Components of E. Coli Energy-Transducing Complexes, ExbB and TolQ, Display Ion Channels

Abstract

ExbBD/TonB and TolQR/TolA complexes of the cytoplasmic membrane transduce energy stored in the electrochemical proton gradient to drive cellular import of siderophores or maintain outer membrane barrier function. Both complexes are utilized for cellular import of colicins. It is unknown how they exert energy transducing and import functions.We explore the idea that energy-transduction by ExbBD/TonB and TolQRA is coupled to ion- translocation through ion channels formed by transmembrane helices of ExbB and TolQ. Plasmid-expressed ExbB and TolQ were extracted with detergent from membranes, purified, and reconstituted into liposomes. For channel measurements proteoliposomes were fused to planar lipid membranes. Changes in protein tertiary structure upon membrane reconstitution were detected by thermal melting of alpha-helices using far-UV circular dichroism. ExbB reconstituted into liposomes, in contrast to its behavior in detergent, melted cooperatively, implying inter-helix interactions. ExbB and TolQ displayed cation-selective ion channels of small conductance (Figure). Divalent cations decreased channel conductance. Channel formation was more prominent at pH<6. It is proposed that transmembrane ion current through ExbB and TolQ channels is transduced into conformational changes of periplasmic domains of the membrane-anchored TonB and TolA components of the complexes.View Large Image | View Hi-Res Image | Download PowerPoint Slide