Comparative structural studies on the light chains of human immunoglobulins. I. Protein Ka with the Inv(3) allotypic marker.

The primary structure of the variable region of the human type K Bence-Jones Protein Ka with Inv(3) genetic marker was determined by sequence analysis of 25 major tryptic peptides covering 214 residues isolated from completely reduced and aminoethylated protein. The complete sequences of 18 of these peptides were determined. These comprised the entire variable NH2-terminal half and 7 peptides from the COOH-terminal half of the protein. For the remaining peptides covering the COOH-terminus, only partial sequences or the amino acid compositions were determined. On the basis of these results all the tryptic peptides could be arranged in order. The sequence of the variable region differed from other previosly reported in 21 to 51 residues, but no variation was found in the sequence of the last 107 residues. On the basis of sequence homology the protein was classified in the kI subgroup. A brief discussion of the possible genetic mechanism of sequence variability in relation to subgroup specificity is presented.