Evolution of immunoglobulin subclasses. Primary structure of a murine myeloma gamma1 chain.

The amino acid sequences of the first six CNBr fragments (303 residues) of MOPC 21 heavy chain have been determined, using conventional techniques. This completes the determination of the covalent structure of MOPC 21 immunoglobulin (IgG1, K). MOPC 21 gamma1 chain is a unique sequence of 440 amino acid residues. The variable region is homologous with human VHIII sequences. A comparison of the constant region with homologous sequences shows that mouse gamma1 and gamma2 proteins differ much more than the gamma subclasses in man and in some other species and supports earlier suggestions that mouse gamma1 and the four human gamma subclasses have a common ancestor different from the mouse gamma2 ancestor. Unlike some other subclasses, but like those in the guinea pig, mouse gamma1 and gamma2 genes have diverged before speciation. An expansion-contraction mechanism is proposed for the evolution of gamma genes.