remains unclear. Here we show that the UBX domain protein Ubx2 is an integral ER membrane protein that recruits Cdc48 to the ER. Moreover, Ubx2 mediates binding of Cdc48 to the ubiquitin ligases Hrd1 and Doa10, and to ERAD substrates. In addition, Ubx2 and Cdc48 interact with Der1 and Dfm1, yeast homologues of the putative dislocation pore protein Derlin-1 (refs 11,​ 12,​ 13). Lack of Ubx2 causes defects in ERAD that are exacerbated under stress conditions. These findings are consistent with a model in which Ubx2 coordinates the assembly of a highly efficient ERAD machinery at the ER membrane.