Structure of the membrane-pore-forming fragment of colicin A

Abstract

Colicins are antibiotic proteins produced by and active against sensitive Escherichia coli and closely related bacteria. They can adsorb to specific receptors located at the external surface of the outer membrane of sensitive cells, and are then translocated to their specific targets within these cells. The largest group of colicins comprises those which can form voltage-dependent channels in membranes, thereby destroying the cell's energy potential. Colicin molecules are organized in structural domains, each domain carrying one function associated with the toxin's lethal activity. The pore-forming activity seems to be located at the carboxyl terminus. A thermolytic fragment comprising amino acids 389-592 from colicin A has pore-forming properties very similar to those of the entire molecule. This fragment is soluble in aqueous medium and spontaneously inserts into lipid bilayers. We have determined the structure of the pore-forming fragment of colicin A by X-ray crystallography and refinement at 2.5 A resolution. The protein consists of ten alpha-helices organized in a three-layer structure. Two of the helices are completely buried within the structure and form a hydrophobic hairpin loop similar to that proposed for signal sequences which function in translocation. We present a model for insertion of the protein into lipid bilayers the features of which may be applicable in other biological systems involving protein insertion or translocation across membranes.