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Many pathogens present highly variable surface proteins to their host as a means of evading immune responses. The structure of a peptide antigen corresponding to the subtype P1.7 variant of the porin PorA from the human pathogen Neisseria meningitidis was determined by solution of the X-ray crystal structure of the ternary complex of the peptide (ANGGASGQVK) in complex with a Fab fragment and a domain from streptococcal protein G to 1.95 A resolution. The peptide adopted a beta-hairpin structure with a type I beta-turn between residues Gly4P and Gly7P, the conformation of the peptide being further stabilised by a pair of hydrogen bonds from the side-chain of Asn2P to main-chain atoms in Val9P. The antigen binding site within the Fab formed a distinct crevice lined by a high proportion of apolar amino acids. Recognition was supplemented by hydrogen bonds from heavy chain residues Thr50H, Asp95H, Leu97H and Tyr100H to main-chain and side-chain atoms in the peptide. Complementarity-determining region (CDR) 3 of the heavy chain was responsible for approximately 50 % of the buried surface area formed by peptide-Fab binding, with the remainder made up from CDRs 1 and 3 of the light chain and CDRs 1 and 2 of the heavy chain. Knowledge of the structures of variable surface antigens such as PorA is an essential prerequisite to a molecular understanding of antigenic variation and its implications for vaccine design.

Original publication




Journal article


J Mol Biol

Publication Date





81 - 91


Amino Acid Sequence, Animals, Antibodies, Monoclonal, Antigenic Variation, Antigens, Bacterial, Binding Sites, Crystallography, X-Ray, Hydrogen Bonding, Immunoglobulin Fab Fragments, Immunoglobulin Heavy Chains, Immunoglobulin Light Chains, Immunoglobulin Variable Region, Mice, Models, Molecular, Molecular Sequence Data, Neisseria meningitidis, Nerve Tissue Proteins, Porins, Protein Structure, Secondary, Sequence Alignment