The Vibrio cholerae porins OmpU and OmpT have distinct channel properties

Numerous environmental signals regulate the production of virulence factors and the composition of the outer membrane of Vibrio cholerae. In particular, bile promotes the ToxR-dependent expression of the porin OmpU. Strains expressing solely OmpU are more resistant to bile, are better able to colonize the intestine, and produce more cholera toxin than strains expressing solely the OmpT porin. To gain some understanding in the physiological relevance and the molecular mechanism underlying these porin-dependent phenotypes, we have undertaken a thorough electrophysiological characterization of the channel properties of the two porins. Purified OmpU or OmpT was reconstituted in liposomes suitable for patch clamp and in planar lipid bilayers. The high resolution of the patch clamp technique allowed us to analyze in detail the behavior of single OmpU and OmpT channels. Both channels exhibit closing transitions to various conductance states. OmpT is a much more dynamic channel than OmpU, displaying frequent and prolonged closures, even at low transmembrane potentials. With a critical voltage for closure V(c) of approximately +/-90 mV, OmpT is much more voltage-sensitive than OmpU (with a V(c) of approximately +/-160 mV), a feature that is also readily apparent in the voltage dependence of closing probability observed in patch clamp in the +/-100 mV range. OmpT has low ionic selectivity (P(K)/P(Cl) = approximately 4), whereas OmpU is more cation-selective (P(K)/P(Cl) = approximately 14). The distinct functional properties of the two porins are likely to play an integrated role with environmental regulation of their expression. For example, the higher selectivity of OmpU for cations provides a possible explanation for the protective role played by this porin in a bile-containing environment, because this type of selectivity would restrict the flux of anionic bile salts through the outer membrane and thus would reduce the exposure of the cytoplasmic membrane to this natural detergent.