J. Liu et al., PERMEABILITY PROPERTIES OF A LARGE GATED CHANNEL WITHIN THE FERRIC ENTEROBACTIN RECEPTOR, FEPA, Proceedings of the National Academy of Sciences of the United Statesof America, 90(22), 1993, pp. 10653-10657
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.