Identical VL Region Sequences of Two Antibodies from Two Outbred Rabbits Exhibiting Complete Idiotypic Cross-Reactivity and Probably the Same Antigen-Binding Site Fine Structure

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Abstract
The amino acid sequences of the VL domains of two antibodies (4422 and 311) that exhibited the same isoelectric focusing (IEF) spectrotype, a1/b4 allotype, idiotype, and association constant for a nonasaccharide hapten have been determined. Two large fragments were obtained upon tryptic digestion of the citraconylated, fully reduced, and alkylated light chains owing to the presence of only two arginine residues per molecule of L chain. By homology with other rabbit k chain sequences, these peptides encompassed residues 1 to 63 and 64 to 214, respectively. The sequence of the N-terminal 60 residues was analyzed by automated Edman degradation with the whole L chain and by carboxypeptidases A + B digestions performed with the N-terminal 63-residue fragment. The citraconyl-peptide 64 to 214 was successfully degraded for 50 steps, which covered the sequence of the third hypervariable region. Both VL domains of antibodies 4422 and 311 were structurally identical with the possible exception of a Glu/Gln substitution at position 91. To our knowledge, this is the first report of two identical rabbit VK regions derived from antibodies expressing identical idiotypic determinants. In contrast, light chains from two other anti-SII antibodies, exhibiting IEF spectrotypes different from that of antibody 4422 (311), differed markedly both in their framework and first hypervariable regions from the leading sequence 4422. Taken together with the immunochemical data presented in the accompanying paper, these sequence data provide further strong support for the great similarity if not identity between antibodies 4422 and 311. Genetic implications of these results are discussed.