Spontaneous missense mutation of an immunoglobulin in a mouse myeloma cell line

Abstract

Abstract The characterisation of the alteration in amino acid sequence of the immuno globulin heavy chain of IF4, a charge mutant of the myeloma line MOPC 21, is described. This was achieved by comparing the sequence of mutant IF4 heavy chain with the known sequence of the wild type. The peptic fragment (Fab′)2 from whole immunoglobulin, and all the ten CNBr fragments of MOPC 21 wild-type and mutant IF4 heavy chains, were identified and characterised. The only difference was in a tryptic peptide of the C-terminal CNBr fragment which had the same amino acid composition, but different electrophoretic mobilities. Thermolysin digestion products showed that asparagine 415 of wild-type heavy chain had been replaced by an aspartate in the mutant. Analysis of newly synthesized immunoglobulins from wild type and mutant showed the same charge difference, which did not seem therefore to result from deamidation. Fingerprints of the [32P]mRNA of IF4 heavy chain were prepared. The T1 ribonuclease oligonucleotide that includes the coding sequence for residue 415 in wild type was not found in mutant IF4. The mechanism is most likely a missense point mutation (A to G transition) in the MOPC 21 heavy chain structural cistron.