Calcium activated proteolysis and protein modification in the U18666A cataract.

Proteolytic modifications of specific water soluble lens crystallins during U18666A cataract formation in young rats were identified by two dimensional gel electrophoresis and contrasted with those produced by incubating control lens homogenates with calcium. Protein changes which began in clear precataractous lenses at 12 days age included a decrease in 31 and 27 kDa (likely to be beta B1a and beta A3, respectively) crystallin polypeptides, increase in 25 kDa basic polypeptide, appearance of new polypeptide at 30 kDa and modification of alpha A-crystallin. Further modification of both alpha- and beta-crystallins occurred as cataracts formed; they progressed from early to advanced stage within a span of 4 days. During this period polypeptides beta B1a and beta A3 almost completely disappeared and several new components of 23-26 kDa in beta-crystallin region appeared. Extensive modification of alpha A resulted in appearance of new components of less than 20 kDa. Most of the gamma-crystallins disappeared from the water soluble proteins in advanced cataract lenses of 18 day old rats presumably by leaking out of the lens. The water insoluble proteins which accumulated in the cataract were very similar to modified crystallins which appeared in the water soluble fraction. In vitro incubation of normal lens water soluble proteins with calcium duplicated most of the protein changes seen during cataract progression. Immunoblotting studies with antisera to rat alpha- and beta-crystallins revealed the identity of most of the modified water soluble proteins.(ABSTRACT TRUNCATED AT 250 WORDS)