Proteolysis by m-calpain enhances in vitro light scattering by crystallins from human and bovine lenses

Purpose. To determine if proteolysis by the calcium-activated protease m-calpain (EC 34.22.17) enhances in vitro light scattering in crystallins from human and bovine lenses. Methods. Total soluble proteins from bovine, human, and rodent lenses, betaH crystallin, or recombinant beta B1 polypeptide were pre-incubated in the presence or absence of activated m-calpain. Heat-induced light scattering was assayed by measuring changes in optical density at 405 rim, Proteolysis and cleavage sites were detected by SDS-PAGE, two dimensional electrophoresis, and N-terminal Edman sequencing. Results. The in vitro cleavage sites produced by m-calpain on the N-termini of human beta B1, beta A3, and beta B2-crystallins were similar to some of those on bovine and rat crystallins. Proteolysis of alpha- and beta -crystallins was associated with enhanced, heat-induced light scattering by human and bovine tens proteins. Conclusions: Proteolysis may be a contributing factor in the insolubilization of crystallins occurring during normal maturation of lens or during cataract formation in such species as man and cows.