DIMER STRUCTURE AS A MINIMUM COOPERATIVE SUBUNIT OF SMALL HEAT-SHOCK PROTEINS

Recently, it has been shown that small heat-shock proteins (Hsp25, Hsp 27) are molecular chaperones, They bind to thermally unfolded proteins and can also assist refolding: of denatured proteins. Mammalian small Hsps can form oligomeric structures of about 32 subunits. Until now, no data about cooperativity and stability of the interactions between the subunits of sHsps are available. To analyze these interactions we studied mouse Hsp25 and human Hsp27 by difference adiabatic scanning m icrocalorimetry (DASM) and circular dichroism (CD). Here we show that, according to DASM data, the minimum cooperatively melting structure i s a sHsp-dimer. CD data indicate that Hsp25 major secondary structure, the beta-pleated conformation, is resistant to acidic influence up to pH 4.5 and, at neutral pH values, to heat treatment up to 60 degrees C. The melting pattern of Hsp25/27 bears resemblance to alpha-crystall ins. CD data indicate similar secondary, tertiary and quaternary struc tures of the proteins compared. This finding is in agreement with the revealed homology of primary structure of these proteins and their com mon chaperone function.