Human alpha‐crystallin chaperone suppresses the aggregation of partially folded intermediates of human gamma D‐crystallin and its deamidated forms.

The chaperone α‐crystallin is one of the ubiquitous crystallins in vertebrate lenses. It is a polydisperse, hetero‐oligomeric complex ( 800 kDa) composed of αA‐ and αB‐crystallin subunits ( 20 kDa). It is a member of the small heat shock family and an ATP‐independent chaperone that suppresses aggregation by forming stable complexes. HγD‐crys, another major structural crystallin, is localized in the lens nucleus, where it must remain stable and soluble throughout the adult lifetime. Aggregated and oxidatively damaged forms are recovered from mature‐onset cataract. Human HγD‐crys can be refolded in vitro at 37 ° C, but an aggregated fibrillar state competes with productive refolding. Deamidated forms have been recovered from cataract. Two of these alterations—Q54E and Q143E at the domain interface—destabilize purified HγD‐crys and increase the tendency to aggregate upon refolding. These reactions of HγD‐crys have been used to investigate the reaction of α‐crystallin with its physiological substrates. Upon dilution of unfolded HγD‐crys into buffer in the presence of HαA‐ or HαB‐crystallin, the aggregation reaction, detected by light scattering, was effectively suppressed. Subunit ratios of 1:10 (WT or deamidated HγD‐crys monomer to HαA/B‐crys monomer) reduced the aggregation to less than 10% of control levels. A high molecular weight complex containing both the chaperone and its substrate was recovered from size exclusion columns. Whether the partially folded chains are bound to the surface of the chaperone oligomer, or are internalized within the lumen, remains to be determined.