Abstract
The chaperone activity and biophysical properties of recombinant human alphaA- and alphaB-crystallins were studied by light scattering and spectroscopic methods. While the chaperone function of alphaA-crystallin markedly improves with an increase in temperature, the activity of alphaB homopolymer appears to change very little upon heating. Compared with alphaB-crystallin, the alphaA-homopolymer is markedly less active at low temperatures, but becomes a more active species at high temperatures. At physiologically relevant temperatures, the alphaB homopolymer appears to be modestly (two times or less) more potent chaperone than alphaA homopolymer. In contrast to very similar thermotropic changes in the secondary structure of both homopolymers, alphaA- and alphaB-crystallins markedly differ with respect to the temperature-dependent surface hydrophobicity profiles. Upon heating, alphaA-crystallin undergoes a conformational transition resulting in the exposure of additional hydrophobic sites, whereas no such transition occurs for alphaB-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 alpha-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.
Highlights
␣-Crystallin, a major lens protein, plays an important role in maintaining the transparency and refractive properties of the eye lens [1,2,3]
We present a detailed comparison of the recombinant human ␣A- and ␣B-crystallins, with a special emphasis on the chaperone activity and the surface hydrophobicity of both proteins
While the lens protein contains both ␣A and ␣B subunits [1,2,3], non-lenticular distribution of ␣-crystallin is clearly dominated by the ␣B chain [3, 5]
Summary
␣-Crystallin, a major lens protein, plays an important role in maintaining the transparency and refractive properties of the eye lens [1,2,3]. The lenticular ␣-crystallin consists of two 20kDa polypeptide chains, ␣A and ␣B, that share about 60% sequence homology. These chains self-associate, forming large oligomeric complexes that contain 30 –50 subunits at the ␣A to ␣B ratio of approximately 3:1. The rapidly growing data on non-lenticular expression of ␣A and ␣B chains was accompanied by the finding that ␣-crystallin belongs to the family of small heat shock proteins [3, 5, 8]. The apparent difference in the tissue specificity of ␣A- and ␣B-crystallins strongly suggests that these two proteins might have evolved to play distinct physiological functions. We present a detailed comparison of the recombinant human ␣A- and ␣B-crystallins, with a special emphasis on the chaperone activity and the surface hydrophobicity of both proteins
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