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Siderophores and colicins enter bacterial cells through TonB-dependent outer membrane proteins. Using site-directed substitution mutagenesis, we studied ligand recognition by a prototypic Escherichia coli siderophore receptor, FepA, that binds the iron chelate ferric enterobactin and colicins B and D. These genetic experiments identified a common binding site for two of the three ligands, containing multiple positive charges, within cell surface residues of FepA. Elimination of single residues in this region did not impair the adsorption or transport of ferric enterobactin, but double mutagenesis in the charge cluster identified amino acids (Arg-286 and Arg-316) that participate in siderophore binding and function in FepA-mediated killing by colicins B and D. Ferric enterobactin binding, furthermore, prevented covalent modification of FepA within this domain by either a fluorescent probe or an arginine-specific reagent, corroborating the involvement of this site in ligand recognition. These results identify, for the first time, residues in a TonB-dependent outer membrane protein that participate in ligand binding. They also explain the competition between ferric enterobactin and the colicins on the bacterial cell surface: all three ligands interact with the same arginine residues within FepA during their penetration through the outer membrane.
ferric enterobactin, siderophore, colicin, TonB, outer membrane
Double mutagenesis of a positive charge cluster in the ligand-binding site of the ferric enterobactin receptor, FepA
Salete M. C. Newton,*†‡ Jennifer S. Allen,*‡ Zhenghua Cao,* Zengbiao Qi,* Xunqing Jiang,* Cathy Sprencel,* John D. Igo,* Samuel B. Foster,* Marvin A. Payne,* and Phillip E. Klebba*§
1997 Apr 29
Colicins B-K260 and D-CA23 were purified by ammonium sulfate precipitation, gel filtration, and ion-exchange chromatography and were compared with respect to a number of physical and chemical properties. Both colicins were shown to be proteins and were found to have similar molecular weights, isoelectric points and amino acid compositions. The two colicins also have substantial antigenic similarities but are distinguished by the presence of non-cross-reacting antigens and by differences in stability and in sensitivity to heat and reducing conditions. In addition, strains of Escherichia coli K-12 producing colicins B-K260 and D-CA23 are not cross-immune. The similarities noted between the two colicins are compatible with their use of a common cell surface receptor while having different modes of action.
Comparison of Colicins B-K260 and D-CA23: Purification and Characterization of the Colicins and Examination of Colicin Immunity in the Producing Strains
Anthony P. Pugsley and Peter Reeves
In the title complex C34H20O6Os2 or (μ-η4-C4Ph4)Os2(CO)6, one Os atom is part of a metallacyclopentadiene ring, while the second Os atom is π-bonded to the organic portion of this ring. The distance of 2.7494?(2)?a between the two Os atoms is typical of an Os?Os single bond. Three carbonyl ligands are attached to each Os atom and these six carbonyls adopt an eclipsed conformation. There are no bridging or semibridging CO groups. Two carbonyl ligands and all four phenyl groups are disordered over two slightly different positions for which each atom in the minor components is displaced less than 1?a from the corresponding atom in the major components. The refined occupancies of the major components of the carbonyl ligands are 0.568?(16) and 0.625?(13), while those for the phenyl rings are 0.50?(3), 0.510?(12), 0.519?(18), and 0.568?(12).
crystal structure, disorder, osmole complex, diene ligand, osmium carbonyl, microwave heating
Crystal structure of [μ-1κ2 C 1,C 4:2(1,2,3,4-η)-1,2,3,4-tetraphenylbuta-1,3-diene-1,4-diyl]bis(tricarbonylosmium)(Os?Os)
Erin F. Rutledge,a Kylie M. Wilson,a Stephanie M. Martin,a John W. Swartout,a Ashley K. Archambeau,a Emily R. Mikeska,a Gregory L. Powell,a Eric W. Reinheimer,b and Cynthia B. Powella,*
2018 Sep 1