ISOLATION OF NON‐IDENTICAL POLYPEPTIDE CHAINS OF α‐CRYSTALLIN

Abstract

α‐Crystallin – a water‐soluble lens protein – was dissociated into subunits by means of urea treatment at pH 3. The dissociated protein was resolved into three distinct polypeptide species by chromatography on SE‐Sephadex columns at pH 3.2 equilibrated with 7 M urea. In the presence of urea the isolated polypeptides, designated as I‐a, I‐b, and II, have different electrophoretic mobilities at acid and alkaline pH. It could be demonstrated that two cysteine residues are present in polypeptide I‐a whereas cysteine is absent in polypeptide II. Moreover, amino‐acid analysis reveals that both polypeptides have a different amino‐acid composition. The third polypeptide has shown to be a dimer of one of the polypeptides. This dimer arises during isolation by disulphide bond formation. The results indicate that a‐crystallin is composed of two distinctly different kinds of subunits. Evidence is presented that both types are built up by two different polypeptide chains.