ScienceWeek

CELL BIOLOGY: ON CRYSTAL STRUCTURES OF ION CHANNELS

The following points are made by Frances M. Ashcroft (Nature 2006 440:440):

1) The cell membrane is a major barrier to ion movement, and specific proteins -- the ion channels, transporters and pumps --have evolved to transport ions across it. Ion channels are gated pores that permit the passive flow of ions down their electrochemical gradients. Ion pumps, by contrast, use the energy of ATP hydrolysis to transport ions against their electrochemical gradient. In between are the coupled transporters (antiporters and symporters), in which movement of one ion species against its electrochemical gradient is powered by the downhill movement of another ion species.

2) Over 340 human genes are thought to encode ion channels. They have important roles in such diverse processes as nerve and muscle excitation, hormone secretion, cell proliferation, sensory transduction, learning and memory, regulation of blood pressure, salt and water balance, lymphocyte proliferation, fertilization and cell death[1]. Your ability to read and understand words depends on the activity of ion channels in your eye and brain. Consequently, defects in ion-channel function often have profound physiological effects.

3) Because of their important functional roles, their membrane location, structural heterogeneity and the restricted tissue expression of some channel types, ion channels are attractive targets for drug therapy. Indeed, many existing drugs, such as local anaesthetics[2], sedatives[3], anti-anxiety agents[3], antidiabetic drugs and even antiviral therapies[1,4], exert their effects by interacting with ion channels. Our understanding of how ion channels function has been illuminated by recent breakthroughs in high-resolution structure determination and by studies of diseases that result from impaired channel function (the channelopathies).

4) In summary: Ion channels are membrane proteins, found in virtually all cells, that are of crucial physiological importance. In the past decade, an explosion in the number of crystal structures of ion channels has led to a marked increase in our understanding of how ion channels open and close, and select between permeant ions. There has been a parallel advance in research on channelopathies (diseases resulting from impaired channel function), and mutations in over 60 ion-channel genes are now known to cause human disease. Characterization of their functional consequences has afforded unprecedented and unexpected insights into ion-channel mechanisms and physiological roles.

References (abridged):

1. Ashcroft, F. M. Ion Channels and Disease (Academic Press, New York, 2000)

2. Ulbricht, W. Sodium channel inactivation: molecular determinants and modulation. Physiol. Rev. 85, 1271 1301 (2005)

3. Johnston, G. A. GABA(A) receptor channel pharmacology. Curr. Pharm. Des. 11, 1867 1868 (2005)

4. Fischer, W. B. & Sansom, M. S. Viral ion channels: structure and function. Biochim. Biophys. Acta 1561, 27 45 (2002)

5. Doyle, D. A. et al. The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science 280, 69 77 (1998)

Nature http://www.nature.com/nature

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NEUROBIOLOGY: TRIGGERS AND THE OPENING OF ION CHANNELS

The following points are made by Cynthia Czajkowski (Nature 2005 438:167):

1) Chemical signalling in the brain involves the rapid opening and closing of channels known as ligand-gated ion channels, which lie in the membranes of nerve cells. Binding of a specific activator (a ligand) to these proteins triggers the opening of an integral pore through the membrane in as little as tens of microseconds[1]. Although we know a fair amount about the structure of ligand-gated ion channels, the mechanisms by which the binding of a ligand triggers channel opening are still under debate. New work[2] identifies a network of interacting amino-acid residues in one such protein, and reveals a pathway by which changes at the protein's ligand-binding site can be propagated to its channel region. Further new work[3] identifies a proline residue that acts as a molecular switch to control channel opening. Together, the two reports provide a compelling description of the structural machinery that couples ligand binding to channel gating.

2) Communication between nerve cells takes place at junctions called synapses. When a presynaptic cell is activated, it releases neurochemicals (neurotransmitters) across the synapse that bind to ligand-gated ion channels on the surface of the postsynaptic cell. Binding of neurotransmitter causes the channels to open, allowing ions to flood across the postsynaptic-cell membrane and change the cell's activity. So ligand-gated ion channels can be thought of as transducers that rapidly convert chemical signals into an electrical output. Their opening and closing regulate information flow throughout the brain, and mutations in these channels are responsible for a number of "channelopathies", such as congenital myasthenic syndromes, epileptic disorders, and hereditary hyperekplexia.

