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Topology of outer membrane porins in pathogenic Neisseria spp
P van der Ley1, J E Heckels, M Virji
1National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands.
Abstract:
In Escherichia coli, membrane-spanning amphipathic beta-sheet structures are characteristic of many outer membrane proteins. By applying the principles that have been recognized for them to the four classes of neisserial porins, we have constructed a model for the topology of the porins within the outer membrane. This model predicts eight surface-exposed loops, both in the meningococcal class 1 and 2 proteins and in the gonococcal PIA and PIB proteins. The transmembrane sequences are highly conserved among these porins and are able to form an amphipathic beta-sheet structure. The surface-exposed hydrophilic loops show extensive variation in both length and sequence. Experimental evidence in support of this model has been obtained by using antisera against synthetic peptides which correspond to surface-exposed loops in class 1 and 2 proteins. Thus, binding to the cell surface was observed with antibodies against loops 1, 4, and 5 of class 1 and loops 1 and 5 of class 2. In class 1, these loops are the longest ones and show the highest sequence diversity among strains of different subtypes. Mapping of epitopes recognized by monoclonal antibodies with bactericidal activity has also provided strong support for the model. The epitopes are located in loops 1 and 4 of class 1 protein, loop 5 of PIB, and loop 6 of PIA. A nonbactericidal antibody that binds only weakly to whole cells was shown to recognize loop 3 of PIB. These results suggest that the longest loops are immunodominant, provide the binding sites for bactericidal antibodies, and display the greatest variation among different strains.
Insights
A new model for neisserial porins reveals eight surface-exposed loops and conserved transmembrane beta-sheets. These loops vary in length and sequence, influencing antibody binding and bacterial strain diversity.
Area of Science:
- Structural biology
- Microbiology
- Immunology
Background:
- Outer membrane proteins in Escherichia coli feature membrane-spanning amphipathic beta-sheet structures.
- Neisserial porins, crucial outer membrane proteins, share structural similarities with E. coli outer membrane proteins.
Purpose of the Study:
- To construct a topological model for neisserial porins within the bacterial outer membrane.
- To investigate the structural basis for variations in neisserial porins and their immunogenicity.
Main Methods:
- Applied principles of E. coli outer membrane protein structure to model neisserial porin topology.
- Utilized experimental evidence with antisera against synthetic peptides corresponding to surface-exposed loops.
- Mapped epitopes recognized by monoclonal antibodies with bactericidal activity.
Main Results:
- The model predicts eight surface-exposed loops and conserved amphipathic beta-sheet transmembrane structures for neisserial porins.
- Experimental data confirmed antibody binding to specific surface-exposed loops (e.g., loops 1, 4, 5 in class 1).
- Longest surface-exposed loops demonstrated the greatest sequence diversity among strains and contained epitopes for bactericidal antibodies.
Conclusions:
- The proposed model accurately describes neisserial porin topology in the outer membrane.
- Surface-exposed loops, particularly the longest ones, are immunodominant, variable, and critical for bactericidal antibody recognition.