Heavy-chain mutants derived from gamma 2b mouse myeloma: characterization of heavy-chain messenger ribonucleic acid, proteins, and secretion in delection mutants and messenger ribonucleic acid in gamma2a mutant progeny.

Mouse myeloma mutants isolated from cell line 45.6 (gamma 2b) producing structurally altered immunoglobulin heavy (H) chains have been characterized. The mutant 10-1 synthesizes an H chain of 47 000 daltons containing a CH1 deletion; two mutants, G251 and I17, derived from 10-1 synthesize H chains of 40 000 and 35 000 daltons, respectively. The messenger ribonucleic acids (mRNAs) in these mutants have been shown to be smaller in molecular weight than mRNAs produced in 45.6 cells and lack a portion, but not all, of the CH1 domain. The H chains of G251 and I17 no longer express IgG subclass-specific determinants, are not secreted, and are structurally altered in the carboxyl-terminal portion of the molecule. In vitro the mRNAs of the mutants code for the synthesis of a polypeptide precursor characteristic of secreted proteins; the shortened proteins are apparently glycosylated intracellularly. Somatic cell hybrids between a structurally altered nonsecretor and a drug-marked wild-type myeloma cell secret only the wild-type protein. Reversion to secretion for G251 or I17 is accompanied by a change in the amino acid composition of the H chain such that gamma 2a subclass-specific determinants are expressed. Therefore, the primary structure of the H chain is an important factor in determining secretion. The gamma 2a-secreted chains from G251 and I17 fall into two classes: (1) those synthesizing proteins of approximately 47 000 daltons producing H-chain mRNAs of approximately 1.66 kilobases that are deleted for a portion, but not all, of CH1; (2) those synthesizing gamma2a proteins of approximately 55 000 daltons that are encoded in mRNAs of apparently wild-type size and that have regained CH1 sequences. The molecular explanations for the production of these alterations is discussed.