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

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Abstract
Mouse myeloma mutants isolated from cell line 45.6 (.gamma.2b) producing structurally altered Ig H chains were characterized. The mutant 10-1 synthesizes an H chain of 47,000 daltons containing a CH1 deletion; 2 mutants, G251 and I17, derived from 10-1 synthesize H chains of 40,000 and 35,000 daltons, respectively. The mRNA in these mutants were smaller in MW than mRNA 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 carboxy-terminal portion of the molecule. In vitro the mRNA 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 nonsecretory and a drug-marked wild type myeloma cell secrete 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 the following 2 classes: those synthesizing proteins of .apprx. 47,000 daltons producing H chain mRNA of .apprx. 1.66 kilobases that are deleted for a portion, but not all, of CH1; those synthesizing .gamma.2a proteins of .apprx. 55,000 daltons that are encoded in mRNA of apparently wild-type size and that have regained CH1 sequences. The molecular explanations for the production of these alterations is discussed.