Douglas M. Cyr

The Hsc70 co-chaperone CHIP targets immature CFTR for proteasomal degradation

The folding of both wild-type and mutant forms of the cystic-fibrosis transmembrane-conductance regulator (CFTR), a plasma-membrane chloride-ion channel, is inefficient. Most nascent CFTR is retained in the endoplasmic reticulum and degraded by the ubiquitin proteasome pathway. Aberrant folding and defective trafficking of CFTRΔF508 is the principal cause of cystic fibrosis, but how the endoplasmic-reticulum quality-control system targets CFTR for degradation remains unknown. CHIP is a cytosolic U-box protein that interacts with Hsc70 through a set of tetratricorepeat motifs. The U-box represents a modified form of the ring-finger motif that is found in ubiquitin ligases and that defines the E4 family of polyubiquitination factors. Here we show that CHIP functions with Hsc70 to sense the folded state of CFTR and targets aberrant forms for proteasomal degradation by promoting their ubiquitination. The U-box appeared essential for this process because overexpresion of CHIPΔU-box inhibited the action of endogenous CHIP and blocked CFTR ubiquitination and degradation. CHIP is a co-chaperone that converts Hsc70 from a protein-folding machine into a degradation factor that functions in endoplasmic-reticulum quality control.

DNAJ-LIKE PROTEINS : MOLECULAR CHAPERONES AND SPECIFIC REGULATORS OF HSP70

The folding of proteins and the assembly of protein complexes within subcompartments of the eukaryotic cell is catalysed by different members of the Hsp70 protein family. The chaperone function of Hsp70 proteins in these events is regulated by members of the DnaJ-like protein family, which occurs through direct interaction of different Hsp70 and DnaJ-like protein pairs that appear to be specifically adapted to each other. This review highlights the diversity of functions of DnaJ-like proteins by using specific examples of DnaJ-Hsp70 interactions with polypeptides in yeast protein-biogenesis pathways.

Sequential Quality-Control Checkpoints Triage Misfolded Cystic Fibrosis Transmembrane Conductance Regulator

Cystic fibrosis arises from the misfolding and premature degradation of CFTR Delta F508, a Cl- ion channel with a single amino acid deletion. Yet, the quality-control machinery that selects CFTR Delta F508 for degradation and the mechanism for its misfolding are not well defined. We identified an ER membrane-associated ubiquitin ligase complex containing the E3 RMA1, the E2 Ubc6e, and Derlin-1 that cooperates with the cytosolic Hsc70/CHIP E3 complex to triage CFTR and CFTR Delta F508. Derlin-1 serves to retain CFTR in the ER membrane and interacts with RMA1 and Ubc6e to promote CFTRs proteasomal degradation. RMA1 is capable of recognizing folding defects in CFTR Delta F508 coincident with translation, whereas the CHIP E3 appears to act posttranslationally. A folding defect in CFTR Delta F508 detected by RMA1 involves the inability of CFTRs second membrane-spanning domain to productively interact with amino-terminal domains. Thus, the RMA1 and CHIP E3 ubiquitin ligases act sequentially in ER membrane and cytosol to monitor the folding status of CFTR and CFTR Delta F508.

From the cradle to the grave: Molecular chaperones that may choose between folding and degradation

Molecular chaperones are known to facilitate cellular protein folding. They bind non‐native proteins and orchestrate the folding process in conjunction with regulatory cofactors that modulate the affinity of the chaperone for its substrate. However, not every attempt to fold a protein is successful and chaperones can direct misfolded proteins to the cellular degradation machinery for destruction. Protein quality control thus appears to involve close cooperation between molecular chaperones and energy‐dependent proteases. Molecular mechanisms underlying this interplay have been largely enigmatic so far. Here we present a novel concept for the regulation of the eukaryotic Hsp70 and Hsp90 chaperone systems during protein folding and protein degradation.

Protein quality control: U-box-containing E3 ubiquitin ligases join the fold.

Molecular chaperones act with folding co-chaperones to suppress protein aggregation and refold stress damaged proteins. However, it is not clear how slowly folding or misfolded polypeptides are targeted for proteasomal degradation. Generally, selection of proteins for degradation is mediated by E3 ubiquitin ligases of the mechanistically distinct HECT and RING domain sub-types. Recent studies suggest that the U-box protein family represents a third class of E3 enzymes. CHIP, a U-box-containing protein, is a degradatory co-chaperone of heat-shock protein 70 (Hsp70) and Hsp90 that facilitates the polyubiquitination of chaperone substrates. These data indicate a model for protein quality control in which the interaction of Hsp70 and Hsp90 with co-chaperones that have either folding or degradatory activity helps to determine the fate of non-native cellular proteins.

