Significance of alpha-crystallin heteropolymer with a 3:1 alphaA/alphaB ratio: chaperone-like activity, structure and hydrophobicity.

The small heat-shock protein alpha-crystallin isolated from the eye lens exists as a large (700 kDa) heteropolymer composed of two subunits, alphaA and alphaB, of 20 kDa each. Although trace amounts of alphaA-crystallin are found in other tissues, non-lenticular distribution of alpha-crystallin is dominated by the alphaB homopolymer. In most vertebrate lens, the molar ratio of alphaA to alphaB is generally 3:1. However, the importance of this ratio in the eye lens is not known. In the present study, we have investigated the physiological significance of the 3:1 ratio by determining the secondary/tertiary structure, hydrophobicity and chaperone-like activity of alphaA- and alphaB-homopolymers and heteropolymers with different ratios of alphaA to alphaB subunits. Although, under physiologically relevant conditions, the alphaB-homopolymer (37-40 degrees C) has shown relatively higher activity, the alphaA-homopolymer or the heteropolymer with a higher alphaA proportion (3:1 ratio) has shown greater chaperone-like activity at elevated temperatures (>50 degrees C) and also upon structural perturbation. Furthermore, higher chaperone activity at elevated temperatures as well as upon structural perturbation is mainly mediated through increased hydrophobicity of alphaA. Although homopolymers and heteropolymers of alpha-crystallin did not differ in their secondary structure, changes in tertiary structure due to structural perturbations upon pre-heating are mediated predominantly by alphaA. Interestingly, the heteropolymer with higher alphaA proportion (3:1) or the alphaA-homopolymer seems to be better chaperones in protecting lens beta- and gamma-crystallins at both normal and elevated temperatures. Thus lens might have favoured a combination of these qualities to achieve optimal protection under both native and stress (perturbed) conditions for which the heteropolymer with alphaA to alphaB in the 3:1 ratio appears to be better suited.