3) Lee and Sine[2] and Lummis et al[3] examined the structures of two members of the "Cys-loop" family of ligand-gated ion channels. This family includes channels that respond to the neurotransmitters acetylcholine, serotonin, gamma-aminobutyric acid (GABA), and glycine. The receptors are large transmembrane proteins (molecular weight 300,000) consisting of five similar subunits arranged around a central ion-conducting channel, with each subunit contributing to the lining of the transmembrane channel. The neurotransmitter binds to the extracellular interface between two subunits. But what has long puzzled researchers is how the binding of a neurotransmitter, which is around 6 angstroms long, is translated so rapidly into the opening of an ion channel more than 50 angstroms away in the transmembrane domain of the receptor.

4) Lee and Sine[2] set out to answer this question. They used the nicotinic acetylcholine receptor, whose structure was recently refined to 4-angstrom resolution[4], to identify receptor amino acids that could physically link the binding site to the channel. They then created a series of mutations, by substituting amino acids, to break these potential links, and analyzed the mutations' effects, both individually and in combinations, on channel activity. As a result, they identified a set of interacting residues that functionally and structurally link the binding site to the channel.

References (abridged):

1. Chakrapani, S. & Auerbach, A. Proc. Natl Acad. Sci. USA 102, 87-92 (2004)

2. Lee, W. Y. & Sine, S. M. Nature 438, 243-247 (2005)

3. Lummis, S. C. R. et al. Nature 438, 248-252 (2005)

4. Unwin, N. J. Mol. Biol. 346, 967-989 (2005)

5. Hu, X. Q., Zhang, L., Stewart, R. R. & Weight, R. R. Nature 421, 272-275 (2003)

Nature http://www.nature.com/nature

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NEUROBIOLOGY: ION CHANNELS AND MEMBRANE LIPIDS

The following points are made by Donald W. Hilgemann (Science 2004 304:223):

1) Ion channels spend their entire lives nestled in the lipid environment of the plasma membrane. Originally, it was thought that to operate reliably ion channels must insulate their moving parts (the "gates") from the effects of membrane lipids. The only exception to this rule seemed to be small peptide channels, such as gramicidin (1). But then, more and more ion channels turned out to be modulated by their lipid environments (2,3). The latest debate swirls around whether the voltage-sensor of voltage-gated potassium (Kv) channels can interact directly with membrane lipids (4,5).

2) The amino-terminal domain of Kv channels mediates channel inactivation by plugging the open pore from the cytoplasmic side of the plasma membrane in a ball-and-chain-like mechanism. The "ball" inactivation domain of Kv channels is positively charged and is hydrophobic. It swings freely on its tether, and can be attacked readily by proteases on the cytoplasmic side. The tether is long enough to reach the plasma membrane, and its ability to bind to negatively charged phospholipids has been characterized. Hence, there is no evident reason why the inactivation domain should not interact with anionic phospholipids on the cytoplasmic side of the plasma membrane.

3) Oliver et al (Science 2004 304:265) have demonstrated that when bound by the polyanionic phospholipid PIP2 (phosphatidylinositol 4,5-bisphosphate), the Kv amino-terminal domain can no longer plug the open Kv channel and so inactivation of the channel is relieved. This result with the Kv channel in which the ball and chain are part of the subunit, is similar to findings with other types of ion channel in which the ball and chain are part of the pore-forming subunit.

4) Oliver et al have demonstrated profound effects of another lipid involved in signal transduction, the lipid arachidonic acid (AA). AA appears to reverse the effects of PIP2 on Kv channel fast inactivation, and it can convert slowly inactivated Kv channels to rapidly inactivated channels. Antagonism between PIP2 and AA has been described for other types of K+ channels. However, Oliver et al argue that AA is acting at a site completely distinct from that of PIP2 activity. Regardless of the details, Kv channel function must now be considered a possible termination point for multiple lipid signaling pathways. This role might explain why different structures are involved in the fast N-type inactivation (ball-and-chain) of Kv channels and the fast inactivation of Na+ channels, which are not sensitive to anionic lipids.

References (abridged):

1. T. C. Hwang et al., Biochemistry 42, 13646 (2003)

2. D. W. Hilgemann et al., Sci. STKE 2001, re19 (2001)

3. R. C. Hardie, Annu. Rev. Physiol. 65, 735 (2003)

4. Y. Jiang et al., Nature 423, 33 (2003)

5. D. M. Starace et al., Nature 427, 548 (2004)

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