No abstract

In eukaryotic cells, incorrectly folded proteins in the endoplasmic reticulum (ER) are exported into the cytosol and degraded by the proteasome. This pathway is co-opted by some viruses. For example, the US11 protein of the human cytomegalovirus targets the major histocompatibility complex class I heavy chain for cytosolic degradation. How proteins are extracted from the ER membrane is unknown. In bacteria and mitochondria, members of the AAA ATPase family are involved in extracting and degrading membrane proteins. Here we demonstrate that another member of this family, Cdc48 in yeast and p97 in mammals, is required for the export of ER proteins into the cytosol. Whereas Cdc48/p97 was previously known to function in a complex with the cofactor p47 (ref. 5) in membrane fusion, we demonstrate that its role in ER protein export requires the interacting partners Ufd1 and Npl4. The AAA ATPase interacts with substrates at the ER membrane and is needed to release them as polyubiquitinated species into the cytosol. We propose that the Cdc48/p97-Ufd1-Npl4 complex extracts proteins from the ER membrane for cytosolic degradation.

The ATP-driven chaperone valosin-containing protein (VCP)/p97 governs critical steps in ubiquitin-dependent protein quality control and intracellular signalling pathways. It cooperates with diverse partner proteins to help process ubiquitin-labelled proteins for recycling or degradation by the proteasome in many cellular contexts. Recent studies have uncovered unexpected cellular functions for p97 in autophagy, endosomal sorting and regulating protein degradation at the outer mitochondrial membrane, and elucidated a role for p97 in key chromatin-associated processes. These findings extend the functional relevance of p97 to lysosomal degradation and reveal a surprising dual role in protecting cells from protein stress and ensuring genome stability during proliferation.

p97, an abundant hexameric ATPase of the AAA family, is involved in homotypic membrane fusion. It is thought to disassemble SNARE complexes formed during the process of membrane fusion. Here, we report two structures: a crystal structure of the N-terminal and D1 ATPase domains of murine p97 at 2.9 A resolution, and a cryoelectron microscopy structure of full-length rat p97 at 18 A resolution. Together, these structures show that the D1 and D2 hexamers pack in a tail-to-tail arrangement, and that the N domain is flexible. A comparison with NSF D2 (ATP complex) reveals possible conformational changes induced by ATP hydrolysis. Given the D1 and D2 packing arrangement, we propose a ratchet mechanism for p97 during its ATP hydrolysis cycle.

The ATPase valosin-containing protein (VCP)/p97 has emerged as a central and important element of the ubiquitin system. Together with a network of cofactors, it regulates an ever-expanding range of processes that stretch into almost every aspect of cellular physiology. Its main role in proteostasis and key functions in signaling pathways are of relevance to degenerative diseases and genomic stability. In this Cell Science at a Glance and the accompanying poster, we give a brief overview of this complex system. In addition, we discuss the pathogenic basis for VCP/p97-associated diseases and then highlight in more detail new exciting links to the translational stress response and RNA biology that further underscore the significance of the VCP/p97 system.