Similarities of the Light Chains and the Variable Regions of the Heavy Chains of the IgG2 λ and IgA1 λ Myeloma Proteins from a Single Patient

High concentrations of an IgA1 and IgG2 myeloma proteins were found in the serum of a single individual. Both light chains were of λ type, had similar amino acid compositions, and could not be distinguished by peptide mapping. Their N-terminal amino acid sequences were identical for the first 30 residues and typical of the V λ III subclass. The heavy chains had the same N-terminal pentapeptide and a comparison of their peptide maps showed the existence of several peptides common to both heavy chains, suggesting similarity of their variable regions. These results, taken together with the previous demonstration of both γ and α chains in the same plasma cell, support the conclusion that during plasma cell differentiation a “switch” may occur which alters the expression of the CH gene although the expression of the VL, CL, and VH genes remains unchanged.

Partial duplication in the "hinge" region of IgA 1 myeloma proteins

The aminoacid sequence of the "hinge" region of two human IgA(1) myeloma proteins was found to be identical. A 30-residue fragment contains 3 cysteines, 12 prolines, and carbohydrate, and the sequence of a small segment of the hinge region is repeated at least twice. Since a deletion of 12-13 residues was found in the same region of an IgA(2) myeloma protein, it is possible that IgA(1) molecules are the result of the insertion of a partially duplicated gene segment in the immunoglobulin genes.