Porins of Escherichia coli: unidirectional gating by pressure.

OmpC and PhoE porins of Escherichia coli were examined by the patch-clamp technique following reconstitution in liposomes, and were observed primarily in the open (conducting) state. With application of negative voltage and positive hydrostatic pressure, OmpC exhibited marked gating towards a more closed state whereas PhoE remained largely unaffected by pressure application. Hybrid chimeric OmpC-PhoE proteins showed an increased tendency for pressure-dependent gating as the OmpC proportion in the chimeric molecule increased. In addition, several PhoE mutants with amino acid substitutions and insertions in either the L3 or L4 loop of the monomer exhibited pressure sensitivity comparable with the wild-type OmpC porin. Our data support the structural plasticity model of porins and are consistent with the 'charge-screening-unscreening' hypothesis that describes how these proteins may exist in distinct conformations.