Chaperone Functions of the E3 Ubiquitin Ligase CHIP

Meredith F.N Rosser,Erin Washburn,Paul J Muchowski,Cam Patterson,Douglas M Cyr

doi:10.1074/jbc.m700513200

Abstract

The carboxyl terminus of the Hsc70-interacting protein (CHIP) is an Hsp70 co-chaperone as well as an E3 ubiquitin ligase that protects cells from proteotoxic stress. The abilities of CHIP to interact with Hsp70 and function as a ubiquitin ligase place CHIP at a pivotal position in the protein quality control system, where its entrance into Hsp70-substrate complexes partitions nonnative proteins toward degradation. However, the manner by which Hsp70 substrates are selected for ubiquitination by CHIP is not well understood. We discovered that CHIP possesses an intrinsic chaperone activity that enables it to selectively recognize and bind nonnative proteins. Interestingly, the chaperone function of CHIP is temperature-sensitive and is dramatically enhanced by heat stress. The ability of CHIP to recognize nonnative protein structure may aid in selection of slow folding or misfolded polypeptides for ubiquitination.

Highlights

Partitioning of nonnative polypeptides between folding and degradation pathways appears to be influenced by the folding kinetics of individual proteins as well as by a network of cochaperones that bind and regulate polypeptide binding and release by Hsp70 family members (8 –13)
We report that carboxyl terminus of the Hsc70-interacting protein (CHIP) exhibits intrinsic chaperone function that enables it to bind nonnative substrates and suppress protein aggregation
Characterization of CHIP Activity during Heat Stress and Recovery—To expand our knowledge of CHIP action in protein triage, we sought to understand the enigmatic observation that overexpression of CHIP in cultured cells enhances refolding of heat-denatured luciferase [18]

Summary

Introduction

Partitioning of nonnative polypeptides between folding and degradation pathways appears to be influenced by the folding kinetics of individual proteins as well as by a network of cochaperones that bind and regulate polypeptide binding and release by Hsp family members (8 –13). The ability of CHIP to utilize its U-box domain to attract UbcH5 to ubiquitinate Hsp70-bound substrates is established [11, 16, 17, 27], the mechanism by which different Hsp clients are selected for degradation by CHIP is not understood. This is an important question, because productive folding intermediates and misfolded proteins that are bound by Hsp appear to have similar conformations, and it does not appear that folding kinetics are the sole determinant of a nonnative protein’s fate [5, 15, 28]. These data suggest that the ability of CHIP to function as a chaperone is an important feature of its action in protein triage and cell stress protection

Methods

Results

Conclusion