PERMEABILITY PROPERTIES OF A LARGE GATED CHANNEL WITHIN THE FERRIC ENTEROBACTIN RECEPTOR, FEPA

FepA is an Escherichia coli outer membrane receptor protein for the si derophore ferric enterobactin. Prior studies conducted in vivo suggest ed that FepA and other TonB-dependent outer membrane proteins transpor t ligands by a gated-channel mechanism. To corroborate and extend thes e findings we have determined the permeability properties of the FepA channel in vitro, by measuring the diffusion rates of hydrophilic none lectrolytes through the FepA channel in liposome swelling experiments. Like porins, the FepA deletion mutant DELTARV showed a size-dependent permeability to oligosaccharides, indicating that it forms a nonspeci fic, hydrophilic pore. Unlike OmpF and other E. coli porins, however, DELTARV proteoliposomes transported stachyose (666 Da) and ferrichrome (740 Da). These data, and other uptake results with a series of malto dextrins of increasing size, confirm the existence of a channel domain within FepA that is considerably larger than OmpF-type pores. These r esults represent a reconstitution of the channel function of a TonB-de pendent receptor protein and establish that FepA contains the largest channel that has been characterized in the E. coli outer membrane.