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These findings support the hypothesis that membranes are deranged in cataractous tissue, which should lead to altered levels of calcium.

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

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