SecA, an essential component of the general protein secretion pathwayof bacteria, is present in Escherichia coli as soluble andmembrane-integral forms. Hereweshow by electron microscopy thatSecA assumes two characteristic forms in the presence of phospholipidmonolayers: dumbbell-shaped elongated structures and ring-likepore structures. The ring-like pore structures with diameters of 8 nmand holes of 2nm are found only in the presence of anionic phospholipids.These ring-like pore structures with larger 3- to 6-nm holes(without staining) were also observed by atomic force microscopicexamination. They do not form in solution or in the presence ofuncharged phosphatidylcholine. These ring-like phospholipidinducedpore-structures may form the core of bacterial proteinconductingchannels through bacterial membranes.