Gene Segments Encoding Transmembrane Carboxyl Termini of Immunoglobulin Gamma Chains

In cell lines producing IgM, secreted and membrane bound forms of immunoglobulin mu heavy chains are produced from two separate mu mRNAs that are identical except for alternative 3' coding sequences. We now show that cell lines producing IgG likewise contain two mRNA species for immunoglobulin gamma chains. The major, 1.7 kilobase (kb) species encodes secreted gamma chains. A less abundant species of 3-4 appears to encode membrane bound gamma chains, in that it contains an alternative 3' end encoded in separate exons 3' to the remainder of the gene. The first exon of this M gene segment has been identified in chromosomal gamma 1 and gamma 2b gene clones by its sequence homology with the corresponding exon in the mu gene. Like the mu M exon, it encodes a probable transmembrane polypeptide segment. The flanking DNA sequences show a patchwork pattern of homology between genes that suggests a checkered evolutionary history.