LIGAND-SPECIFIC OPENING OF A GATED-PORIN CHANNEL IN THE OUTER-MEMBRANE OF LIVING BACTERIA

Ligand-gated membrane channels selectively facilitate the entry of iro n into prokaryotic cells. The essential role of iron in metabolism mak es its acquisition a determinant of bacterial pathogenesis and a targe t for therapeutic strategies. In Gram-negative bacteria, TonB-dependen t outer membrane proteins form energized, gated pores that bind iron c helates (siderophores) and internalize them. The time-resolved operati on of the Escherichia coli ferric enterobactin receptor FepA was obser ved in vivo with electron spin resonance spectroscopy by monitoring th e mobility of covalently bound nitroxide spin labels. A ligand-binding surface loop of FepA, which normally closes its transmembrane channel , exhibited energy-dependent structural changes during iron and toxin (colicin) transport. These changes were not merely associated with lig and binding, but occurred during ligand uptake through the outer membr ane bilayer. The results demonstrate by a physical method that gated-p orin channels open and close during membrane transport in vivo.