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

To probe the structure of the gamma heavy chain of membrane IgG and the mRNA and gene segments that encode it, we have analyzed cDNA clones derived from a gamma 1 membrane RNA of B lymphoma 2PK-3. The nucleotide sequence of the clones indicated that membrane gamma 1 chains bear a COOH-terminal 71-residue segment that is absent from secretory gamma 1 chains. This terminus includes a 26-residue hydrophobic transmembrane region homologous to that of membrane mu chains and, significantly, a 28-residue intracellular domain found only on gamma 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 gamma 1 membrane terminus is encoded by two gene segments 1.5 and 2.4 kilobase pairs downstream from the C gamma 1 gene, and homologous segments occur 3' to the C gamma 2a and C gamma 3 genes. Small amounts of membrane gamma mRNAs persist in plasmacytomas secreting IgG1, IgG2a, or IgG2b, suggesting that competition between alternative RNA processing pathways governs the synthesis of membrane and secretory gamma chain mRNAs.