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ScienceWeek
SCIENCEWEEK
July 4, 2003
Vol. 7 - Number 27B
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CONTENTS:
1. Validity and Ethics in Science
2. Supercooled Water
3. Jupiter's Moons
4. Repair Mechanisms of Damaged Cell Membranes
5. Plant Stem Cells
6. ClC Chloride Ion Channels
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1. VALIDITY AND ETHICS IN SCIENCE
The following points are made by R. Stephen Berry (Science 2003 300:1341):
1) Recent events have forced scientists and others to confront the issue of ethical behavior in scientific work, leading some to question the validity of the body of accumulated scientific knowledge. But fraud and validity are separable matters, and it is important that the public understand the differences between them. Yes, the scientific enterprise may occasionally fall short in dealing with unethical behavior, but that has not threatened the reliability of our accumulated scientific knowledge.
2) The knowledge structure produced by science has a quality unique among the creations of the human species. Its uniqueness lies in its capacity to provide reliable quantitative predictions of phenomena within its own domain; no other aspect of human experience has that kind of capability. This predictive power is a consequence of the way scientific studies evolve -- and science's validation processes, themselves unique, guarantee that power.
3) Science advances by trial and error, guided by past observations and their interpretations. Establishing the validity of each new result is essential. Some new findings, such as the measurement of a quantity predicted by a well-established theory, call for only modest efforts to establish validity. At the opposite extreme are results that challenge established concepts. Some apparently idiosyncratic ideas cannot be tested rigorously at the time when they are proposed. For example, continental drift, eventually called "plate tectonics", could not be validated until many years after it was proposed. But the celebrated cases of "cold fusion" and "polywater" immediately produced major efforts at validation, because both would have been important were they correct. That stimulated many researchers to examine each and, in months, to discredit them.
4) In these examples, the researchers doubtless believed that their results were valid. In some recent cases, it has become clear that the scientists knew their results were not valid. These new cases of fraudulent research, though hardly the first, raise the question of whether the validation process that corrects honest error works as well for deliberate fraud.
5) When any new result is presented to a scientific community, the tacit presumption is that the presenter is honest. But whether that presumption holds or not, the result will be subject to the standard validation processes that make science work. And these processes have kept the body of scientific knowledge remarkably self-consistent. Thus, it is especially important to separate two questions: how and whether science succeeds at self-validation, and how to recognize and deal with misconduct.
6) Scientific self-correction is alive and well, and it serves to maintain the validity of the body of scientific knowledge. That process may work slowly, partly because of the procedures required for validation and partly because scientists may feel little urgency to validate a particular result. Nonetheless the validation must eventually occur if the result is to be used in building further science. It is important that, despite the furor over research misconduct, the public understand that the validation process is working as it should. Cultivating, even demanding, ethical behavior in the scientific enterprise is important for other reasons, quite distinct from our need to have confidence in the enterprise itself.
Related Material:
ABDERHALDEN'S FRAUD REVISITED
The following points are made by U. Deichmann and B. Muller-Hill (Nature 1998 393:109):
Emil Abderhalden (1877-1950) was a biochemist involved in designing tests for various clinical disease entities, the tests involving what he called "defense enzymes" (Abwehrfermente), enzymes which he claimed to have identified, and which according to his analyses were specific proteases produced when humans were challenged by foreign proteins. During the years 1912 to 1950, Abderhalden enjoyed the status of one of the most eminent scientists in Germany, was professor of physiology and physiological chemistry at Halle University, president of the oldest German academy of science (the Leopoldina), editor of several journals, and author of several books and more than 1000 research papers -- and all of this notwithstanding, the consensus today is that nearly all of his research on the so-called "defense enzymes" was completely fraudulent, with scores of colleagues and underlings either explicitly or implicitly colluding in the fraud over a period of decades.
2) It is an ugly story with political tangents (e.g., Joseph Mengele, the notorious Auschwitz doctor, was one of Abderhalden's proteges). In their commentary, Deichmann and Muller-Hill conclude: "The elite of today [the biomedical elite in Germany] are loyal students of the old elite, and they have learned and internalized the old values. Has medical clinical science in Germany today really changed that much? We doubt it. The Brach-Herrmann-Mertelsmann affair [a recent biomedical research fraud scandal in Germany] provides a brief glimpse into the abyss of medical science in Germany. Will it be soon forgotten by the German medical elite, or will there be real change in the spirit of true science?"
