The structure of the mouse immunoglobulin in γ3membrane gene segment

The genomic region containing the mouse immunoglobulin gamma 3 heavy chain membrane (M) exons has been located and sequenced. The exon structure is highly similar to that of the other mouse gamma chains, with strong sequence conservation in the coding regions and the intron 5' to the M1 exon. The intron between M1 and M2 shows moderate sequence homology but very strong conservation of size. RNA blots suggest that gamma 3 membrane exon usage is similar to that seen in other immunoglobulin membrane heavy chain mRNAs. The transmembrane region contains the invariant residues which have been noted in all other heavy chain sequences and which were previously proposed to be interactive in a two-chain model for insertion through the lymphocyte membrane. Conserved residues with similar spacing have been seen in class II histocompatibility antigens, which are also two-chain transmembrane molecules, but not in class I antigens, which span cell membranes with a single chain.