An atypical form of alpha B-crystallin is present in high concentration insome human cataractous lenses - Identification and characterization of aberrant N- and C-terminal processing

Two unique polypeptides, 22.4 and 16.4 kDa, were prominent in some human ca taracts. Both proteins mere identified as modified forms of the small heat shock protein, alpha B-crystallin. The concentration of total alpha B-cryst allin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alpha B-g. Mass spectrometric analys es of tryptic and Asp-N digests showed alpha B-g is alpha B-crystallin minu s the C-terminal lysine. alpha B-g constituted 10-90% of the total alpha B- crystallin in these cataracts and was preferentially phosphorylated over th e typical form of alpha B-crystallin. Human alpha B-g and alpha B-crystalli n were cloned and expressed in Escherichia coli. The differences in electro phoretic mobility and the large difference in native pI values suggest some structural differences exist. The chaperone-like activity of recombinant h uman alpha B-g was comparable to that of recombinant human alpha B-crystall in in preventing the aggregation of lactalbumin induced by dithiothreitol, The mechanism involved in generating alpha B-g is not known, but a prematur e termination of the alpha B-crystallin gene was ruled out by sequencing th e polymerase chain reaction products of the last exon for the alpha B-cryst allin gene from lenses containing alpha B-g. The 16.4-kDa protein was an N- terminally truncated fragment of alpha B-g. The high concentration of alpha B-crystallin in these cataracts is the first observation of this kind in h uman lenses.