Unlocking family secrets: K+ channel transmembrane domains.

The selectivity of ion channels is not enough to allow organisms to move, sense, and think. Evolutionary tinkering has attached various valves and levers onto the channel in order that voltage changes, various molecules and ions, and time can all turn on and off the flow of ions. Although there are various cytoplasmic domains whose secrets will soon be revealed by crystallography (some functions, such as intersubunit and accessory protein binding, already have; e.g.5xDoyle, D.A, Lee, A, Lewis, J, Kim, E, Sheng, M, and MacKinnon, R. Cell. 1996; 85: 1067–1076Abstract | Full Text | Full Text PDF | PubMed | Scopus (815)See all References, 9xKreusch, A, Pfaffinger, P.J, Stevens, C.F, and Choe, S. Nature. 1998; 392: 945–948Crossref | PubMed | Scopus (240)See all References, 15xMorais, J.H, Lee, A, Cohen, S.L, Chait, B.T, Li, M, and MacKinnon, R. Cell. 1998; 95: 649–655Abstract | Full Text | Full Text PDF | PubMedSee all References), little is known about the transmembrane structural components that control gating. In the case of the pH-gated bacterial K+ channel, proton binding at the extracellular face of KcsA induces a large conformational change in the C-terminal end of TM2, increasing the diameter of the internal vestibule with channel opening (Perozo et al. 1998xPerozo, E, Cortes, D.M, and Cuello, L.G. Nat. Struct. Biol. 1998; 5: 459–469Crossref | PubMed | Scopus (248)See all ReferencesPerozo et al. 1998). In KcsA, the four M2 helices cross in a bundle, like the top of a teepee, leaving a small aperture at the bundle crossing. This crossing site may be the channel gate, but of course one crystal structure is static and alone cannot answer the gating question. In fact, KcsA was crystallized at pH 7.5, most likely in its closed gating conformation. In recent experiments Holmgren et al. 1998xHolmgren, M, Shin, K.S, and Yellen, G. Neuron. 1998; 21: 617–621Abstract | Full Text | Full Text PDF | PubMed | Scopus (156)See all ReferencesHolmgren et al. 1998 substituted cysteine into the Shaker K+ channel S6 to create high-affinity Cd2+-binding sites on the intracellular end of the helical bundle. The metal formed a bridge between the engineered cysteine in one S6 subunit and a native histidine in an adjacent subunit, trapping the channel in an open state. Holmgren et al. suggest that gating occurs at this bundle crossing by slightly twisting (or perhaps scissoring) the helices to expand the ring of the pore defined by the intersection of the S6 helices (see Yellen 1998xYellen, G. Quart. Rev. Biophys. 1998; 31: 239–296Crossref | PubMed | Scopus (342)See all ReferencesYellen 1998).For the simple Kir channels, gating is primitive and in part controlled by Mg2+ (Logothetis et al. 1987xLogothetis, D.E, Kurachi, Y, Galper, J, Neer, E.J, and Clapham, D.E. Nature. 1987; 325: 321–326Crossref | PubMed | Scopus (408)See all ReferencesLogothetis et al. 1987) or polyamine block (Lopatin et al. 1994xLopatin, A, Makhina, E, and Nichols, C. Nature. 1994; 372: 366–369Crossref | PubMed | Scopus (618)See all ReferencesLopatin et al. 1994) of the cytoplasmic mouth of the pore. In keeping with the proposed slightly different structural classification for inward rectifier K+ channels, indirect structural data from Lu et al. 1999xLu, L, Nguyen, B, Zhang, X, and Jian, Y. Neuron. 1999; 22: 571–580Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesLu et al. 1999 suggest that the cytoplasmic opening of Kir 2.1 channels is 10 A in diameter, widening to more than 12 A in the center of the membrane. At a position known to be important for inward rectification, the pore may be sufficiently wide to bind three Mg2+ ions or polyamine molecules simultaneously. For another large pore channel, the nonselective, mechanically gated bacterial channel (MscL), crystallography reveals a pentameric structure surrounding a large water-filled opening that narrows through a hydrophilic pore to an occluded hydrophobic gate (Chang et al. 1998xChang, G, Spencer, R.H, Lee, A.T, Barclay, M.T, and Rees, D.C. Science. 1998; 282: 2220–2226Crossref | PubMed | Scopus (717)See all ReferencesChang et al. 1998). For the ligand-gated, nonselective, pentameric nicotinic channels, Unwin has provided structural data at 9 A resolution on the putative open and closed conformations (Unwin 1995xUnwin, N. Nature. 1995; 373: 37–43Crossref | PubMedSee all ReferencesUnwin 1995). Binding of acetylcholine initiates small rotations of the protein subunits that trigger a change in configuration of α helices lining the pore (Unwin 1998xUnwin, N. J. Struct. Biol. 1998; 121: 181–190Crossref | PubMed | Scopus (73)See all ReferencesUnwin 1998), but higher resolution is needed to understand the most interesting aspects of the nicotinic receptor channel gating.The next hurdle in the field of voltage-gated channels will be to determine how movements of S4 are translated into twisting and opening of the S6 helical bundle. Recent creative fluorescent molecule tagging of residues (19xSiegel, M.S and Isacoff, E.Y. Neuron. 1997; 19: 735–741Abstract | Full Text | Full Text PDF | PubMed | Scopus (237)See all References, 2xCha, A and Bezanilla, F. J. Gen. Physiol. 1998; 112: 391–408Crossref | PubMed | Scopus (104)See all References), fluorescence resonance energy transfer, and simultaneous gating charge movement measurements will certainly be brought to bear. Alternatively, this second central problem of ion channels may be solved in one fell swoop by crystallography of voltage-gated ion channels trapped in open and in closed states.