Asymmetry of orientation and voltage gating of the Acidovorax delafieldii porin Omp34 in lipid bilayers

Abstract

The functional properties of the major outer-membrane protein of Acidovorax delafieldii, the anion-selective porin Omp34, were investigated in artificial membranes. Detergent-solubilized porin incorporates into the membrane in a undirectional orientation solely determined by protein features. This enabled us to characterize the vectorial properties of the porin channels. Omp34 is electrostatically asymmetric regarding both the ion conductivity of a single trimer and the macroscopic ion conductance of multiple porin molecules. Voltage-dependent closing occurred at negative potentials; 50% of the channels were already closed at -10 mV (switching voltage). Our experimental results suggest that protein charges situated on flexible parts inside the channel are involved in the gating mechanism. A simple model is proposed illustrating the mechanism of voltage-dependent opening and closing of the porin channels. This model explains the functional characteristics of Omp34 and the dependence of the switching voltage on the electrolyte concentration in particular. Further factors influencing voltage gating include the buffer concentration as well as the technique used for membrane formation. Altogether these factors may explain the relatively high voltages needed to obtain voltage gating with other porins.