Identification of origin of two polypeptides of 4 and 5 kD isolated from human lenses

To purify crystallin fragments (degraded polypeptides molecular weight < 18 kD) and identify their parent crystallins.

The purification of polypeptides with apparent molecular weights of 4 and 5 kD was carried out using three sequential steps: Sephadex G-50 chromatography under denaturing conditions, preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and high-performance liquid chromatography using a C-18 column. The parent crystallins of the two polypeptides were identified by the Western blotting method using polyclonal antibodies raised against individual 4 and 5 kD polypeptides and by comparing N-terminal amino acid sequences of the polypeptides with crystallins.

Two polypeptides of 4 and 5 kD were purified by the three sequential steps as described from water-soluble proteins of lenses from 60-80-year-old donors. Both purified polypeptides showed a single major peak during high-performance liquid chromatography on a C-18 column and also a single band during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The Western blot analyses showed maximum immunoreactivity of the anti-4 kD polypeptide antibody to a 22 kD species of beta-crystallin, whereas the anti-5 kD polypeptide antibody showed maximum reactivity to only the alpha B crystallin. These results were further confirmed during comparison of the N-terminal amino acid sequences of the two polypeptides with crystallins. Such comparison showed that the 4 kD polypeptide originated from beta A 3/A1 crystallin after cleavage at His187-His188 bond. Further, the 5 kD polypeptide was a fragment of alpha B crystallin that originated after cleavage at Val145-Asn146 bond.

These results showed that specific bonds of beta A3/A1 and alpha B crystallins are posttranslationally cleaved in vivo to produce 4 kD and 5 kD polypeptides, respectively.