The membrane channel-forming colicin A: synthesis, secretion, structure, action and immunity

The study of colicin release from producing cells has revealed a novel mechanism of secretion. Instead of a built-in 'tag', such as a signal peptide containing information for secretion, the mechanism employs coordinate expression of a small protein which causes an increase in the envelope permeability, resulting in the release of the colicin as well as other proteins. On the other hand, the mechanism of entry of colicins into sensitive cells involves the same three stages of protein translocation that have been demonstrated for various cellular organelles. They first interact with receptors located at the surface of the outer membrane and are then transferred across the cell envelope in a process that requires energy and depends upon accessory proteins (TolA, TolB, TolC, TolQ, TolR) which might play a role similar to that of the secretory apparatus of eukaryotic and prokaryotic cells. At this point, the type of colicin described in this review interacts specifically with the inner membrane to form an ion channel. The pore-forming colicins are isolated as soluble proteins and yet insert spontaneously into lipid bilayers. The three-dimensional structures of some of these colicins should soon become available and site-directed mutagenesis studies have now provided a large number of modified polypeptides. Their use in model systems, particularly those in which the role of transmembrane potential can be tested for polypeptide insertion and ionic channel gating, constitutes a powerful handle with which to improve our understanding of the dynamics of protein insertion into and across membranes and the molecular basis of membrane excitability. In addition, their immunity proteins, which exist only in one state (membrane-inserted) will also contribute to such an understanding.