Structural analysis of a protective epitope of the Francisella tularensis O-polysaccharide

Biochemistry. 2012 Jul 17;51(28):5684-94. doi: 10.1021/bi201711m. Epub 2012 Jul 2.


Francisella tularensis (Ft), the Gram-negative facultative intracellular bacterium that causes tularemia, is considered a biothreat because of its high infectivity and the high mortality rate of respiratory disease. The Ft lipopolysaccharide (Ft LPS) is thought to be a main protective antigen in mice and humans, and we have previously demonstrated the protective effect of the Ft LPS-specific monoclonal antibody Ab52 in a mouse model of respiratory tularemia. Immunochemical characterization has shown that the epitope recognized by Ab52 is contained within two internal repeat units of the O-polysaccharide [O-antigen (OAg)] of Ft LPS. To further localize the Ab52 epitope and understand the molecular interactions between the antibody and the saccharide, we determined the X-ray crystal structure of the Fab fragment of Ab52 and derived an antibody-antigen complex using molecular docking. The docked complex, refined through energy minimization, reveals an antigen binding site in the shape of a large canyon with a central pocket that accommodates a V-shaped epitope consisting of six sugar residues, α-D-GalpNAcAN(1→4)-α-D-GalpNAcAN(1→3)-β-D-QuipNAc(1→2)-β-D-Quip4NFm(1→4)-α-D-GalpNAcAN(1→4)-α-D-GalpNAcAN. These results inform the development of vaccines and immunotherapeutic/immunoprophylactic antibodies against Ft by suggesting a desired topology for binding of the antibody to internal epitopes of Ft LPS. This is the first report of an X-ray crystal structure of a monoclonal antibody that targets a protective Ft B cell epitope.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Antibodies, Monoclonal / chemistry*
  • Antigen-Antibody Complex / chemistry
  • Carbohydrate Sequence
  • Crystallography, X-Ray
  • Epitopes
  • Francisella tularensis / metabolism*
  • Immunoglobulin Fab Fragments / chemistry*
  • Molecular Docking Simulation
  • Molecular Sequence Data
  • O Antigens / chemistry*
  • O Antigens / immunology
  • Protein Conformation


  • Antibodies, Monoclonal
  • Antigen-Antibody Complex
  • Epitopes
  • Immunoglobulin Fab Fragments
  • O Antigens

Associated data

  • PDB/3UJT