VOLTAGE SENSING IN THE PHOE AND OMPF OUTER-MEMBRANE PORINS OF ESCHERICHIA-COLI - ROLE OF CHARGED RESIDUES

The porins PhoE and OmpF form anion and cation-selective pores, respec tively. In the outer membrane of Escherichia coli. Each monomer of the se trimeric proteins consists of a 16-stranded beta-barrel, which cont ains a constriction at half the height of the channel. The functional significance of a transverse electrical field that is formed by charge d amino acid residues within the constriction zone was investigated. F or this purpose, the PhoE residues R37, R75, K18 and E110 were substit uted by neutral amino acids. The mutant pores allowed an increased per meation of beta-lactam antibiotics across the outer membrane in vivo, although the single channel conductance, measured in planar lipid bila yers, was not increased or even slightly decreased. Replacement of the positively charged residues resulted in a decreased voltage sensitivi ty, whereas the substitution of a negatively charged residue resulted in an increased voltage sensitivity. Similar substitutions in OmpF cau sed the opposite effects, i.e. the substitution of positive and negati ve charges resulted in increased and decreased voltage sensitivity, re spectively. Together, the results suggest that opposite charges, i.e. positive charges in anion-selective and negative charges in cation-sel ective porins, act as sensors for voltage gating. (C) 1997 Academic Pr ess Limited.