Voltage gating of porins from Haemophilus influenzae type b

The major outer membrane protein of Haemophilus influenzae type b (Hib) is porin (M(r) 37,782; 341 amino acids). Porins were purified from Hib strains representative of the three outer membrane protein subtypes 1H, 2L and 6U, reconstituted into artificial planar bilayers, and tested for their voltage dependency. At membrane potentials of 50-80 mV, individual Hib 2L and 6U porin channels showed a high probability of undergoing a reversible change to one of several lower conducting substates. Such behaviour was not observed for Hib 1H porin with transmembrane potentials up to 80 mV. The voltage dependence of Hib 2L and 6U porins was asymmetric: it occurred at only one polarity. The asymmetry was also observed for membranes with numerous porins incorporated, suggesting that Hib porin inserted asymmetrically into the bilayer. At macroscopic levels the voltage gating reduced the conductance by 25-50%, implying that the channels closed only partially. Hib 2L porin differs from Hib 1H porin by the substitution Arg166Gln and Hib 6U porin differs from Hib 1H porin by substitutions at ten amino acids including the change Arg166Leu. We conclude that substitutions at Arg166 residue, which is localized to surface-exposed loop number four, are associated with a lowered threshold potential for the voltage gating of Hib porin. This surface-exposed loop may play some role in the conformational changes that occur during voltage gating.