Crystallins vary in their tendency to precipitate after proteolysis

Purpose: Study the mechanism of crystailin precipitation in cataracts associated with proteolysis. Methods: Total soluble proteins from human, rat, bovine, and chicken embryo lenses were partially proteolyzed by calpain II enzyme. Subsequent precipitation of crystallins was measured by light scattering at A₄₀₅, SDS-PAGE and 2D-electrophoresis assessed proteolysis on the crystallins, and size-exclusion HPLC detected formation of high MW aggregates. Results: Most rapid precipitation occurred with lens proteins from young rat, either by adding purified calpain or by activating endogenous lens calpain with calcium. The precipitate was composed primarily of β-crystallins and occurred without formation of soluble, intermediate high MW aggregates, suggesting direct transition to insoluble states. Crystallins from older rat lens and human cortex showed some precipitation, but only when purified calpain was added. Chick embryo and bovine lenses showed minimal or no precipitation. β-crystallins from rat, bovine, human, but not chick, were substrates for calpain II. Significance: Compared to similarly proteolyzed crystallins from other animals, our results indicate an unusually high susceptibility of crystallins from young rat lens to precipitation after limited proteolysis. We hypothesize that certain polypeptides, such as relatively low amounts of stabilizing βB₂ in young rat lens, allow precipitation of other polypeptide fragments. Thus, young rat lens provides a unique opportunity to investigate how interactions between crystallin polypeptides influence susceptibility to precipitation found in cataract.