A survey of ray finned fish PSMB8 sequences reveal novel holostean alleles that may impact MHCI function
The evolutionary history and scope of haplotypic diversity of this critical piece of the immunoproteasome is only beginning to be understood. Based on a significant amino acid difference at the 31st residue of the mature protein, PSMB8 haplotypes are classified into A and F types. All jawed vertebrates possess PSMB8A. In contrast, the PSMB8F lineage has been lost by most vertebrates except for chondrichthyans and early diverging actinopterygians, leading to type conversions in multiple species that lost the F lineage. Here we investigate type conversions in 68 ray-finned fishes, and identify two new PSMB8 types, PSMB8S and PSMB8K. These previously unknown types are exclusive to holosteans (gar and bowfin), and introduce novel biochemical changes to the binding pocket suggesting altered cleavage properties. These results expand our knowledge of vertebrate PSMB8 haplotypes and the processing of peptide antigens for display by MHC class I.