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Colicin import and pore formation: a system for studying protein transport across membranes?

Pore‐forming colicins are a family of protein toxins (Mr40–70kDa) produced by Escherichia coli and related bacteria. They are bactericidal by virtue of their ability to form ion channels in the inner membrane of target cells. They provide a useful means of studying questions such as toxin action, polypeptide translocation across and into membranes, voltage‐gated channels and receptor function. These colicins bind to a receptor in the outer membrane before being translocated across the cell envelope with the aid of helper proteins that belong to nutrient‐uptake systems and the so‐called‘Tol’proteins, the function of which has not yet been properly defined. A distinct domain appears to be associated with each of three steps (receptor binding, translocation and formation of voltage‐gated channels). The Tol‐dependent uptake pathway is described here. The structures and interactions of TolA, B, Q and R have by now been quite clearly defined. Transmembrane α‐helix interactions are required for the functional assembly of the E. coli Tol complex, which is preferentially located at contact sites between the inner and outer membranes. The number of colicin translocation sites is about 1000 per cell. The role and the involvement of the OmpF porin (with colicins A and N) have been described in a recent study on the structural and functional interactions of a colicin‐resistant mutant of OmpF. The X‐ray crystal structure of the channel‐forming fragment of colicin A and that of the entire colicin la have provided the basis for biophysical and site‐directed muta‐genesis studies. Thanks to this powerful combination, it has been established that the interaction with the receptor in the outer membrane leads to a very substantial conformational change, as a result of which the N‐terminal domains of colicins interact with the lumen of the OmpF pore and then with the C‐terminal domain of TolA. A molten globular conformation of colicins probably constitutes the intermediate translocation/insertion competent state. Once the pore has formed, the polypeptide chain spans the whole cell envelope. Three distinct steps occur in the last stage of the process: (i) fast binding of the C‐terminal domain to the outer face of the cytoplasmic membrane; (ii) a slow insertion of the polypeptide chain into the outer face of the inner membrane in the absence of Δψ and (iii) a profound reorganization of the helix association, triggered by the transmembrane potential and resulting in the formation of the colicin channel.