Small heat shock proteins are molecular chaperones.

TLDR

Small heat shock proteins (sHsps) are ubiquitous 15–30 kDa chaperones whose function has been unclear, but their heat‑induced upregulation and protective effects on cell viability imply a role in maintaining native protein conformation. To test this hypothesis we studied the influence of murine Hsp25, human Hsp27, and bovine alpha‑B‑crystallin on the unfolding and refolding of citrate synthase and alpha‑glucosidase in vitro. We examined how these sHsps affect unfolding and refolding of citrate synthase and alpha‑glucosidase in vitro, assessing aggregation prevention and refolding promotion. All tested sHsps acted as molecular chaperones, maximally preventing aggregation of citrate synthase and alpha‑glucosidase at stoichiometric ratios, stabilizing the proteins, promoting functional refolding after urea denaturation, and operating in an ATP‑independent manner.

Abstract

Small heat shock proteins (sHsp) with a molecular mass of 15-30 kDa are ubiquitous and conserved. Up to now their function has remained enigmatic. Increased expression under heat shock conditions and their protective effect on cell viability at elevated temperatures suggest that they may have a function in the formation or maintenance of the native conformation of cytosolic proteins. To test this hypothesis we studied the influence of murine Hsp25, human Hsp27, and bovine alpha-B-crystallin (an eye lens protein homologous to sHsps) on the unfolding and refolding of citrate synthase and alpha-glucosidase in vitro. Here we show that all sHsps investigated act as molecular chaperones in these folding reactions. At stoichiometric amounts they maximally prevent the aggregation of citrate synthase and alpha-glucosidase under heat shock conditions and stabilize the proteins. Furthermore, they promote the functional refolding of these proteins after urea denaturation similar to GroE and Hsp90. The interaction both with unfolding and refolding proteins seems to be ATP-independent.