An Evolutionarily Conserved Family of Hsp70/Hsc70 Molecular Chaperone Regulators

Shinichi Takayama,Zhihua Xie,John C Reed

doi:10.1074/jbc.274.2.781

Shinichi Takayama, Zhihua Xie + Show 1 more

Open Access

https://doi.org/10.1074/jbc.274.2.781

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Abstract

Heat Shock Protein 70 kDa (Hsp70) family molecular chaperones play critical roles in protein folding and trafficking in all eukaryotic cells. The mechanisms by which Hsp70 family chaperones are regulated, however, are only partly understood. BAG-1 binds the ATPase domains of Hsp70 and Hsc70, modulating their chaperone activity and functioning as a competitive antagonist of the co-chaperone Hip. We describe the identification of a family of BAG-1-related proteins from humans (BAG-2, BAG-3, BAG-4, BAG-5), the invertebrate Caenorhabditis elegans (BAG-1, BAG-2), and the fission yeast Schizosaccharomyces pombe (BAG-1A, BAG-1B). These proteins all contain a conserved approximately 45-amino acid region near their C termini (the BAG domain) that binds Hsc70/Hsp70, but they differ widely in their N-terminal domains. The human BAG-1, BAG-2, and BAG-3 proteins bind with high affinity (KD congruent with 1-10 nM) to the ATPase domain of Hsc70 and inhibit its chaperone activity in a Hip-repressible manner. The findings suggest opportunities for specification and diversification of Hsp70/Hsc70 chaperone functions through interactions with various BAG-family proteins.

Highlights

The chaperone activity of mammalian Hsc70/Heat Shock Protein 70 kDa (Hsp70) is regulated by partner proteins that either modulate the peptide binding cycle or that target the actions of these chaperones to specific proteins and subcellular compartments
The DnaJ-family proteins Ydj1p of yeast and human hdj2 have C-terminal CAAX prenylation motifs that account for their post-translational modification with lipids which anchor them in intracellular membranes, thereby allowing them to recruit Hsp70/Hsc70-family chaperones into protein-folding reactions involved in mitochondrial protein import [7, 8]
Using the ATPase domain of Hsc70 as a “bait” in yeast two-hybrid screens, several human cDNAs were cloned that encode portions of BAG-1 or two other BAG-1-like proteins, which were arbitrarily named BAG-2 and BAG-3

Summary

Introduction

The chaperone activity of mammalian Hsc70/Hsp is regulated by partner proteins that either modulate the peptide binding cycle or that target the actions of these chaperones to specific proteins and subcellular compartments. DnaJ-family proteins (Hdj-1/Hsp; Hdj-2; Hsj-1) stimulate the ATPase activity of Hsc70/Hsp, resulting in the ADP-bound state that binds tightly to peptide substrates [1,2,3]. The Hip protein collaborates with Hsc70/Hsp and DnaJ homologues in stimulating ATP hydrolysis and, stabilizes Hsc70/Hsp complexes with substrate polypeptides, whereas the Hop protein may provide co-chaperone functions through interactions with the C-terminal peptide binding domain (4 – 6). Hdj-1 and its yeast counterpart Sis1p are uniquely associated with ribosomes among known DnaJ-family members, collaborating with Hsp70/Hsc70-family proteins in folding of nascent polypeptides during translation [11, 12]. The findings reveal unexpected diversity in the protein networks available for targeting Hsp70/Hsc functions in cells

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