|
ScienceWeek
CELL BIOLOGY: ACTIVE ION TRANSPORT VS. PASSIVE ION CHANNELS
The following points are made by Louis J. Defelice (Nature 2004 432:279):
1) To function properly, living cells must maintain ions and small molecules at concentrations that are far different from those in their local environments. The electrochemical gradients formed in this way can be used by cells as a source of energy to drive other processes, for example membrane transport or the generation of electrical signals in specialized cells -- such as those in nerves and muscle. Internal sodium ions are kept at roughly one-tenth of their external concentration, and the reverse is true for potassium ions. These ion gradients are maintained by a transmembrane protein (the sodium-potassium pump, an enzymic transporter), which actively pumps out sodium ions and pumps in potassium ions. Then, as an electrical signal (action potential) propagates along a nerve cell, sodium and potassium channels open, allowing the rapid flow of sodium ions into and potassium ions out of the cell. From many such examples, the concept emerges that transporters generate ionic gradients and channels dissipate them: transporters and channels seem as different as oil and water.
2) But this distinction depends on what we classify as "transporter". For enzymatic transporters, the difference seems clear -- they use chemical energy to generate gradients. But for another class of transporters, known as co-transporters, the distinction is more difficult. Co-transporters can also create gradients, but they do so by using the energy stored in ionic gradients established by pumps -- and are thus secondarily active. For example, co-transporters would use the sodium gradient set up by the sodium pump to power the transport of another substrate, such as a neurotransmitter, against its own gradient. However, researchers tend to model co-transporters as if they were enzymes, even though they have authentic channel properties. What are the origins of this riddle and how can it be resolved?
3) Channels and transporters did not originate as separate entities. Alan Hodgkin (1914-1998) and colleagues unravelled the ionic basis of action potentials in the 1940s. But to explain sodium and potassium permeability they initially postulated transporters, not channels -- ions were carried across the membrane rather than moving through a pore. Soon afterwards, prominently in the work of Hodgkin and Richard Keynes, the notion of electrodiffusion through narrow pores (channels) emerged. Concurrently, researchers struggled with electrodiffusion as a means to concentrate metabolites "against the gradient". Wilfred F. Widdas, in particular, introduced the harbinger of co-transport, in which the gradient of one species drives the transport of another. The adoption of enzymatic theory and carrier kinetics for co-transporters soon followed, and two camps evolved: electrodiffusion theory governed channels, and enzyme theory governed transporters. Different methodologies contributed to this separation, as channel biophysicists relied mainly on electrical measurements, whereas transport physiologists preferred radiolabelled uptake experiments. Indeed, early attempts at an electrical description of active transport were disappointing because of the comparatively low signal.
4) As a result, the study of transporters (including co-transporters) and the study of channels grew apart. But in the past decade the cloning of co-transporters, combined with the measurement of tiny currents through individual channel and transporter proteins by high-resolution electrophysiology (patch clamp) has begun to bridge this historical gap. Almost every co-transporter studied in this way exhibits ion channel properties. Glutamate and dopamine co-transporters harbor chloride-selective channels. GABA, serotonin and norepinephrine co-transporters contain sodium and lithium channels. Recently, a presumedchloride channel has been shown to be a co-transporter, but a simple mutation returns it to pure chloride selectivity. In another case, an iron co-transporter naturally mutates into a calcium channel. We may expect many other such examples as the application of ion channel techniques are applied to transporters. Even the sodium-potassium pump has been shown to have ion-channel properties under special conditions.(1-3)
References:
1. DeFelice, L.J. Trends Neurosci. 27, 352-359 (2004)
2. Hodgkin, A.L. Chance and Design (Cambridge Univ. Press, 1992)
3. Stein, W.D. Channels, Carriers, and Pumps (Academic Press, New York, 1990)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
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)
Science http://www.sciencemag.org
--------------------------------
Related Material:
CELL BIOLOGY: ION CHANNELS AND CALMODULIN
The following points are made by Irwin B. Levitan (Science 2004 304:394):
1) Ion channels are composed of intrinsic membrane proteins that permit the regulated flow of ions across the lipid bilayer of the plasma membrane (and of intracellular membranes). They are essential for a wide range of biological processes, including the electrical excitability of neurons, muscle contraction, and cellular secretion (1). The activity of many ion channels is calcium dependent -- that is, ion flow through these channels is regulated by the concentration of intracellular free calcium ions. In many cases, calcium regulation of ion-channel activity is mediated by the ubiquitous calcium sensor protein, calmodulin (2,3).
2) Plasma membrane voltage-dependent calcium channels, as their name implies, selectively allow the flow of calcium ions down an electrochemical gradient, from a high concentration outside the cell to a low concentration inside the cell. Many genes encode calcium channels, and the different gene products exhibit different functional properties. The "L-type" calcium channel (CaV1.2) is regulated by intracellular calcium ions (5) through their attachment to calmodulin. Calmodulin is tethered constitutively to the intracellular carboxyl-terminal tail domain of this channel in a calcium-independent manner. When the channel is opened by membrane depolarization, calcium ions rush in through the open channel and bind to the tethered calmodulin, altering its conformation and thereby causing channel inactivation and feedback inhibition of calcium entry. A similar picture is seen with one kind of calcium-dependent, potassium-selective ion channel, although in this case the binding of calcium to the constitutively tethered calmodulin results in channel activation rather than inactivation. The calcium ions that enter through L-type channels also bind to free calmodulin and cause it to move to the nucleus, where it influences gene expression.
3) Mori et al (4) constructed a series of chimeric proteins in which calmodulin is fused via a polyglycine linker to the carboxyl terminus of the 1C subunit that encodes the L-type calcium channel. They then used these chimeras to investigate two key questions. The first concerns functional stoichiometry -- how many calmodulin molecules are required to inactivate the L-type calcium channel? By the clever use of a chimera with mutant calmodulin that cannot bind to calcium ions, and therefore cannot mediate calcium-dependent channel inactivation, Mori et al (4) demonstrate that a single tethered calmodulin molecule is both necessary and sufficient to produce calcium-dependent inactivation. Although the answer itself is not surprising, this clear and unequivocal elucidation of the functional stoichiometry is a necessary prelude to further understanding the molecular details of calmodulin's regulation of the L-type calcium channel.
References (abridged):
1. B. Hille, Ion Channels of Excitable Membranes (Sinauer, Sunderland, MA, ed. 3, 2001)
2. I. B. Levitan, Neuron 22, 645 (1999)
3. Y. Saimi, C. Kung, Annu. Rev. Physiol. 64, 289 (2002)
4. M. X. Mori, M. G. Erickson, D. T. Yue, Science 304, 432 (2004)
5. P. Brehm, R. Eckert, Science 202, 1203 (1978)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
|