Gene segments encoding transmembrane carboxyl termini of immunoglobulin γ chains

Abstract In cell lines producing lgM, secreted and membrane-bound forms of immunoglobulin μ heavy chains are produced from two separate μ mRNAs that are identical except for alternative 3′ coding sequences. We now show that cell lines producing lgG likewise contain two mRNA species for immunoglobulin γ chains. The major, 1.7 kilobase (kb) species encodes secreted γ chains. A less abundant species of 3–4 kb appears to encode membrane-bound γ 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 γ1 and γ2b gene clones by its sequence homology with the corresponding exon in the μ gene. Like the μ 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.