Molecular Mechanism of Action of Pore-Forming Colicins and of their Immunity Proteins
Résumé
Pore-forming colicins constitute a large family of homologous bacteriocins that are of interest for a variety of problems related to membranes. Topics such as toxin action, polypeptide translocation across and into membranes, voltage-gated channels, receptor function, protein-protein interactions in membranes will benefit from the simple model system provided by the colicins. Mostly colicins A and El have been studied, however, only the three-dimensional structure of the pore-forming C-terminal domain of colicin A has been solved at 2.4 Å resolution. This domain can exist both in a water soluble as well as in a membrane form. Colicins bind to a receptor in the outer membrane and are then translocated across the cell envelope to the inner membrane. Import of colicins involves a cascade of steps which use specific proteins in the outer and inner membranes that are probably required for the acquisition and stabilization of a translocation-competent intermediate state. A membrane potential-dependent movement of a large region of the channel peptide into the membrane bilayer is required for channel opening. Pore-forming colicins span the whole cell envelope even after channel opening. Thus, they probably remain at putative translocation contact sites between the outer and inner membranes. Site-directed mutagenesis studies are now used to probe models concerning the dynamics of conformational changes of the C-terminal domain upon pore-formation. The protective action of immunity proteins against pore-forming colicins is a challenging problem. This action is specifically directed against the C-terminal domain of colicins. Studies using homologous recombination between the genes for colicins A and B provided hybrid colicins which allowed us to demonstrate that the specificity determinant in the pore-forming domain is constituted by the two hydrophobic helices of the structure. Despite extensive studies on the topography and identification of critical regions in the immunity polypeptide which spans the inner membrane four times, it is not yet known whether the formation of the channel or its opening is prevented by the immunity protein.