Temperature-dependent chaperone activity and structural properties of human αA- and αB-crystallins

The chaperone activity and biophysical properties of recombinant human alpha A- and alpha B-crystallins were studied by light scattering and spectroscopic methods. While the chaperone function of alpha A-crystallin markedly improves with an increase in temperature, the activity of alpha B homopolymer appears to change very little upon heating. Compared with alpha B-crystallin, the alpha A-homopolymer is markedly less active at low temperatures, but becomes a more active species at high temperatures. At physiologically relevant temperatures, the alpha B homopolymer appears to be modestly (two times or less) more potent chaperone than alpha A homopolymer, In contrast to very similar thermotropic changes in the secondary structure of both homopolymers, alpha A- and alpha B-crystallins markedly differ with respect to the temperature-dependent surface hydrophobicity profiles, Upon heating, alpha A-crystallin undergoes a conformational transition resulting in the exposure of additional hydrophobic sites, whereas no such transition occurs for alpha B-crystallin, The correlation between temperature-dependent changes in the chaperone activity and hydrophobicity properties of the individual homopolymers supports the view that the chaperone activity of a crystallin is dependent on the presence of surface-exposed hydrophobic patches. However, the present data also show that the surface hydrophobicity is not the sole determinant of the chaperone function of alpha-crystallin.