COVALENT STRUCTURE OF A HUMAN GAMMAG-IMMUNOGLOBULIN .5. PARTIAL AMINO ACID SEQUENCE OF LIGHT CHAIN

The amino acid sequence of the variable region of a χ chain isolated from a yG myeloma protein (Eu) has been determined. In addition, the tryptic peptides of the constant portion have been isolated and partially sequenced. Comparison with Bence-Jones proteins of the same antigenic class and subgroup showed that the variable portion of Eu has 13 new substitutions at positions which have not previously been observed to vary. Seven additional new interchanges were found at positions known to be variable in κ chains. Five of the twenty new amino acid interchanges require a two-base change in the codon specifying amino acids at that position. The data on the constant region suggest that it has the same sequence as that of Inv 3 x chains with one exception. The residue at position 108 of the light chain is glycine rather than arginine. Thus this position may be in the variable region or it may represent a site for allotypic variation in the constant region. In spite of the high degree of variability in the sequence of the Eu χ chain as compared with the sequences of Bence-Jones proteins, the extent of homology is consistent with the previous conclusion that urinary Bence-Jones proteins are light chains. © 1968, American Chemical Society. All rights reserved.