Mutagenesis and Molecular Modeling Reveal Three Key Extracellular Loops of the Membrane Receptor HasR That Are Involved in Hemophore HasA Binding

On the basis of the three-dimensional model of the heme/hemophore TonB-dependent outer membrane receptor HasR, mutants with six-residue deletions in the 11 putative extracellular loops were generated. Although all mutants continued to be active TonB-dependent heme transporters, mutations in three loops abolished hemophore HasA binding both in vivo and in vitro.