The functional importance of VCP to maintaining cellular protein homeostasis

Abstract

The AAA-ATPase (ATPases associated with diverse cellular activities) valosin-containing protein (VCP), is essential for many cellular pathways including but not limited to endoplasmic reticulum-associated degradation (ERAD), DNA damage responses, and cell cycle regulation. VCP primarily identifies ubiquitylated proteins in these pathways and mediates their unfolding and degradation by the 26S proteasome. This review summarizes recent research on VCP that has uncovered surprising new ways that this ATPase is regulated, new aspects of recognition of substrates and novel pathways and substrates that utilize its activity.

Keywords: autophagy; neurodegeneration; organelles; protein quality control; ubiquitin.

Figure 1.. Structural organization of VCP and mechanism of substrate unfolding.

(A) VCP protein structure. VCP assembles primarily as a homo-hexamer comprised of six monomers, each of which contains a globular N-domain and two ATPase domains (D1 and D2). Pathogenic mutations within each domain are indicated. (B) Directionality of substrate unfolding by Cdc48p. Cdc48p encounters a type I branch point when the initiator ubiquitin reaches the pore of the hexamer. Unfolding continues toward the C-terminus of the ubiquitin. At a type II branch point, Cdc48p can move toward the N- or C-terminus to eventually unfold the substrate. (C) Structure and substrate processing by VCP. VCP interacts with adaptor proteins to identify ubiquitylated substrates for degradation by the proteasome. Through interaction with p37, VCP can unfold substrates such as I3 independently of ubiquitylation. Phosphorylation of VCP by ULK1/2 increases the ATPase activity of VCP to enhance stress granule disassembly, while phosphorylation by PLK1 regulates VCP function in mitosis. Trimethylation of VCP monomers at Lys315 occurs by the action of METTL21D/C/E. The ASPL adaptor disassembles VCP hexamers to form ASPL–VCP hetero-tetramers (structure modeled in PyMOL using PDB 5IFS). Two hexamers that interact at the C-terminal tail provide an interface for the formation of dodecamers (structure modeled in PyMOL using PDB 7K57).

Figure 2.. Novel VCP-dependent cellular functions.

(A) VCP is hijacked by flaviviruses to disassemble stress granules to free the viral replication machinery through a direct interaction between the NPL4 adaptor and viral protein, NS4B. VCP–UBXD8 also regulates stress granule disassembly at the ER. (B) The interaction between VCP and ataxin-3 stimulates the deubiquitylation of the Ptdlns3K complex component Beclin1, stabilizing the complex and initiating phagophore formation. (C) The VCP–UBXD8 complex regulates ER-mitochondria contact sites by maintaining SCD1 protein levels and thereby membrane lipid saturation and composition. VCP an also directly modulate tethering proteins. (D) VCP plays multiple roles in several steps of autophagy. VCP–U–N plays a role in autophagosome-lysosome fusion, and VCP–UBXD1 functions in the clearance of damaged lysosomes (lysophagy). The VCP–SVIP complex maintains the integrity of the tubular lysosome network.