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MOMP from Campylobacter jejuni Is a Trimer of 18-Stranded β-Barrel Monomers with a Ca2+ Ion Bound at the Constriction
Luana G M Ferrara1, Gregor D Wallat1, Lucile Moynié1
1Biomedical Sciences Research Complex, University of St Andrews, 09042 St Andrews, UK.
Abstract:
The Gram-negative organism Campylobacter jejuni is the major cause of food poisoning. Unlike Escherichia coli, which has two major porins, OmpC and OmpF, C. jejuni has one, termed major outer membrane protein (MOMP) through which nutrients and antibiotics transit. We report the 2.1-Å crystal structure of C. jejuni MOMP expressed in E. coli and a lower resolution but otherwise identical structure purified directly from C. jejuni. The 2.1-Å resolution structure of recombinant MOMP showed that although the protein has timeric arrangement similar to OmpC, it is an 18-stranded, not 16-stranded, β-barrel. The structure has identified a Ca2+ bound at the constriction zone, which is functionally significant as suggested by molecular dynamics and single-channel experiments. The water-filled channel of MOMP has a narrow constriction zone, and single-molecule studies show a monomeric conductivity of 0.7±0.2 nS and a trimeric conductance of 2.2±0.2 nS. The ion neutralizes negative charges at the constriction zone, reducing the transverse electric field and reversing ion selectivity. Modeling of the transit of ciprofloxacin, an antibiotic of choice for treating Campylobacter infection, through the pore of MOMP reveals a trajectory that is dependent upon the presence metal ion.
Insights
The structure of Campylobacter jejuni's major outer membrane protein (MOMP) reveals a unique 18-stranded beta-barrel. A bound calcium ion influences antibiotic transit through this key channel for nutrients and drugs.
Area of Science:
- Microbiology
- Structural Biology
- Biophysics
Background:
- Campylobacter jejuni is a leading cause of foodborne illness.
- Its outer membrane protein (MOMP) is crucial for nutrient and antibiotic transport.
- MOMP is the sole porin in C. jejuni, unlike related bacteria.
Purpose of the Study:
- To determine the high-resolution crystal structure of C. jejuni MOMP.
- To investigate the functional role of ions within the MOMP channel.
- To understand the mechanism of antibiotic passage through MOMP.
Main Methods:
- X-ray crystallography (2.1 Å resolution) of recombinant C. jejuni MOMP.
- Purification and structural analysis of native C. jejuni MOMP.
- Molecular dynamics simulations and single-channel electrophysiology experiments.
Main Results:
- The C. jejuni MOMP adopts an 18-stranded beta-barrel structure, distinct from E. coli porins.
- A calcium ion (Ca2+) was identified at the channel's constriction zone.
- Ion binding neutralizes negative charges, altering the electric field and ion selectivity.
- Single-molecule studies quantified monomeric and trimeric channel conductances.
Conclusions:
- The unique structure of C. jejuni MOMP, including the Ca2+ binding site, is critical for its function.
- Ion presence significantly impacts the channel's properties and antibiotic permeability.
- Structural insights into MOMP may inform strategies against Campylobacter infections.
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