Fig. 3.

Evolution of immunoproteasomes and thymoproteasomes

Panel A shows that the majority of genes coding for catalytic β subunits are located in the MHC and MHC-paralogous regions. Para 1, 2 and 3 stand for MHC-paralogous regions 1, 2 and 3. In humans, PSMB10 and PSMB6 map outside of the paralogous regions. In Xenopus tropicalis, all six catalytic β subunit genes are encoded in the MHC and MHC-paralogous regions; psmb10 maps to the class III region of the MHC and psmb6 maps to the region syntenic to human para 3 (19p13.1–13.3). In zebrafish, psmb12 and psmb13, which originated from psmb6 and psmb7, respectively, are in the class I region of the MHC. Panel B shows three possible scenarios accounting for the emergence of the PSMB11 gene (indicated in white) from the PSMB5 gene (indicated in black). In bony fishes, psmb11 and psmb5 are located in a tail-to-head orientation as shown in the figure. In contrast, they are located in a head-to-head orientation in tetrapods (Sutoh et al. 2012). Therefore, PSMB11 appears to have been inverted in a tetrapod lineage (not shown in the figure).