Intracellular protein unfolding and aggregation: The role of small heat-shock chaperone proteins

Molecular chaperones are a diverse group of proteins that interact with partially folded protein states to stabilize and prevent their mutual ( illicit) association. Proteins require involvement with molecular chaperones throughout their lifespan: from their synthesis and folding through intracellular transport, membrane translocation, and to their ultimate degradation. Small heat-shock proteins (sHsps) are a ubiquitous family of molecular chaperones that are found in all organisms. Unlike many of the well-characterized chaperones, for example from the Hsp60 and Hsp70 families, sHsps are not involved in regulating protein folding. Instead, under conditions of cellular stress, such as elevated temperatures, they interact and stabilize partially folded target proteins to prevent their aggregation and precipitation. Because of this ability, their expression is elevated in many protein diseases that are characterized by protein aggregation and precipitation, including Alzheimer's, Creutzfeldt-Jakob, and Parkinson's diseases. The principal lens protein, alpha-crystallin, is a sHsp. Its chaperone ability is important in preventing lens protein precipitation and hence in maintaining lens transparency. This review summarizes the salient structural features of sHsps that enable them to act as highly efficient chaperones to prevent protein precipitation under stress conditions. The mechanism of chaperone action and the state of the target protein when interacting with sHsps are also discussed. Finally, diseases in which sHsp expression is elevated are discussed including the potential roles of sHsps and their therapeutic uses in the treatment of these diseases.