1. Introduction 100

2. Molecular architecture 107

2.1 Primary structure 108

2.1.1 Homologous regions 109

2.1.2 Chain typing 115

2.1.3 Post-translational modifications 117

2.2 Secondary structure 118

2.2.1 Central rod domain 118

2.2.2 Head and tail domains 119

2.3 Tertiary structure 123

2.3.1 Coiled-coil rod domain 123

2.3.1.1 Specificity through salt bridges 124

2.3.1.2 Specificity through apolar interactions 127

2.3.1.3 A consensus trigger sequence for two-stranded coiled-coils 128

2.3.2 Discontinuities in the rod domain 128

2.3.2.1 Links 129

2.3.2.2 Stutter 131

2.3.3 Head and tail domains 131

2.4 Electron microscope observations 133

3. Assembly 136

3.1 Role of the coiled-coil rod domain 137

3.2 Role of end domains 141

3.3 Experimentally induced crosslinks and modes of assembly 145

3.4 Naturally occurring crosslinks for tissue coordination 154

3.5 STEM data 154

4. Quaternary structure 160

4.1 Protofilaments and protofibrils 160

4.2 Head and tail domains 163

4.3 Surface lattice structure 164

4.4 Crystal studies on intermediate filament fragments 168

5. Polymorphism 169

5.1 Variations on a theme 170

5.1.1 Axial structure 170

5.1.2 Lateral structure 171

6. Keratin intermediate filament chains in diseases 172

7. Concluding remarks 175

8. Acknowledgments 176

9. References 176

Three types of intracellular filament have been identified in eukaryotic cells, and together they constitute the key elements of the cytoskeleton. They are the microtubules, the actin-containing microfilaments and the intermediate filaments. The uniqueness of the former two types of filament in cells has been well known for a long time but, in contrast, the intermediate filaments have been a relative new-comer to the scene. The microtubules and the microfilaments have always been easy to distinguish from one another on the grounds of their respective sizes (microtubules are about 25 nm in diameter and microfilaments are about 7–10 nm in diameter). Additionally, microtubules were capable of being disaggregated by the action of colchicine, and microfilaments could be disassembled by other drugs or be decorated with heavy meromyosin to generate arrowhead-like structures. Importantly, in both microtubules and microfilaments the constituent protein subunits were arranged to give the filaments a directionality, and the ability of these filaments to function in vivo depended crucially on this feature of their structure. Microtubules, for example, are involved in mitosis, motility and transport within the cell: each of these functions is clearly a ‘directional’ one. With this background the discovery and characterization of the intermediate filaments can begin.

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.