A novel liver-specific immunoglobulin heavy chain-like gene in a cartilaginous fish.

We identified a novel immunoglobulin (Ig) heavy chain-like gene (tsIgH) expressed in the liver of the banded houndshark by preliminary transcriptomic analysis. The tsIgH gene showed less than 30% of amino acid identities to Ig genes of the shark. The gene encodes one variable domain (VH) and three conserved domains (CH1-CH3) with a predicted signal peptide. Interestingly, this protein has only one cysteine residue in a linker region between VH and CH1 other than those required for the formation of the immunoglobulin domain. Genome sequencing revealed that each of the domains was encoded by a corresponding single exon, and the exon-intron structures of the homologues are conserved in the other cartilaginous fishes. By RT-qPCR analysis, the transcript of the tsIgH gene was observed only in the liver, while that of the IgM was mainly detected in the epigonal organ, liver, and spleen. The novel Ig-heavy chain-like gene in cartilaginous fish may provide new clues to the evolution of immunoglobulin genes.