Significance of α-crystallin heteropolymer with a 3:1 αA/αB ratio: chaperone-like activity, structure and hydrophobicity

The small heat-shock protein α-crystallin isolated from the eye lens exists as a large (700 kDa) heteropolymer composed of two subunits, αA and αB, of 20 kDa each. Although trace amounts of αA-crystallin are found in other tissues, non-lenticular distribution of α-crystallin is dominated by the αB homopolymer. In most vertebrate lens, the molar ratio of αA to αB 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 αAand αB-homopolymers and heteropolymers with different ratios of αA to αB subunits. Although, under physiologically relevant conditions, the αB-homopolymer (37–40 ◦C) has shown relatively higher activity, the αA-homopolymer or the heteropolymer with a higher αA proportion (3:1 ratio) has shown greater chaperonelike activity at elevated temperatures (>50 ◦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 αA. Although homopolymers and heteropolymers of α-crystallin did not differ in their secondary structure, changes in tertiary structure due to structural perturbations upon pre-heating are mediated predominantly by αA. Interestingly, the heteropolymer with higher αA proportion (3:1) or the αA-homopolymer seems to be better chaperones in protecting lens βand γ -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 αA to αB in the 3:1 ratio appears to be better suited.