Structural and functional changes in the alpha A-crystallin R116C mutant in hereditary cataracts.
alpha-Crystallin, the major protein component of vertebrate lenses, forms a large complex comprised of two homologous subunits, alphaA- and alphaB-crystallin. It has the ability to suppress stress-induced protein aggregation in vitro, bind saturably to lens plasma membranes, and aid in light refraction through short-range ordering. Recently, a missense mutation in alphaA-crystallin that changes arginine 116 to a cysteine residue (R116C) was genetically linked to one form of autosomal dominant congenital cataracts. This point mutation is reported to cause structural alterations at many levels as well as a 4-fold reduction in chaperone-like activity. To extend these findings, we examined the quaternary stability of the alphaA R116C mutant protein and its effect on chaperone-like activity, subunit exchange, and membrane association. Homocomplexes of mutant subunits become highly polydisperse following incubation at 37 degrees C, reflecting the likely in vivo distribution of the complexes. Chaperone-like activity of the alphaA R116C mutant is approximately 4-fold lower than wild type, whether measured before or after conversion to a polydisperse population with incubation. alphaA R116C complexes also have a 4-fold reduced ability to exchange subunits with wild-type complexes. Finally, membrane binding capacity measurements of mutant subunits showed a 10-fold increase over wild type. Our results, in conjunction with previous reports, suggest that the changes in complex polydispersity, the reduction of subunit exchange, and increased membrane binding capacity are all potential factors in the pathogenesis of alphaA R116C associated congenital cataracts.