A pore-forming protein with a metal-actuated switch.

Staphylococcal alpha-hemolysin, a pore-forming exotoxin, is a polypeptide of 293 amino acids that is secreted by Staphylococcus aureus as a water-soluble monomer. It assembles to form hexameric pores in lipid bilayers. Previous studies of pore formation have established the involvement of a central glycine-rich loop. Here, we show that when five consecutive histidine residues replace amino acids 130-134 at the midpoint of the loop, they provide a switch with which pore activity can be (i) turned off by micromolar concentrations of divalent zinc ions and (ii) turned back on with the chelating agent EDTA. Planar bilayer recordings show that Zn2+ and EDTA can act on open channels from either side of the bilayer and thus demonstrate that the central loop lines part of the conductive pathway. Our results suggest that genetically-engineered pore-forming proteins might make useful components of metal ion sensors.