Hsp20 is one of the newly described members of the mammalian small heat-shock protein (sHsp) family. It occurs most abundantly in skeletal muscle and heart. We isolated clones for Hsp20 from a rat heart cDNA library, and expressed the protein in Escherichia coli to characterize this little known sHsp. Recombinant Hsp20 displayed similar far-ultraviolet circular dichroism spectra as the most closely related sHsp, AB-crystallin, but was less heat stable, denaturing upon heating to 50°C. While other mammalian recombinant sHsps form large multimeric complexes, Hsp20 occurs in two complex sizes, 43-kDa dimers and 470-kDa multimers. The ratio between the two forms depends on protein concentration. Moreover, Hsp20 has a much lower chaperone-like activity than AB-crystallin, as indicated by its relatively poor capacity to diminish the reduction-induced aggregation of insulin B chains. Hsp20 is considerably shorter at the C-terminus and less polar than other sHsps, but 1 H-NMR spectroscopy reveals that the last 10 residues are flexible, 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 dimers. These properties, together with the shorter and less polar C-terminal extension, may contribute to the less effective chaperone-like activity.Keywords : small heat-shock protein; dimer ; oligomer; chaperone-like activity ; protein structure.In mammals, six members of the ubiquitous superfamily of small heat-shock proteins (sHsp) are known to occur [1]. These are AA-and AB-crystallin, Hsp25 (depending on species, also indicated as Hsp27 or Hsp28), the more recently discovered p20 [2], (now dubbed Hsp20 [3]), and the latest additions, HSPB2[4] and HspB3 [5]. A-Crystallin is a major eye lens protein, composed of two types of subunits, AA-crystallin and AB-crystallin (for reviews see [6,7]). The latter also occurs at high levels in other tissues, notably in heart and striated muscle [8]. AB-crystallin is stress-inducible [9] and its level is increased in various neurodegenerative disorders and tumors [10Ϫ13]. Hsp25 occurs at low levels in various tissues and is also inducible upon stress (for review see [14]). Both A-crystallins and Hsp25 display ATP-independent chaperone-like properties and confer thermotolerance upon expression in cultured cells. Mammalian A-crystallins and Hsp25 have been studied extensively. In contrast, Hsp20 has been the subject of only four reports [2,3,15,16], while HSPB2 and HspB3 are, as yet, the least known [4,5]. Hsp20 was originally isolated in mixed complexes with AB-crystallin and Hsp25 from rat and human skeletal muscle, and its amino acid sequence is most similar to AB-crystallin [1, 2]. Hsp20 was detected in all rat tissues examined, reaching the highest levels (up to 1.3 % of total protein) in striated muscle, heart and diaphragm, similar to AB-crystallin and Hsp25. Like Hsp25, it is also conspicuously present in smooth muscle of the bladder and rectum. Hsp20 exists in muscle extracts in a multimeric and a disso...
