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Bactericidal antibody recognition of meningococcal PorA by induced fit. Comparison of liganded and unliganded Fab
J van den Elsen1, L Vandeputte-Rutten, J Kroon
1Department of Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
Abstract:
MN12H2 is a bactericidal antibody directed against outer membrane protein PorA epitope P1.16 of Neisseria meningitidis. Binding of MN12H2 to PorA at the meningococcal surface activates the classical complement pathway resulting in bacterial lysis. We have determined the crystal structure of the unliganded MN12H2 Fab fragment in two different crystal forms and compared it with the structure of the Fab in complex with a P1.16-derived peptide. The unliganded Fabs have elbow bend angles of 155 degrees and 159 degrees, whereas the liganded Fab has a more closed elbow bend of 143 degrees. Substantial differences in quaternary and tertiary structure of the antigen binding site are observed between the unliganded and liganded MN12H2 Fab structures that can be attributed to peptide binding. The variable light and heavy chain interface of the liganded Fab is twisted by a 5 degrees rotation along an axis approximately perpendicular to the plane of the interface. Hypervariable loops H1, H2, and framework loop FR-H3 follow this rotation. The hypervariable loop H3 undergoes conformational changes but remains closely linked to hypervariable loop L1. In contrast with the binding site expansion seen in other Fab-peptide structures, the MN12H2 binding site is narrowed upon peptide binding due to the formation of a "false floor" mediated by arginine residue 101 of the light chain. These results indicate that PorA epitope P1.16 of N. meningitidis is recognized by the complement-activating antibody MN12H2 through induced fit, allowing the formation of a highly complementary immune complex.
Insights
The antibody MN12H2 targets Neisseria meningitidis outer membrane protein PorA. Structural analysis reveals an induced fit mechanism upon peptide binding, narrowing the antigen-binding site for effective complement activation and bacterial lysis.
Area of Science:
- Immunology
- Structural Biology
- Microbiology
Background:
- Neisseria meningitidis is a significant bacterial pathogen.
- The outer membrane protein PorA is a key target for bactericidal antibodies.
- Antibody MN12H2 targets the PorA epitope P1.16, activating complement and causing bacterial lysis.
Purpose of the Study:
- To determine the crystal structure of the MN12H2 Fab fragment in both unliganded and peptide-bound states.
- To elucidate the structural basis of MN12H2 recognition of the PorA epitope P1.16.
- To understand the mechanism of complement activation induced by MN12H2 binding.
Main Methods:
- X-ray crystallography was used to determine the structures of unliganded and peptide-liganded MN12H2 Fab fragments.
- Comparison of structural differences in the antigen-binding site and elbow bend angles.
- Analysis of quaternary and tertiary structure changes upon peptide binding.
Main Results:
- Unliganded MN12H2 Fab fragments exhibited elbow bend angles of 155 and 159 degrees.
- The peptide-liganded Fab showed a more closed angle of 143 degrees.
- Peptide binding induced significant structural changes, including a narrowed binding site due to a "false floor" formed by Arg101 of the light chain, and a 5-degree rotation at the variable light and heavy chain interface.
Conclusions:
- MN12H2 recognizes the PorA epitope P1.16 of N. meningitidis via an induced fit mechanism.
- The induced fit involves conformational changes in hypervariable loops and the formation of a unique "false floor" in the binding site.
- This structural adaptation allows for the formation of a highly complementary immune complex, leading to potent complement activation and bacterial lysis.