Ion transport features of a biological nanopore, the bacterial porin OmpF from Escherichia coli, have been investigated by patch-clamp experiments performed at the single channel level. Membrane potential measurements done under asymmetric conditions of pH and electrolyte concentration provide important evidences about the charge regulation exerted by the channel that cannot be extracted from the rectification displayed in current-voltage curves. The pH gradient imposed across the pore induces an asymmetric fixed-charge distribution that resembles the structure of synthetic bipolar membranes. This particular arrangement demonstrates that the ionic selectivity,of a non-uniformly charged Structure is not an intrinsic quality of the system but depends crucially on several external factors. Amazingly, changing the direction of the salt concentration gradient can turn a cation selective channel into an anion selective one. (C) 2009 Elsevier B.V. All rights reserved.