COMPLEX INHIBITION OF OMPF AND OMPC BACTERIAL PORINS BY POLYAMINES

The effects of four polyamines (putrescine, cadaverine, spermidine, an d spermine) on the activity of bacterial porins OmpC and OmpF were inv estigated by electrophysiology. Membrane vesicles made from the outer membrane of Escherichia coli strains expressing only OmpC or OmpF were reconstituted into liposomes probed by patch clamp. The channel activ ity was recorded in control solutions and in the presence of increasin g concentrations of a specific polyamine. In all cases, concentration- and voltage-dependent inhibitory effects were observed. They include both the suppression of channel openings and the enhancement of channe l closures as well as the promotion of blocked or inactivated states, OmpF and OmpC, although highly homologous, have distinct sensitivities to modulation, especially by spermine. This compound inhibits OmpF in the nanomolar range, which is in agreement with its potency on eukary otic channels. Putrescine was the least effective (upper millimolar ra nge) and also had inhibitory effects qualitatively distinct from those exerted by the other polyamines. The compounds appear to bind to at l east two distinct binding sites, one of which resides within the pore. The potencies to this site are lower when the polyamines are applied from the extracellular side than from the periplasmic side, suggesting an asymmetric binding site.