CTX, a Xenopus thymocyte receptor, defines a molecular family conserved throughout vertebrates

CTX, a cortical thymocyte marker in Xenopus, is an immunoglobulin superfami ly (Igsf) member comprising one variable and one constant C2-type Igsf doma in, a transmembrane segment and a cytoplasmic tail. Although resembling tha t of the TCR and immunoglobulins, the variable domain is not encoded by som atic rearrangement of the gene but by splicing of two half-domain exons. Th e C2 domain, also encoded by two exons, has an extra pair of cysteines. The transmembrane segment is free of charged residues, and the cytoplasmic tai l (70 amino acids) contains one tyrosine and many glutamic acid residues. C hT1, a chicken homologue of CTX, has the same structural and genetic featur es, and both molecules are expressed on the thymocyte surface. We cloned ne w mouse (CTM) and human (CTH) cDNA and genes which are highly homologous to CTX/ChT1 but not lymphocyte specific. Similarity with recently described h uman cell surface molecules. A33 antigen and CAR (coxsackie and adenovirus 5 receptor), and a number of expressed sequence tags leads us to propose th at CTX defines a novel subset of the Igsf, conserved throughout vertebrates and extending beyond the immune system. Strong homologies within vertebrat e sequences suggest that the V and C2 CTX domains are scions of a very anci ent lineage.