mRNA for surface immunoglobulin γ chains encodes a highly conserved transmembrane sequence and a 28-residue intracellular domain

To probe the structure of the γ heavy chain of membrane IgG and the mRNA and gene segments that encode it, we have analyzed cDNA clones derived from a γ₁ membrane RNA of B lymphoma 2PK-3. The nucleotide sequence of the clones indicated that membrane γ₁ chains bear a COOH-terminal 71-residue segment that is absent from secretory γ₁ chains. This terminus includes a 26-residue hydrophobic transmembrane region homologous to that of membrane μ chains, significantly, a 28-residue intracellular domain found only on γ chains. The extra domain suggests that receptor IgG, on memory B cells, may generate a different signal on binding antigen than does receptor IgM, on virgin B cells. The γ₁ membrane terminus is encoded by two gene segments 1.5 and 2.4 kilobase pairs downstream from the C(γ1) gene, and homologous segments occur 3' to the C(γ2a) and C(γ3) genes. Small amounts of membrane γ mRNAs persist in plasmacytomas secreting IgG1, IgG2a, or IgG2b, suggesting that competition between alternative RNA processing pathways governs the synthesis of membrane and secretory γ chain mRNAs.