Cloning expression, and chaperone-like activity of human alpha A-crystallin

One of the major protein components of the ocular lens, alpha-crystallin, is composed of alpha A and alpha B chain subunits that have structural homology to the family of mammalian small heat shock proteins, Like other small heat shock proteins, alpha-crystallin subunits associate to form large oligomeric aggregates that express chaperone-like activity, as defined by the ability to suppress nonspecific aggregation of proteins destabilized by treatment with a variety of denaturants including heat, UV irradiation, and chemical modification, It has been proposed that age-related loss of sequences at the C terminus of the alpha A chain subunit may be a factor in the pathogenesis of cataract due to diminished capacity of the truncated crystallin to protect against nonspecific aggregation of lens proteins, To evaluate the functional consequences of alpha-crystallin modification, two mutant forms of alpha A subunits were prepared by site-directed mutagenesis, Like wild type (WT), aggregates of similar to 540 kDa were formed from a tryptophan-free alpha A mutant (W9F), When added in stoichiometric amounts, both WT and W9F subunits completely suppressed the heat-induced aggregation of aldose reductase, In contrast, subunits encoded by a truncation mutant in which the C-terminal 17 residues were deleted (R157STOP), despite having spectroscopic properties similar to WT, formed much larger aggregates with a marked reduction in chaperone-like activity, Similar results were observed when the chaperone-like activity was assessed through inhibition of gamma-crystallin aggregation induced by singlet oxygen, These results demonstrate that the structurally conservative substitution of Phe for Trp-9 has a negligible effect on the functional interaction of alpha A subunits, and that deletion of C-terminal sequences from the alpha A subunit results in substantial loss of chaperone-like activity, despite overall preservation of secondary structure.