The primary structure of a 49-residue CNBr fragment from the N-terminal quarter of strain 13 guinea pig immunoglobulin G(2) heavy chain has been determined. This fragment, which spans residues N-35 to N-83 of the γ2 chain, has two regions of almost constant sequence joined by a markedly variable region spanning 12 residues. Automated sequential degradation of C-1-a1 from strain 13 and strain 2 animals and from anti-dinitrophenyl antibody purified from strain 13 animals confirmed the presence of a constant sequence for residues N-36 to N-47 of γ2 chain. There was greatly restricted variability in purified antibody within the region shown to be markedly variable in normal C-1-a1 of both strains. Positions N-49, N-51, and N-58 within this variable region seem constant in all these sources of C-1-a1.
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