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

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.