Structure of calf lens β-crystallins

An investigation of the polypeptide composition of calf lens β-crystallin was conducted. Two β-crystallin fractions, previously designated fractions B and C, were separated by gel filtration on Sephadex G-200. Fraction C was found to have a molecular weight of approximately 52,000. Electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate (SDS) of this fraction yielded two bands with molecular weights of 24,000 and 27,500, respectively, calculated on the basis of their mobilities. Electrophoresis in polyacrylamide gels in 6 m urea, however indicated that eight distinct polypeptides compose Fraction C. The molecular weight of Fraction B was calculated to be approximately 210,000. SDS electrophoresis of this fraction produced four bands. Over 80% of the protein material was of either 24,000 or 27,500 molecular weight, however two lesser components had molecular weights of 31,000 and 35,000. Electrophoresis in 6 m urea gels yielded eight bands corresponding in mobility to the bands found for Fraction C, plus at least five other bands of lesser mobility. On the basis of these figures Fraction C apparently exists as a dimer in the native state and Fraction B in the native state consists of eight polypeptide chains. The percentage composition of the SDS polypeptides of Fraction B suggests that each native molecule may contain one polypeptide chain from the 31,000 or 35,000 molecular weight species. We suggest that the evidence strongly indicates that the two fractions of calf lens β-crystallin, while being quite different in molecular weight, are composed to a great extent of similar polypeptide chains. This fact would make the great similarity in amino acid composition of the two fractions more easily explicable. Furthermore the large number of non-identical polypeptides which were found to exist in each of these fractions would produce the high degree of heterogeneity observed in the β-crystallin by several previous investigators.