Identification and Molecular Characterization of Z/ZE Lineage MHC Class I Heavy Chain Homologue and ^|^beta;2-microglobulin from Rock Bream Oplegnathus fasciatus

We identified and characterized full-length cDNAs of rock bream Oplegnathus fasciatus major histocompatibility complex (RbMHC) class I and beta 2-microglobulin (Rb beta 2m) molecules. The full-length RbMHC class I cDNA was 2,667 bp with a 1,227 bp open reading frame encoding 409 amino acids (aa). The Rb beta 2m full-length cDNA (1,339 bp) consisted of a 354 bp coding region, encoding 118 aa. RbMHC class I alpha was clustered with the Z/ZE lineage of known MHC class I sequences, suggesting a novel molecule belonging to the Z/ZE lineage. Although characteristic features (residues and motifs) known to be conserved in classical and non-classical class I molecules were conserved in RbMHC class I alpha with few modifications, it is more closely resembled to classical MHC class I. RbMHC class I and Rb beta 2m were transcribed constitutively in healthy fish, but expression differed among the different organs and was highest in whole blood. Low sequence variation in the RbMHC class I alpha 1 region makes it uncertain whether the molecule belongs to the non-classical group. However, because transcription was induced by megalocytivirus, the molecule is most likely a member of the classical group. Our results provide important information for future studies on antigen presentation and cytotoxic CD8 super(+) T cell responses in rock bream against infectious pathogens.