Effects of pore mutations and permeant ion concentration on the spontaneous gating activity of OmpC porin

Porins are trimers of beta-barrels that form channels for ions and other hy drophilic solutes in the outer membrane of Gram-negative bacteria. The X-ra y structures of OmpF and PhoE show that each monomeric pore is constricted by an extracellular loop that folds into the channel vestibule, a motif tha t is highly conserved among bacterial porins, Electrostatic calculations ha ve suggested that the distribution of ionizable groups at the constriction zone (or eyelet) may establish an intrinsic transverse electrostatic field across the pore, that is perpendicular to the pore axis. In order to study the role that electrostatic interactions between pore residues may have in porin function, we used spontaneous mutants and engineered site-directed mu tants that have an altered charge distribution at the eyelet and compared t heir electrophysiological behavior with that of wild-type OmpC, We found th at some mutations lead to changes in the spontaneous gating activity of Omp C porin channels. Changes in the concentration of permeant ions also altere d this activity. These results suggest that the ionic interactions that exi st between charged residues at the constriction zone of porin may play a ro le in the transitions between the channel's closed and open states.