Channel-forming colicins: translocation (and other deviant behaviour) associated with colicin Ia channel gating.

Abstract

1. Introduction 1892. Channel properties 1912.1 Voltage-dependent gating 1912.2 Ion permeability 1932.2.1 Selectivity between potassium and chloride 1932.2.2 Permeability to large cations and large anions 1932.3 Single-channel characteristics 1942.4 Molecularity of the channel 1953. Colicin Ia channel topology and protein translocation 1953.1 Channels formed by whole colicin Ia 1953.1.1 General channel topology 1963.1.2 The translocated region 1993.1.3 The nonuniqueness of the upstream membrane-inserted segment 1993.2 Channels formed by the C-terminal domain of colicin Ia 2004. Concluding remarks 2025. Acknowledgement 2036. References 203Colicins are plasmid-encoded proteins, produced by some strains of E. coli, that kill other strains lacking the specific immunity protein encoded by the same plasmid. Most of the colicins have a three-domain structure: a central domain that binds to a receptor in the outer membrane of the target cell; an N-terminal domain that interacts with target cell proteins to move the C-terminal domain across the outer membrane and periplasmic space to the inner membrane; and a C-terminal domain that carries the toxic activity. In some colicins the C-terminal domain is an enzyme that kills the cell by entering the cytoplasm and attacking its DNA (e.g. colicin E2), its ribosomal RNA (e.g. colicin E3), or another target (Schaller et al. 1982; Ogawa et al. 1999). In other colicins, the C-terminal domain forms an ion-conducting channel in the inner membrane that ultimately leads to cell death by allowing essential solutes to leak out of the cell. These colicins, or their isolated C-terminal domains, can also form voltage-dependent channels in planar phospholipid bilayers. (For a review of the E colicins, including enzymatic colicins, see James et al. 1996; for a review of channel-forming colicins, see Cramer et al. 1995; and for a review of colicin import into E. coli, see Lazdunski et al. 1998.) The channel-forming colicins are the subject of this review, with particular emphasis on one member of this group, colicin Ia, and the protein translocation associated with the gating of its channel.