In vitro approaches to investigation of the early steps of colicin-OmpF interaction.

The OmpF porin plays a central role during the colicin uptake by sensitive Escherichia coli cells. Lipopolysaccharide-OmpF complexes (-1bLPS-OmpF), which contain one tightly bound and no loosely bound LPS molecules for each porin trimer, is able to recognize and bind to immobilized colicins. This association is specific to colicins A and N, which both use the OmpF porin as receptor, and depends on the presence of the porin-receptor domain on the bacteriocin molecule. The -1bLPS-OmpF complex protects sensitive cells against colicin A and N activity. The protection level depends on the native conformation, as demonstrated by heat denaturation of the trimeric porin which abolishes the protection. This indicates that the purified OmpF trimer presents an affinity site for the colicin which efficiently mimics the native cellular receptor site. These results are discussed with regard to the conformation of the receptor site and to the early steps of colicin uptake.