A molecule bearing an immunoglobulin-like V region of the CTX subfamily in amphioxus.

An expressed sequence tag with significant similarity to a vertebrate T-cell receptor (Tcr) sequence was found in a cDNA library prepared from the posterior part of the adult amphioxus, Branchiostoma lanceolatum ( Brla). Characterization of the corresponding cDNA clone revealed the presence of an open reading frame encoding a 351 amino acid residue-long polypeptide. The putative protein, tentatively designated Brla-VDB for "V-domain bearing", appears to consist of two domains. The N-terminal domain begins with a putative leader peptide followed by a sequence resembling the V domain of the CTX protein originally found to be expressed in the cortical thymocytes of the clawed frog Xenopus. The C-terminal domain of the VDB protein does not show significant sequence similarity to any entry in the databases and contains five hydrophobic segments separated by short intervening hydrophilic stretches. It may therefore belong to a protein that crosses the plasma membrane five times. These findings support the notion that V domains resembling those found in Tcrs evolved in nonvertebrates before the emergence of the adaptive immune system and may have participated in functions not involved directly in immunity.