Molecular cloning and characterization of sea bass (Dicentrarchus labrax, L.) Tapasin

Mammalian tapasin (TPN) is a key member of the major histocompatibility complex (MHC) class I antigen presentation pathway, being part of the multi-protein complex called the peptide loading complex (PLC). Several studies describe its important roles in stabilizing empty MHC class I complexes, facilitating peptide loading and editing the repertoire of bound peptides, with impact on CD8⁺ T cell immune responses. In this work, the gene and cDNA of the sea bass (Dicentrarchus labrax) glycoprotein TPN have been isolated and characterized. The coding sequence has a 1329 bp ORF encoding a 442-residue precursor protein with a predicted 24-amino acid leader peptide, generating a 418-amino acid mature form that retains a conserved N-glycosylation site, three conserved mammalian tapasin motifs, two Ig superfamily domains, a transmembrane domain and an ER-retention di-lysine motif at the C-terminus, suggestive of a function similar to mammalian tapasins. Similar to the human counterpart, the sea bass TPN gene comprises 8 exons, some of which correspond to separate functional domains of the protein. A three-dimensional homology model of sea bass tapasin was calculated and is consistent with the structural features described for the human molecule. Together, these results support the concept that the basic structure of TPN has been maintained through evolution. Moreover, the present data provides information that will allow further studies on cell-mediated immunity and class I antigen presentation pathway in particular, in this important fish species.