An atypical form of alphaB-crystallin is present in high concentration in some 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 cataracts. Both proteins were identified as modified forms of the small heat shock protein, alphaB-crystallin. The concentration of total alphaB-crystallin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alphaB(g). Mass spectrometric analyses of tryptic and Asp-N digests showed alphaB(g) is alphaB-crystallin minus the C-terminal lysine. alphaB(g) constituted 10-90% of the total alphaB-crystallin in these cataracts and was preferentially phosphorylated over the typical form of alphaB-crystallin. Human alphaB(g) and alphaB-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 alphaB(g) was comparable to that of recombinant human alphaB-crystallin in preventing the aggregation of lactalbumin induced by dithiothreitol. The mechanism involved in generating alphaB(g) is not known, but a premature termination of the alphaB-crystallin gene was ruled out by sequencing the polymerase chain reaction products of the last exon for the alphaB-crystallin gene from lenses containing alphaB(g). The 16.4-kDa protein was an N-terminally truncated fragment of alphaB(g). The high concentration of alphaB-crystallin in these cataracts is the first observation of this kind in human lenses.
Jimenez-Asensio
J
J
NEI, National Institutes of Health, Bethesda, Maryland 20892, USA.
Colvis
C M
CM
Kowalak
J A
JA
Duglas-Tabor
Y
Y
Datiles
M B
MB
Moroni
M
M
Mura
U
U
Rao
C M
CM
Balasubramanian
D
D
Janjani
A
A
Garland
D
D
D000595:Amino Acid Sequence; D002386:Cataract / Q000473:pathology*; D002851:Chromatography, High Pressure Liquid; D003001:Cloning, Molecular; D003459:Crystallins / Q000737:chemistry*; D015180:Electrophoresis, Gel, Two-Dimensional; D006801:Humans; D007525:Isoelectric Focusing; D007908:Lens, Crystalline / Q000473:pathology*; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D008970:Molecular Weight; D011994:Recombinant Proteins / Q000737:chemistry
D003459:Crystallins; D011994:Recombinant Proteins