Hsp70 protein complexes as drug targets

Abstract

Heat shock protein 70 (Hsp70) plays critical roles in proteostasis and is an emerging target for multiple diseases. However, competitive inhibition of the enzymatic activity of Hsp70 has proven challenging and, in some cases, may not be the most productive way to redirect Hsp70 function. Another approach is to inhibit Hsp70's interactions with important co-chaperones, such as J proteins, nucleotide exchange factors (NEFs) and tetratricopeptide repeat (TPR) domain-containing proteins. These co-chaperones normally bind Hsp70 and guide its many diverse cellular activities. Complexes between Hsp70 and co-chaperones have been shown to have specific functions, including roles in pro-folding, pro-degradation and pro-trafficking pathways. Thus, a promising strategy may be to block protein- protein interactions between Hsp70 and its co-chaperones or to target allosteric sites that disrupt these contacts. Such an approach might shift the balance of Hsp70 complexes and re-shape the proteome and it has the potential to restore healthy proteostasis. In this review, we discuss specific challenges and opportunities related to these goals. By pursuing Hsp70 complexes as drug targets, we might not only develop new leads for therapeutic development, but also discover new chemical probes for use in understanding Hsp70 biology.

Figure 1

Hsp70 forms the core of a multi-protein complex and associates with numerous co-chaperones. Three distinct classes of co-chaperones, NEFs, J proteins, and TPR domain-containing proteins, interact with Hsp70 and regulate its activities. The J proteins and NEFs interact with the NBD, while the TPR domain-containing proteins bind the C-terminal region. Representative structures from each class are shown with the corresponding PDB code. Images were prepared in PyMol.

Figure 2

Structure and ATPase cycle of Hsp70. (A) Hsp70 is composed of a 45 kDa N-terminal nucleotide binding domain (NBD) connected to a 25 kDa substrate binding domain (SBD) by a short hydrophobic linker. The SBD is composed of a β-sandwich and an α-helical “lid” domain. The structure of the prokaryotic Hsp70, DnaK, is shown (PDB code 2KHO), but the general architecture appears to be conserved amongst prokaryotic and eukaryotic family members. (B) Schematic of ATP hydrolysis and the role of co-chaperones. Substrate binding in the SBD coupled with J-domain co-chaperone interactions in the NBD promotes ATP hydrolysis. Conformational changes associated with ATP conversion close the “lid” and enhance affinity for the substrate. The cycle is completed when a nucleotide exchange factor interacts with the NBD and assists with ADP release.

Figure 3

J protein co-chaperones fall into three structural classes. (A) The domain architecture of each class of J protein is depicted as a schematic beginning with the N-terminus to the left. The domain types are J domain, GF (glycine-phenylalanine rich region), ZFLR (zinc finger-like region), CTDI and II (C-terminal domain) and DD (dimerization domain). (B) The crystal structures of the C-terminal portions of Ydj1 (yeast class A J protein) and Sis1 (yeast class B J protein) are shown with corresponding PDB codes. Images were prepared in PyMol.

Figure 4

Structures of chemical modulators of the Hsp70-J protein system.

Figure 5

Structures of Hsp70-NEF complexes. (A) Crystal structure of yeast Hsp110, Sse1, and human Hsp70 NBD. Complex formation between Hsp70 and Hsp110 leads to a rotation in lobe IIB allowing nucleotide release. (B) Crystal structure of HspBP1 and lobe II of Hsp70’s NBD. HspBP1 wraps around lobe IIB displacing lobe I and opening the nucleotide cleft. (C) Crystal structures of Hsp70 NBD in complex with the BAG domain of BAG1 and BAG2. Association between Hsp70 and the BAG proteins cause an outward rotation of lobe II, promoting nucleotide exchange. In all figures Hsp70 is colored in light grey and NEFs are colored in dark grey with PDB codes indicated. Images were prepared in PyMol.

Figure 6

Domain architecture of the BAG family of co-chaperones.