Effects of Divalent Cations on the Single-Channel Conductance of the OmpF Channel: Linearity, Saturation and Blocking

Abstract

Previous studies on the bacterial porin OmpF (Biophys. J. 96 (2009) 55-66) have shown that the channel moderate cationic selectivity of OmpF in NaCl and KCl solutions is reversed to anionic selectivity in solutions of CaCl2 and MgCl2. We study here the effect of salts of divalent cations on the channel conductance with a particular emphasis in dissecting the role of the electrolyte, the role of the counterion accumulation induced by the protein channel charges and any other effect not found in salts of monovalent cations. Single-channel conductance measurements are performed over a wide range of salt concentrations (up to 3 M).We find that the change of channel conductance with salt activity in bulk solution exhibits different features in salts of monovalent cations and in salts of divalent cations. In order to separate channel and electrolyte effects we analyse the correlation between channel conductance and bulk solution conductivity. While one scales with the other in solutions of NaCl and KCl over the whole concentration range studied, the conductance for CaCl2 and MgCl2 has two regimes. At salt concentrations below 1 M we found the same pattern as in solutions of NaCl and KCl. However, for higher concentrations such linearity between conductivity and conductance vanishes. Moreover, surprisingly, at high concentrations of MgCl2 and CaCl2 the conductance decreases as conductivity increases. Once, one accounts for the big variation of activity coefficients of divalent salts at high concentrations, the experimental results suggest that at low concentrations the pore conductance is controlled mainly by the electrolyte properties. The current recordings in salts of divalent cations reveal the existence of substates of lower conductance as one of the causes of the conductance decrease in the high concentration regime.