1. Introduction 189

2. Channel properties 191

2.1 Voltage-dependent gating 191

2.2 Ion permeability 193

2.2.1 Selectivity between potassium and chloride 193

2.2.2 Permeability to large cations and large anions 193

2.3 Single-channel characteristics 194

2.4 Molecularity of the channel 195

3. Colicin Ia channel topology and protein translocation 195

3.1 Channels formed by whole colicin Ia 195

3.1.1 General channel topology 196

3.1.2 The translocated region 199

3.1.3 The nonuniqueness of the upstream membrane-inserted segment 199

3.2 Channels formed by the C-terminal domain of colicin Ia 200

4. Concluding remarks 202

5. Acknowledgement 203

6. References 203

Colicins 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.