Spontaneous somatic mutations. Structural studies on mutant immunoglobulins.

The precise alterations in the protein amino acid sequences of the immunoglobulin heavy chains of spontaneously arisen MOPC 21 mutant clones IF2 and IF1 have been determined. All the cyanogen bromide fra-ments of both heavy chains have been isolated and compared to the wild type CNBr fragments. For IF2, there is an internal deletion, from the wild type sequence, of 96 amino acids, from residues 121 to 215 inclusive. Moreover, in IF2, there are no disulfide bonds formed between heavy and light chains, presumably because of the deletion of the CH1 pseudosubunit. There are no other alterations in its covalent structure. For IF1, there is a deletion of the COOH-terminal 83 amino acids (residues 358 to 440, inclusive). Although IF1 heavy chain behaves on dodecyl sulfate-polyacryl-amide gels as if it were only 10 to 15 residues shorter than wild type, no other amino acid sequence differences from wild type are found. IF1 arose most likely by a nonsense mutation of a serine codon. For IF2, whose deletion is like that seen in some human heavy chain disease proteins, the most likely explanation is an error of recombination. The structure of IF2 suggests that the heavy chain variable region ends at a position homologous to residue 120 of the MOPC 21 heavy chain.