A Small Heat Shock Protein Cooperates with Heat Shock Protein 70 Systems to Reactivate a Heat-Denatured Protein

TLDR

Small heat shock proteins are conserved, heat‑induced proteins abundant in plants that act as chaperones to bind non‑native intermediates and prevent thermal aggregation, allowing ATP‑dependent refolding by other chaperones. The study aimed to determine the minimal chaperone system required to refold a heat‑denatured protein bound to the pea Hsp18.1 small heat shock protein. Using heat‑denatured firefly luciferase bound to Hsp18.1, the authors tested refolding with eukaryotic Hsc70/Hsp70 plus DnaJ homologs Hdj1 and Ydj1, or with bacterial DnaK, DnaJ, and GrpE. Luciferase bound to Hsp18.1 was refolded to 97 % with eukaryotic Hsc70/Hsp70 plus Hdj1/Ydj1 and to 100 % with bacterial DnaK/DnaJ/GrpE, and Hsp18.1 alone more effectively prevented aggregation and increased refolding yields even when other chaperones were present, demonstrating that sHsps cooperate with Hsp70 systems to reactivate heat‑denatured proteins.

Abstract

Small heat shock proteins (sHsps) are a diverse group of heat-induced proteins that are conserved in prokaryotes and eukaryotes and are especially abundant in plants. Recent in vitro data indicate that sHsps act as molecular chaperones to prevent thermal aggregation of proteins by binding non-native intermediates, which can then be refolded in an ATP-dependent fashion by other chaperones. We used heat-denatured firefly luciferase (Luc) bound to pea (Pisum sativum) Hsp18.1 as a model to define the minimum chaperone system required for refolding of a sHsp-bound substrate. Heat-denatured Luc bound to Hsp18.1 was effectively refolded either with Hsc/Hsp70 from diverse eukaryotes plus the DnaJ homologs Hdj1 and Ydj1 (maximum = 97% Luc reactivation with k(ob) = 1.0 x 10(-2)/min), or with prokaryotic Escherichia coli DnaK plus DnaJ and GrpE (100% Luc reactivation, k(ob) = 11.3 x 10(-2)/min). Furthermore, we show that Hsp18.1 is more effective in preventing Luc thermal aggregation than the Hsc70 or DnaK systems, and that Hsp18.1 enhances the yields of refolded Luc even when other chaperones are present during heat inactivation. These findings integrate the aggregation-preventive activity of sHsps with the protein-folding activity of the Hsp70 system and define an in vitro system for further investigation of the mechanism of sHsp action.