Co-Chaperones in Targeting and Delivery of Misfolded Proteins to the 26S Proteasome

Abstract

Protein homeostasis (proteostasis) is essential for the cell and is maintained by a highly conserved protein quality control (PQC) system, which triages newly synthesized, mislocalized and misfolded proteins. The ubiquitin-proteasome system (UPS), molecular chaperones, and co-chaperones are vital PQC elements that work together to facilitate degradation of misfolded and toxic protein species through the 26S proteasome. However, the underlying mechanisms are complex and remain partly unclear. Here, we provide an overview of the current knowledge on the co-chaperones that directly take part in targeting and delivery of PQC substrates for degradation. While J-domain proteins (JDPs) target substrates for the heat shock protein 70 (HSP70) chaperones, nucleotide-exchange factors (NEFs) deliver HSP70-bound substrates to the proteasome. So far, three NEFs have been established in proteasomal delivery: HSP110 and the ubiquitin-like (UBL) domain proteins BAG-1 and BAG-6, the latter acting as a chaperone itself and carrying its substrates directly to the proteasome. A better understanding of the individual delivery pathways will improve our ability to regulate the triage, and thus regulate the fate of aberrant proteins involved in cell stress and disease, examples of which are given throughout the review.

Keywords: chaperone; co-chaperone; misfolding; proteasome; protein quality control; protein stability; ubiquitin.

Figure 1

The substrate binding/release cycle of HSP70. (A) Schematic overview of the HSP70 domain structure. NBD: nucleotide-binding domain, SBD: substrate-binding domain, aa: amino acids. (B) The ATP-bound open conformation of HSP70 engages with a substrate-bound J-domain protein (JDP), which causes ATP hydrolysis and transfer of the substrate to HSP70 leading it to switch into its closed conformation. A nucleotide exchange factor (NEF) mediates ADP release, which permits ATP binding and results in opening of HSP70 and subsequent substrate release.

Figure 2

Delivery pathways of misfolded proteins to the 26S proteasome. (A) The 26S proteasome with one catalytic 20S particle (green) and two regulatory 19S particles (blue) each containing a lid and a base subcomplex. (B) Schematic overviews of BAG-1, BAG-6 and HSP110 domain structures, which bind the regulatory 19S particles (arrows). UBL: ubiquitin-like, BAG: Bcl2-associated athanogene, NLS: nuclear localization signal, NBD: nucleotide-binding domain, SBD, substrate-binding domain, aa: amino acids. (C) Delivery pathways to the 26S proteasome. Left: CHIP binds HSP70 through its C-terminal EEVD motif and ubiquitinates the bound substrate. The substrate-HSP70-CHIP complex engages with BAG-1, which connects the complex to the proteasome through binding of the 19S particle through its UBL domain. The binding is strengthened by CHIP-mediated ubiquitination of the UBL-domain. Substrate release at the proteasome is mediated by BAG-1. Middle: HSP110 binds the 19S regulatory particle, where it associates with a substrate-HSP70 complex. NEF activity by HSP110 releases ADP from HSP70, which in turn mediates substrate release. Right: The substrate-bound JDP, SGTA, binds the BAG-6 complex consisting of BAG-6, UBL4A and TRC35. This promotes transfer of the substrate to BAG-6. Next, RNF126 binds the UBL-domain of BAG-6 and ubiquitinates the substrate. The BAG-6 complex binds the 19S particle of the proteasome, thus enabling substrate degradation.