Porins OmpC and PhoE of Escherichia coli as specific cell-surface targets of human lactoferrin - Binding characteristics and biological effects

The binding of lactoferrin, an iron-binding glycoprotein found in secretion s and leukocytes, to the outer membrane of Gram-negative bacteria is a prer equisite to exert its bactericidal activity. It was proposed that porins, i n addition to lipopolysaccharides, are responsible for this binding. We stu died the interactions of human lactoferrin with the three major porins of E scherichia coli OmpC, OmpF, and PhoE. Binding experiments were performed on both purified porins and porin-deficient E. coli K12 isogenic mutants. We determined that lactoferrin binds to the purified native OmpC or PhoE trime r with molar ratios of 1.9 +/- 0.4 and 1.8 +/- 0.5 and K-d values of 39 +/- 18 and 103 +/- 15 mM, respectively, but not to OmpF, Furthermore, preferen tial binding of lactoferrin was observed on strains that express either Omp C or PhoE, It was also demonstrated that residues 1-5, 28-34, and 39-42 of lactoferrin interact with porins, Eased on sequence comparisons, the involv ement of lactoferrin amino acid residues and porin loops in the interaction s is discussed. The relationships between binding and antibacterial activit y of the protein were studied using E. coli mutants and planar Lipid bilaye rs. Electrophysiological studies revealed that lactoferrin can act as a blo cking agent for OmpC but not for PhoE or OmpF. However, a total inhibition of the growth was only observed for the PhoE-expressing strain (minimal inh ibitory concentration of lactoferrin was 2.4 mg/ml). These data support the proposal that the antibacterial activity of lactoferrin may depend, at lea st in part, on its ability to bind to porins, thus modifying the stability and/or the permeability of the bacterial outer membrane.