CTX, A NOVEL MOLECULE SPECIFICALLY EXPRESSED ON THE SURFACE OF CORTICAL THYMOCYTES IN XENOPUS

CTX, a novel developmentally regulated type-I transmembrane protein is expressed specifically by a large fraction of cortical thymocytes in the amphibian Xenopus. This apparently monomeric 55-kDa glycoprotein i s composed of two immunoglobulin domains, one variable (V) and one con stant (C2 type), followed by a transmembrane and a 64-amino acid cytop lasmic domain. The first immunoglobulin domain is a V-J segment that i s generated without gene rearrangement. In the genome, the V and C2 do mains are both encoded by two half-domain exons. Two CTX loci are foun d per haploid genome, and they exhibit sequence differences with a hig h replacing/silent ratio in the CDR1-like region of the V domain. sugg esting that these differences were selected. The cytoplasmic domain co ntains a motif that is highly conserved evolutionarily in several type s of proteins, including adenylyl cyclases. Based on its unique tissue distribution. the variability of its V region and the motif of its cy toplasmic domain. CTX is a candidate for a new type of specific signal ing molecule involved in thymocyte selection.