Nucleotide sequences of gene segments encoding membrane domains of immunoglobulin gamma chains

The nucleotide sequences of the exons encoding membrane-bound IgG1, IgG2a, and IgG2b of mouse were determined and compared with the sequence of membrane-bound IgM. The sequences indicate that membrane-bound gamma chains bear an additional 71 residues at the COOH termini, including 17-residue acidic, 26-residue hydrophobic, and 28 residue hydrophilic portions. The hydrophobic portion, which seems to be anchored in the lipid bilayer of the membrane, is highly conserved among membrane-bound mu and gamma chains. We propose the presence of a membrane protein that recognizes the conserved hydrophobic segment and anchors the membrane-bound immunoglobulin. Comparison of the nucleotide sequences revealed another example of intervening sequence-mediated domain exchange around one of the membrane exons.