The Hdj-2/Hsc70 chaperone pair facilitates early steps in CFTR biogenesis

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride ion channel constructed from two membrane‐spanning domains (MSDs), two nucleotide‐binding domains (NBD) and a regulatory (R) domain. The NBDs and R‐domain are cytosolic and how they are assembled with the MSDs to achieve the native CFTR structure is not clear. Human DnaJ 2 (Hdj‐2) is a co‐chaperone of heat shock cognate 70 (Hsc70) which is localized to the cytosolic face of the ER. Whether Hdj‐2 directs Hsc70 to facilitate the assembly of cytosolic regions on CFTR was investigated. We report that immature ER forms of CFTR and ΔF508 CFTR can be isolated in complexes with Hdj‐2 and Hsc70. The ΔF508 mutation is localized in NBD1 and causes the CFTR to misfold. Levels of complex formation between ΔF508 CFTR and Hdj‐2/Hsp70 were ∼2‐fold higher than those with CFTR. The earliest stage at which Hdj‐2/Hsc70 could bind CFTR translation intermediates coincided with the expression of NBD1 in the cytosol. Interestingly, complex formation between Hdj‐2 and nascent CFTR was greatly reduced after expression of the R‐domain. In experiments with purified components, Hdj‐2 and Hsc70 acted synergistically to suppress NBD1 aggregation. Collectively, these data suggest that Hdj‐2 and Hsc70 facilitate early steps in CFTR assembly. A putative step in the CFTR folding pathway catalyzed by Hdj‐2/Hsc70 is the formation of an intramolecular NBD1–R‐domain complex. Whether this step is defective in the biogenesis of ΔF508 CFTR will be discussed.

CHIP activates HSF1 and confers protection against apoptosis and cellular stress.

Induction of molecular chaperones is the characteristic protective response to environmental stress, and is regulated by a transcriptional program that depends on heat shock factor 1 (HSF1), which is normally under negative regulatory control by molecular chaperones Hsp70 and Hsp90. In metazoan species, the chaperone system also provides protection against apoptosis. We demonstrate that the dual function co‐chaperone/ubiquitin ligase CHIP (C‐terminus of Hsp70‐interacting protein) regulates activation of the stress‐chaperone response through induced trimerization and transcriptional activation of HSF1, and is required for protection against stress‐induced apoptosis in murine fibroblasts. The consequences of this function are demonstrated by the phenotype of mice lacking CHIP, which develop normally but are temperature‐sensitive and develop apoptosis in multiple organs after environmental challenge. CHIP exerts a central and unique role in tuning the response to stress at multiple levels by regulation of protein quality control and transcriptional activation of stress response signaling.

CHIP-mediated stress recovery by sequential ubiquitination of substrates and Hsp70

Exposure of cells to various stresses often leads to the induction of a group of proteins called heat shock proteins (HSPs, molecular chaperones). Hsp70 is one of the most highly inducible molecular chaperones, but its expression must be maintained at low levels under physiological conditions to permit constitutive cellular activities to proceed. Heat shock transcription factor 1 (HSF1) is the transcriptional regulator of HSP gene expression, but it remains poorly understood how newly synthesized HSPs return to basal levels when HSF1 activity is attenuated. CHIP (carboxy terminus of Hsp70-binding protein), a dual-function co-chaperone/ubiquitin ligase, targets a broad range of chaperone substrates for proteasomal degradation. Here we show that CHIP not only enhances Hsp70 induction during acute stress but also mediates its turnover during the stress recovery process. Central to this dual-phase regulation is its substrate dependence: CHIP preferentially ubiquitinates chaperone-bound substrates, whereas degradation of Hsp70 by CHIP-dependent targeting to the ubiquitin–proteasome system occurs when misfolded substrates have been depleted. The sequential catalysis of the CHIP-associated chaperone adaptor and its bound substrate provides an elegant mechanism for maintaining homeostasis by tuning chaperone levels appropriately to reflect the status of protein folding within the cytoplasm.

YDJ1p facilitates polypeptide translocation across different intracellular membranes by a conserved mechanism

The role of S. cerevisiae YDJ1 protein (YDJ1p) in polypeptide translocation across membranes has been examined. A conditional ydj1 mutant strain (ydj1-151TS) is defective for import of several polypeptides into mitochondria and alpha factor into the endoplasmic reticulum at 37 degrees C. These defects are suppressed by E. coli dnaJ or overexpression of S. cerevisiae SIS1 proteins. A different ydj1 mutant, which cannot be farnesylated (ydj1-S406), displays similar transport defects to the ydj1-151 strain. Furthermore, the ability of purified ydj1-151p to stimulate the ATPase activity of hsp70SSA1 was greatly diminished compared with the wild-type protein. Together, these data suggest that YDJ1p functions in polypeptide translocation in a conserved manner, probably acting at organelle membranes and in association with hsp70 proteins.

Mechanisms for regulation of Hsp70 function by Hsp40

Abstract The Hsp70 family members play an essential role in cellular protein metabolism by acting as polypeptide-binding and release factors that interact with nonnative regions of proteins at different stages of their life cycles. Hsp40 cochaperone proteins regulate complex formation between Hsp70 and client proteins. Herein, literature is reviewed that describes the mechanisms by which Hsp40 proteins interact with Hsp70 to specify its cellular functions.