Structural insights into the evolution feature of a bony fish CD8αα homodimer.

The CD8αα homodimer structures of endotherms demonstrate that despite distinct diversity at the amino acid sequence level, a few conserved key amino acids ensure common structural features. The structure of CD8αα in ancient ectotherms, such as lower bony fish, remains unclear. In this study, the high-resolution structure of the grass carp (Ctenopharyngodon idella) CD8αα (Ctid-CD8αα) homodimer was determined using the single-wavelength anomalous diffraction (SAD) method. The structure of Ctid-CD8αα shows distinct differences from the known CD8αα structures of endotherms, including a distinct topological structure with shorter back β sheets. The configuration and distribution of the hydrophobic core are different from those in endotherms. Interestingly, mutation of the key amino acid F32S, which is very common in fish and lies in the CDR loop region, leads to the absence of the typical cavity that binds to an epitope-MHC I (p/MHC I) in endotherms, yet Ctid-CD8αα can still specifically bind the grass carp peptide-Ctid-UAA-β2m (p/UAA-β2m). Our results indicate that during the evolutionary process, CD8αα has undergone dramatic changes that affect its dimeric structure and may use a new strategy to interact with p/MHC I.