Long-range effects in protein--ligand interactions mediate peptide specificity in the human major histocompatibilty antigen HLA-B27 (B*2701).
B*2701 differs from all other HLA-B27 subtypes of known peptide specificity in that, among its natural peptide ligands, arginine is not the only allowed residue at peptide position 2. Indeed, B*2701 is unique in binding many peptides with Gln2 in vivo. However, the mutation (Asp74Tyr) responsible for altered selectivity is far away from the B pocket of the peptide binding site to which Gln/Arg2 binds. Here, we present a model that explains this effect. It is proposed that a new rotameric state of the conserved Lys70 is responsible for the unique B*2701 binding motif. This side chain should be either kept away from pocket B through its interaction with Asp74 in most HLA-B27 subtypes, or switched to this pocket if residue 74 is Tyr as in B*2701. Involvement of Lys70 in pocket B would thus allow binding of peptides with Gln2. Binding of Arg2-containing peptides to B*2701 is also possible because Lys70 could adopt another conformation, H-bonded to Asn97, which preserves the same binding mode of Arg2 as in B*2705. This model was experimentally validated by mutating Lys70 into Ala in B*2701. Edman sequencing of the B*2701(K70A) peptide pool showed only Arg2, characteristic of HLA-B27-bound peptides, and no evidence for Gln2. This supports the computational model and demonstrates that allowance of B*2701 for peptides with Gln2 is due to the long-range effect of the polymorphic residue 74 of HLA-B27, by inducing a conformational switch of the conserved Lys70.
Krebs
S
S
Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid, Facultad de Ciencias, Cantoblanco, Spain.
Rognan
D
D
López de Castro
J A
JA
D000595:Amino Acid Sequence; D015796:HLA-B27 Antigen / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000737:chemistry / Q000378:metabolism*
D015796:HLA-B27 Antigen; D008024:Ligands; D010455:Peptides