THE CRITICAL ROLE OF A SOLVENT-EXPOSED RESIDUE OF AN MHC CLASS I-RESTRICTED PEPTIDE IN MHC-PEPTIDE BINDING

The immunodominant ovalbumin(257-264) (OVA-8, SIINFEKL) and herpes sim plex virus gB(496-503) (HSV-8, SSIEFARL) peptides share 50% amino acid identity (residues P1, P3, P5 and P8) and bind with comparable effica cy to the murine MHC-encoded class I molecule H-2K(b), However, these two peptides bind differently to H-2K(bm8), a natural H-2K(b) variant with a substitution in four amino acids on the floor of the peptide-bi nding site; HSV-8 binds with high and OVA-8 with a relatively law effi cacy, To investigate which of the non-homologous peptide residues were responsible for this differential binding, we used substituted peptid e variants and the class I thermodynamic stabilization assay, Variatio n at the solvent-exposed peptide residues P6 and P7 did not appreciabl y influence binding, By contrast, variation at the buried P2 and, surp risingly, at the solvent-exposed P4 residue was found to be important, Transplantation of the HSV-8 P2 or P4 residues onto the OVA-8 backbon e created variant peptides O2S (P21 --> S) and O4E (P4N --> E) that bo und considerably better to H-2K(bm8) than OVA-8, Furthermore, the doub le-substituted peptide, O2S4E, bound even better, revealing a cooperat ive effect of the two residues, The reciprocally substituted peptides H2I and H4N, generated by grafting the OVA-8 P2 and P4 residues onto t he HSV-8 backbone respectively, bound to H-2K(bm8) Slightly worse than HSV-8 but the double-substituted peptide H2I4N bound as poorly as OVA -8, Effects exerted by the P4 residue, which is solvent accessible and therefore available for the TCR contact, demonstrated that exposed pe ptide residues can, in certain situations, influence not only the TCR contact but also MHC-peptide binding.