CART: a conserved antigen receptor transmembrane motif.

We have compared the transmembrane sequences of 72 vertebrate antigen receptor (mIg and TCR) polypeptides. This allowed us to identify a Conserved Antigen Receptor Transmembrane (CART) motif which is present in all antigen receptor transmembrane domains from species as far removed as cartilaginous fish. Most of the amino acids in the CART motif are polar or aromatic and may interact with other proteins in the lipid environment. In addition, modeling the antigen receptor transmembrane domain in an alpha helical conformation places the CART residues on one face of the alpha helix. Thus, the CART motif may encode a structural unit which plays a role in the assembly and/or the signaling properties of lymphocyte antigen receptors. We speculate on the potential role of the CART motifs in lymphocyte signaling.