CHIP promotes the degradation of mutant SOD1 by reducing its interaction with VCP and S6/S6′ subunits of 26S proteasome

Abstract

Previously we showed that CHIP, a co-chaperone of Hsp70 and E3 ubiquitin ligase, can promote the degradation of mutant SOD1 linked to familial amyotrophic lateral sclerosis (fALS) via a mechanism not involving SOD1 ubiquitylation. Here we present evidence that CHIP functions in the interaction of mutant SOD1 with 26S proteasomes. Bag-1, a coupling factor between molecular chaperones and the proteasomes, formed a complex with SOD1 in an hsp70-dependent manner but had no direct effect on the degradation of mutant SOD1. Instead, Bag-1 stimulated interaction between CHIP and the proteasome-associated protein VCP (p97), which do not associate normally. Over-expressed CHIP interferedwith the association between mutant SOD1 and VCP. Conversely, the binding of CHIP to mutant SOD1 was inhibited by VCP, implying that the chaperone complex and proteolytic machinery are competing for the common substrates. Finally we observed that mutant SOD1 strongly associated with the 19S complex of proteasomes and CHIP over-expression specifically reduced the interaction between S6/S6? ATPase subunits and mutant SOD1. These results suggest that CHIP, together with ubiquitin-binding proteins such as Bag-1 and VCP, promotes the degradation of mutant SOD1 by facilitating its translocation from ATPase subunits of 19S complex to the 20S core particle.

Mutant Copper-Zinc Superoxide Dismutase (SOD1) Induces Protein Secretion Pathway Alterations and Exosome Release in Astrocytes

“…In a previous study, we found that VCP/p97 (referred to as transitional endoplasmic reticulum ATPase) was recovered in mutant SOD1-rich aggregates from the spinal cord in the G93A SOD1 mouse model, already at a presymptomatic stage of the disease (28). Possibly, VCP/p97, which interacts directly or indirectly with misfolded mutant SOD1 (55, 56), is sorted with it into exosomes as unwanted material to be shuttled out of astrocytes. VCP/p97 mutations have been associated with familial and sporadic ALS cases (57, 58).…”

Structural Properties and Interaction Partners of Familial ALS-Associated SOD1 Mutants

“…It has been reported that the ALS-related SOD1 mutant binds directly to the S6 (Rpt3/PSMC4) and S6′(Rpt5/PSMC3) subunits of the 19S regulatory complex of the proteasome and interferes with their normal functions. Since both S6 and S6′ subunits belong to the AAA ATPase superfamily, it has been suggested that the association between these ATPase subunits and the mutant SOD1 proteins impairs the ATPase-induced activation of the 20S catalytic core and causes proteasomal dysfunction and degradation stalling (128). Moreover, the SOD1 mutant interacts physically with the co-chaperone CHIP (carboxyl terminus of Hsc70 interacting protein) (129, 130) and with the ubiquitin-binding proteins Bag1 and VCP (valosin-containing protein; p97) (128).…”

“…Since both S6 and S6′ subunits belong to the AAA ATPase superfamily, it has been suggested that the association between these ATPase subunits and the mutant SOD1 proteins impairs the ATPase-induced activation of the 20S catalytic core and causes proteasomal dysfunction and degradation stalling (128). Moreover, the SOD1 mutant interacts physically with the co-chaperone CHIP (carboxyl terminus of Hsc70 interacting protein) (129, 130) and with the ubiquitin-binding proteins Bag1 and VCP (valosin-containing protein; p97) (128). Interestingly, CHIP and VCP compete with each other for the binding to SOD1 mutant, indicating that the chaperone complex (CHIP/Hsp70) and the proteolytic machinery (VCP/26S proteasome) compete for the common substrate SOD1 mutant (128).…”

A Decade of Boon or Burden: What Has the CHIP Ever Done for Cellular Protein Quality Control Mechanism Implicated in Neurodegeneration and Aging?

“…It mediates polyubiquitination of Hsp70/Hsc70, which preferentially promotes degradation of SOD1 via 26S proteasome, as shown in Figure 5B (Urushitani et al, 2004 ). The overexpression of CHIP decreases cytotoxicity in cultured human cells by reducing levels of mutant SOD1 together with Hsp70/Hsc70 and Hsp90 (Choi and Lee, 2010 ). The degradation of mutant SOD1 protein through autophagy is carried out by Bag3/HspB8/Hsc70/CHIP complex that inserts mutant protein into autophagosome (Crippa et al, 2010 ).…”