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

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 SOD I ubiquitylation. Here we present evidence that CHIP functions in the interaction of mutant SOD I 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 interfered with the association between mutant SOD I and VCP. Conversely, the binding of CHIP to mutant SOD I was inhibited by VCP, implying that the chaperone complex and proteolytic machinery are competing for the common substrates. Finally we observed that mutant SOD I strongly associated with the 19S complex of proteasomes and CHIP over-expression specifically reduced the interaction between S6/S6' ATPase subunits and mutant SOD I. 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.