The role of the p97 ATPase in protein quality control at the endoplasmic reticulum

In eukaryotic cells, Endoplasmic Reticulum‐Associated Degradation (ERAD) eliminates misfolded proteins from the ER to preserve ER homeostasis. This process can be hijacked by viral or bacterial pathogens to facilitate their invasion into host cells. ERAD requires retrotranslocation of polypeptides into the cytosol by the concerted action of membrane‐bound ubiquitin E3 ligases and the translocation‐driving ATPase p97. To study the mechanism of p97‐mediated retrotranslocation, we developed in vitro assays that recapitulate human cytomegaloviral protein‐induced retrotranslocations. Our studies revealed two distinct modes of p97‐mediated retrotranslocation: one requires the p97‐Ufd1‐Npl4 ternary complex, whereas the other is independent of the cofactor complex Ufd1‐Npl4. We also established Eeyarestatin I as the first chemical inhibitor of p97, and characterized the mechanism that permits it to block ERAD as well as its potential implication in cancer therapy. Our findings highlight an important role played by p97 in ERAD and reveal a class of bifunctional chemical agents that preferentially inhibit membrane‐bound p97 to disrupt ER homeostasis and to induce tumor cell death. These results also indicate p97 as a potential drug target for cancer therapeutics.