CTX, a novel molecule specifically expressed on the surface of cortical thymocytes in Xenopus

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Abstract
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 is composed of two immunoglobulin domains, one variable (V) and one constant (C2 type), followed by a transmembrane and a 64‐amino acid cytoplasmic domain. The first immunoglobulin domain is a V‐J segment that is generated without gene rearrangement. In the genome, the V and C2 domains are both encoded by two half‐domain exons. Two CTX loci are found per haploid genome, and they exhibit sequence differences with a high replacing/silent ratio in the CDR1‐like region of the V domain, suggesting that these differences were selected. The cytoplasmic domain contains a motif that is highly conserved evolutionarily in several types of proteins, including adenylyl cyclases. Based on its unique tissue distribution, the variability of its V region and the motif of its cytoplasmic domain, CTX is a candidate for a new type of specific signaling molecule involved in thymocyte selection.