MOMP from Campylobacter jejuni is a trimer of 18-stranded β–barrel monomers with a Ca²⁺ ion bound at the constriction zone

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 an18-stranded not 16-stranded β-barrel. The structure has identified a Ca²⁺ bound at the constriction zone, which molecular dynamics and single channel experiments suggest is functionally significant. 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. Modelling 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.