Small heat-shock proteins function in the insoluble protein complex

Small heat-shock proteins (sHSPs) represent an abundant and ubiquitous family of molecular chaperones. The current model proposes that sHSPs function to prevent irreversible aggregation of non-native proteins by forming soluble complex. The chaperone activity of sHSPs is usually determined by the capacity to suppress thermally or chemically induced protein aggregation. However, sHSPs were frequently found in the insoluble complex particularly in vivo. In this report, it is clearly revealed that the insoluble sHSP/substrate complex is formed when sHSP is overloaded with non-native substrates, which is the very case under in vivo conditions. The proposal that sHSPs function to prevent the protein aggregation seems misleading. sHSPs appear to promote the elimination of protein aggregates by incorporating into the insoluble protein complex.