Chaperone Functions of the E3 Ubiquitin Ligase CHIP

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.Cells must constantly monitor the folding status of nascent polypeptides and repair or degrade misfolded proteins during protein denaturing stress. To accomplish these tasks, the cell relies on an intricately regulated protein quality control (QC) 5 system. The cytosolic QC system consists of molecular chaperones, such as Hsp70 and Hsp40, that promote the proper folding and refolding of nonnative proteins and the ubiquitin proteasome system, which degrades misfolded or stress-damaged proteins (1-4). Maintenance of cellular homeostasis requires a delicate balance between that activity of protein folding and ubiquitin proteasome systems. Under circumstances where the molecular chaperone system is unable to promote proper folding of a protein substrate to its native state, it is necessary for the substrate protein to be selected for degradation, a process that is often referred to as protein triage (5, 6). Protein triage needs to be tightly regulated, since the escape of toxic proteins from QC systems or overactivity of protein degradation pathways leads to a variety of human diseases (7).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). CHIP is a co-chaperone that functions as an E3 ubiquitin ligase that links the polypeptide binding activity of Hsp70 to the ubiquitin proteasome system. CHIP binds Hsp70 through interactions between its N-terminal TPR domains and the C-terminal EEVD motif found on Hsp70. The binding of CHIP to Hsp70 can stall the folding of Hsp70 client proteins (14 -16) and concomitantly facilitate the U-box dependent ubiquitination of Hsp70-bound substrates (16,17). CHIP appears to play a central role in cell stress protection (18 -20) and is responsible for the degradation of disease-related proteins that include cystic fibrosis transmembrane conductance regulator (15), p53 (21), huntingtin (22), ataxin-3 (22), Tau protein (23-25), and ␣-synuclein (26).The domain ...

MKKS Is a Centrosome-shuttling Protein Degraded by Disease-causing Mutations via CHIP-mediated Ubiquitination

“…It should be emphasized that the G345E mutant was strongly recognized by chaperones and CHIP and that it was heavily polyubiquitinated. It was recently shown by in vitro experiments that CHIP stimulates HSP70 activity to prevent huntingtin-Q53 aggregation and that CHIP can directly inhibit the aggregation of heat-denatured luciferase (Rosser et al, 2007). CHIP may maintain the MKKS G345E mutant in a degradation-competent soluble state via enhancement of HSP70 activity and the intrinsic chaperone-like activity of CHIP.…”

Firefly luciferase mutants as sensors of proteome stress

“…A recent study suggests that CHIP has an intrinsic chaperone like activity that enables it to selectively recognize and bind misfolded proteins. This function of CHIP is temperature sensitive which may allow CHIP to target heat denatured proteins directly for degradation (Rosser et al, 2007). Therefore, CHIP plays a pivotal role in cellular triage decisions that regulate the balance between folding and degradation of chaperone substrates.…”

Human Liver Cytochrome P450 3A4 Ubiquitination

“…Molecular recognition of CHIP substrates is believed to occur largely via its Hsc70-Hsp40 cochaperone complex (47)(48)(49)(50)(51)(52). However, although many UBC7-gp78 heterologous substrates have now been identified (42)(43)(44)(45)(46), to our knowledge little is known about the mechanisms of their molecular recognition as specific cellular targets of this E2-E3 complex.…”