The mammalian small heat-shock protein Hsp20 forms dimers and is a poor chaperone

Hsp20 is one of the newly described members of the mammalian small heat-sho ck protein (sHsp) family. It occurs most abundantly in skeletal muscle and heart. We isolated clones for Hsp20 from a rat heart cDNA library, and expr essed the protein in Escherichia coli to characterize this little known sHs p. Recombinant Hsp20 displayed similar far-ultraviolet circular dichroism s pectra as the most closely related sHsp, alpha B-crystallin, but was less h eat stable, denaturing upon heating to 50 degrees C. While other mammalian recombinant sHsps form large multimeric complexes, Hsp20 occurs in two comp lex sizes, 43-kDa dimers and 470-kDa multimers. The ratio between the two f orms depends on protein concentration. Moreover, Hsp20 has a much lower cha perone-like activity than aB-crystallin, as indicated by its relatively poo r capacity to diminish the reduction-induced aggregation of insulin B chain s. Hsp20 is considerably shorter at the C-terminus and less polar than othe r sHsps, but H-1-NMR spectroscopy reveals that the last 10 residues are fle xible, as in the other sHsps. Our findings suggest that Hsp20 is a special member of the sHsp family in being less heat stable and tending to form dim ers. These properties, together with the shorter and less polar C-terminal extension, may contribute to the less effective chaperone-like activity.