Modulation of Vibrio cholerae porin function by acidic pH

The outer membrane of Gram-negative bacteria contains porins, large pore-forming proteins which allow the traffic of hydrophilic compounds between the external medium and the periplasm. The oral mode of infection of Vibrio cholerae, the agent of cholera, implies that the bacteria must adapt to severe changes in the environment, such as acidic pH and the presence of bile. Because of their localization and the regulation of their expression in response to these external factors, the OmpU and OmpT porins of V. cholerae are thought to be involved in the adaptation of the bacteria to the host environment. Using patch clamp and planar lipid bilayer electrophysiology, we assessed the effect of pH on the channel properties of OmpU and OmpT. OmpT does not show any major modification in its activity between pH 4 and pH 7.2. In the case of OmpU, the effect of acidic pH is manifested by promoting single-step closures, whose duration, frequency and current size increase as pH is lowered, thereby producing a pH-dependent decrease in the channel open probability. Surprisingly, the increase in current size of this single-step closure is not coupled with an increase of the total current through the porin, indicating that the trimeric conductance remains unchanged. This observation suggests that coordinated events take place at the level of the trimer, and various explanations for this peculiar effect of acidic pH on porin gating and conductance are provided.