Previous work indicated that the amino (N)-terminal 34 residues (which includes one hypervariable region) of heavy chains of monotypic immunoglobulins G2-κ and M-κ from a single patient (Til) are identical, and that these two molecules share idiotypic determinants not present in their isolated light chains or in any of a large number of other immunoglobulins tested. Our present data demonstrate that the amino acid sequences of the μ and γ2 chains of this patient are also identical from residues 83 to 108, which includes two other hypervariable regions. These data furnish strong support for the concept that the constant and the variable regions of each immunoglobulin polypeptide chain are synthesized by different structural genes. Examination of amino acid sequences reported for variable regions indicates that tyrosine occurs frequently either within or at the immediate neighborhood of hypervariable regions. Thus, amino acid sequence data of monotypic immunoglobulins support the concept proposed by Singer and his colleagues that tyrosine may play an important role in antigen combining sites. Further amino acid sequence analyses show that the N-terminal 38 residues of the Fcμ fragment of Til IgM are identical with those reported for another IgM, whereas the N-terminal 60 residues of Fcγ2 fragment of Til IgG2 showed approximately 95% amino acid sequence homology at the CH2 domain with the other three γ-chain subclasses. This degree of homology is markedly higher than that of the hinge region, where only 60% homology is observed among the four γ-chain subclasses.
ASJC Scopus subject areas