Sequence analysis of lens β-crystallins suggests involvement of calpain in cataract formation

Abnormal activation of the protease calpain in the lens may be a cause of cataracts. Cataracts were induced in 10-day-old rats by a single overdose of sodium selenite. The water-insoluble protein from the opaque lens nucleus was separated by two-dimensional electrophoresis, electroblotted onto membranes, and the NH₂-terminal sequence of partially degraded β-crystallin polypeptides determined. Selenite cataractous lenses contained four major structural proteins, βB1, βB3, βA3/A1, and βA4 crystallins, missing from 5 to 49 amino acids from their NH₂ termini. Incubation of intact β- crystallins with calpain II in vitro produced identical cleavage sites. This provided further evidence for the role of calpain in the production of light scattering insoluble protein in cataractous lenses and also suggested that a similar process may lead to lens protein insolubilization during aging.