The CD8α from sea bass ( Dicentrarchus labrax L.): Cloning, expression and 3D modelling

In this paper we describe the cloning, expression and structural study by modelling techniques of the CD8α from sea bass ( Dicentrarchus labrax L.). The sea bass CD8α cDNA is comprised of 1490 bp and is translated in one reading frame to give a protein of 217 amino acids, with a predicted 26 amino acids signal peptide, a 88 bp 5′-UTR and a 748 bp 3′-UTR. A multiple alignment of CD8α from sea bass with other known CD8α sequences shows the conservation of most amino acid residues involved in the peculiar structural domains found within CD8α's. Cysteine residues that are involved in disulfide bonding to form the V domain are conserved. In contrast, an extra cysteine residue found in most mammals in this region is not present in sea bass. The transmembrane and cytoplasmic regions are the most conserved regions within the molecule in the alignment analysis. However, the motif (CXCP) that is thought to be responsible for binding p56 lck is missing in the sea bass sequence. Phylogenetic analysis conducted using amino acid sequences showed that sea bass CD8α grouped with other known teleost sequences and that three different clusters were formed by the mammalian, avian and fish CD8α sequences. The thymus was the tissue with the highest CD8α expression, followed by gut, gills, peripheral blood leukocytes and spleen. Lower CD8α mRNA levels were found in head kidney, liver and brain. It was possible to create a partial 3D model using the human and mouse structures as template. The CD8α 11–120 amino acid region was taken into consideration and the best obtained 3D model shows the presence of ten β-strands, involving about 50% of the sequence. The global structure was defined as an immunoglobulin-like β-sandwich made of two anti-parallel sheets. Two cysteines were present in this region and they were at a suitable distance to form an S S bond as seen in the template human and mouse structures.