Outer membrane protein A and OprF: versatile roles in Gram-negative bacterial infections

Abstract

Outer membrane protein A (OmpA) is an abundant protein of Escherichia coli and other enterobacteria and has a multitude of functions. Although the structural features and porin function of OmpA have been well studied, its role in the pathogenesis of various bacterial infections has emerged only during the last decade. The four extracellular loops of OmpA interact with a variety of host tissues for adhesion to and invasion of the cell and for evasion of host-defense mechanisms when inside the cell. This review describes how various regions present in the extracellular loops of OmpA contribute to the pathogenesis of neonatal meningitis induced by E. coli K1 and to many other functions. In addition, the function of OmpA-like proteins, such as OprF of Pseudomonas aeruginosa, is discussed.

Fig. 1. Sequence homology of OmpA from various Gram-negative bacteria

Representative pathogens for which OmpA has been shown to play a role in virulence were subjected to protein sequence alignment using the online ClustalW2 alignment tool (www.ebi.ac.uk/ClustalW2). The 4 loops of OmpA are shown within boxes and difference in amino acid sequences are highlighted in yellow. Cleavage site of the mature protein is indicated by arrowhead.

Fig. 2. C4b-binding protein interaction with E. coli K1 that express mutant OmpA

Two dimensional structure of OmpA depicting regions of three or four amino acids mutated to alanines (A). E. coli K1 expressing either wild type or mutated OmpA were incubated with C4bp for 30 min, washed and the bound proteins were evaluated by flow cytometry. Previously published in J. Biol. Chem. DOI: 10.1074/jbc.M110.178236

Fig. 3. OmpA interaction with CD64 is critical of E. coli K1 survival in macrophages

(A) RAW 264.7 macrophages were infected with OmpA+ E. coli or OmpA− E. coli for various times, washed, fixed and extracellular and intracellular bacteria were stained with anti-S-fimbria antibody. (B) RAW 264.7 macrophages were transfected with control short hairpin RNA (shRNA), FcγRI or CR3 shRNA and the cells were used for bacterial binding and invasion assays. Both the binding and entry of E. coli were significantly reduced in FcγRI⁻ macrophages. (C) FcγRI^−/− infant mice at Day 3 were infected with 10³ cfu of E. coli K1, the brains were collected after 72 h, paraffin embedded and tissue sections stained with H and E. Wild type mice showed gliosis, neuronal apoptosis and neutrophil infiltration, whereas FcγRI^−/− newborn mice showed no such pathology. Previously published in PLoS Pathogens, DOI: 10.1371/journal.ppat. 1001203.

Fig. 4. Simulation snap shots of OmpA interaction with GlcNAc1, 4 GlcNAc moieties

Wild-type OmpA binds to two GlcNAc1, 4GlcNAc moieties (chitobiose), one at the tips of loops 1 and 2, and second one near the membrane region at the base of loops 2 and 4. The simulations studies showed that OmpA structure stabilizes by 10 ns with more favorable free energy. In contrast, mutation of three amino acids in loop 2 (aa 61–64) to alanines renders the chitobiose unable to bind and exhibited less favorable energy. Previously published in J. Biol. Chem. DOI: 10.1074/jbc.M110.122804.

Fig. 5. OmpA+ CS induces NEC in newborn mice by disrupting the tight junctions

(A) Newborn mice were infected with OmpA+ CS or OmpA− CS and intestines were processed for H and E staining. Arrow indicates the disruption of villi structure. (B) Confluent monolayers of Caco-2 cells were infected with OmpA+ CS or OmpA− CS for 4 h, washed, fixed and stained with anti-ZO-1 antibody followed by secondary antibody coupled to Cy3. Stained cells were imaged with confocal microscope LSM710. Previously published by The American Association of Immunologists, Inc. in J. Immunology, DOI: 10.4049/jimmunol.1100108