Identification of SVIP as an Endogenous Inhibitor of Endoplasmic Reticulum-associated Degradation

Abstract

Misfolded proteins in the endoplasmic reticulum (ER) are eliminated by a process known as ER-associated degradation (ERAD), which starts with misfolded protein recognition, followed by ubiquitination, retrotranslocation to the cytosol, deglycosylation, and targeting to the proteasome for degradation. Actions of multisubunit protein machineries in the ER membrane integrate these steps. We hypothesized that regulation of the multisubunit machinery assembly is a mechanism by which ERAD activity is regulated. To test this hypothesis, we investigated the potential regulatory role of the small p97/VCP-interacting protein (SVIP) on the formation of the ERAD machinery that includes ubiquitin ligase gp78, AAA ATPase p97/VCP, and the putative channel Derlin1. We found that SVIP is anchored to microsomal membrane via myristoylation and co-fractionated with gp78, Derlin1, p97/VCP, and calnexin to the ER. Like gp78, SVIP also physically interacts with p97/VCP and Derlin1. Overexpression of SVIP blocks unassembled CD3␦ from association with gp78 and p97/VCP, which is accompanied by decreases in CD3␦ ubiquitination and degradation. Silencing SVIP expression markedly enhances the formation of gp78-p97/VCP-Derlin1 complex, which correlates with increased degradation of CD3␦ and misfolded Z variant of ␣-1-antitrypsin, established substrates of gp78. These results suggest that SVIP is an endogenous inhibitor of ERAD that acts through regulating the assembly of the gp78-p97/VCP-Derlin1 complex.

Protein Folding and Quality Control in the ER

“…gp78 also cooperates with RMA1 to degrade CFTRD508. SVIP is reported to inhibit the assembly of the gp78 ligase complex (Derlin-1, gp78, and p97), which suggests an inhibitory effect on ERAD (Ballar et al 2007). (E) TEB4 (Doa10) ligase complex: TEB4 is known to be a mammalian homolog of yeast Doa10.…”

Distinct mechanisms controlling rough and smooth endoplasmic reticulum-mitochondria contacts

“…Rough mitochondria-associated ER tubules present ribosomes facing mitochondria and might be associated with translocon-mediated ER-mitochondria Ca 2+ coupling (Flourakis et al, 2006). Indeed, as a key component of ERAD, Gp78 interacts with many components of the translocon, including derlin-1, VIMP and PNGase (Ballar et al, 2007;Li et al, 2006;Ye et al, 2005). Selective regulation of RER contact sites by AMF and Gp78 represents, to our knowledge, the first identified regulator of these contact sites.…”

UBX domain proteins: major regulators of the AAA ATPase Cdc48/p97

“…As SVIP comprises only 76 amino acids, it is unlikely to be a classical substrate-recruiting cofactor. Rather, it appears to act as a negative regulator of p97 in the ERAD pathway [91], perhaps by interfering with the formation of a stable p97 complex at the ER.…”