ALPHA-CRYSTALLIN CHAPERONE ACTIVITY IS REDUCED BY CALPAIN-II IN-VITROAND IN SELENITE CATARACT

This study reports the first demonstration of a marked reduction in al pha-crystallin chaperone activity in an experimental model of cataract , and the study implicates activation of the cysteine protease calpain II (EC 3.4.22.17) as the in vivo protease responsible for decreased c haperone activity. Chaperone activity of normal alpha-crystallin from lenses of young rats was assayed by measuring attenuation of heat-indu ced aggregation and scattering of betaL-crystallin. Alpha-crystallin f rom the nucleus of lenses with selenite cataract showed specific selec tive proteolysis, and chaperone activity was diminished. Proteolysis o f alpha-crystallin from selenite cataract lenses was mimicked by incub ating normal alpha-crystallin with calpain II, and this also resulted in loss of chaperone activity. Two-dimensional gel electrophoresis and peptide mapping were used to identify four partially degraded alphaA- and alphaB-crystallin polypeptides following incubation of normal alp ha-crystallin with calpain. Similar partially degraded alphaA and alph aB polypeptides were found in selenite cataract. Previous experiments indicated that alpha-crystallin chaperone activity decreases because o f removal of the COOH terminus. Our experiments support this observati on and suggest that calpain proteolysis of alpha-crystallin at the COO H terminus may result in a loss of chaperone activity in selenite cata ract.