Structural studies on lens proteins.

The sequence around the thiol group in lens proteins has been investigated. The proteins were converted into their carboxy[(14)C]methyl derivatives and submitted to partial acid hydrolysis, or digested with proteolytic enzymes. Acid hydrolysis of bovine alpha-crystallin gives N-seryl-(S-carboxymethyl)cysteine, Ser-CMCys (Waley, 1965a), but this dipeptide is not obtained from beta-crystallin or gamma-crystallin. Trypsin and chymotrypsin also give different peptides from the three crystallins. The radioactive peptide from alpha-crystallin and chymotrypsin has the sequence Ser-CMCys-Ser-Leu; another peptide, Asp-Leu-Leu-Phe, was also identified. The radioactive peptides obtained from bovine alpha-crystallin are probably also obtained from human alpha-crystallin, and from bovine and human albuminoid (the insoluble lens protein). alpha-Crystallin has been fractionated by chromatography in urea on DEAE-cellulose. Comparison of the fractions by peptide ;mapping', and immunochemically, shows that they fall into two classes. The fraction eluted first differs from the later fractions, but the later fractions resemble each other The first fraction may represent impurities, or it may be a structurally different sub-unit of alpha-crystallin.