Related Material:
A SCIENTIFIC FRAUD SCANDAL IN FRANCE
There is often an element of farce in detected scientific fraud, since in hindsight it usually seems ludicrous that the perpetrator or perpetrators of the fraud expected to get away with it. But farce aside, there are serious aspects to scientific fraud, first the damage to the ideals of the scientific community, and second (and perhaps more important), the damage caused to other scientists by the fraud provoking costly strategic decisions based on the faked evidence.
The coelacanth is a primitive *teleost fish that apparently first appeared in the *Devonian Period, with the most recent fossil specimens dating from the *Cretaceous Period. The group was assumed to be extinct, but a living specimen was caught in 1938 in the mouth of the Chalumna River in South Africa and named Latimeria chalumnae. The 1938 specimen was 2 meters in length and weighed 40 kilograms. More specimens have since been caught, all off the coast of South Africa and near the island of Madagascar (Comoros archipelago) in the Indian Ocean.
In July 1998, M.V. Erdmann et al reported the capture and observation of a live coelacanth specimen near the island of Manado Tua (north Sulawesi) in Indonesia, and the find was quickly published in the journal Nature and recognized as of considerable importance. In the published article, a photograph of the captured coelacanth appeared, a lateral view of the complete fish.
In the spring of 2000, three French scientists (Bernard Seret, Laurent Pouyaud, and George Serre), one of whom (Seret) was an ichthyologist at the Museum National d'Histoire Naturelle de Paris (FR), submitted to the journal Nature an article claiming a prior discovery in 1995 of a coelacanth in Indonesian waters (thus preempting the 1998 discovery of M.V. Erdmann et al), the Seret et al submitted article including a photograph of their captured fish. The French group stated they were unable to register their specimen in 1995 because it failed to reach the museum to which it had been sent. They stated they photographed the fish at the time, then lost the photograph while moving house, and only found the photograph again in the year 2000.
In addition to the question of priority of discovery, the scientific implications of the findings of the French team were that it would extend the distribution of living coelacanths, since the French fish was stated to have been caught more than 2000 kilometers from the spot where Erdmann et al found the 1998 specimen, which suggests a larger distribution of the fish in the Indo-West Pacific region.
What happened was that staff people at Nature noticed that the photograph submitted by B. Seret et al and the photograph by M.V. Erdmann et all previously published two years before were virtually identical. The Seret et al paper was refused by Nature, and the French photograph was denounced as an outright fake. Apparently what was done was to use computer graphics software to simply cut and paste the 1998 Erdmann et all fish into another photograph, moving the coelacanth onto a table with two other ordinary fish as though all three fish were in the same catch. When presented with the hard evidence that the two coelacanth photos are identical, the Seret et al response was that the picture was "taken by a friend who later died and whose widow gave it to Serre before moving abroad." In the 13 July 2000 Nature issue in reference below, the apparently faked photo and the original photo published in 1998 by Erdmann et all appear side by side for comparison.
Again, aside from the farcical aspects here (clipping a photograph of an unusual object from a journal article and using the clipped photograph to fake another photograph submitted as "evidence" in another article sent to the very same journal), it should be recognized that a bit more cleverness with computer software might have made the fake unrecognizable as such, and steered coelacanth specialists onto a false trail for decades. That is not amusing.
Commenting on this affair in a recent letter, M.V. Erdmann and R.I. Caldwell (University of California Berkeley, US), Erdmann the discoverer of the 1998 coelacanth, state: "The Indonesians and Comorans are rightfully proud of efforts in their two countries to preserve these rare and very special fish. What pride can we in the Western scientific community take in this affair?"
H. McCabe and J. Wright: Tangled tale of a lost, stolen and disputed coelacanth. (Nature 2000 406:114)
M.V. Erdmann and R.I. Caldwell: How new technology put a coelacanth among the heirs of Piltdown Man. (Nature 27 2000 406:343)
Notes:
teleost: In general, this refers to any of the bony fish, the most advanced in terms of evolution and the largest group of fish. Besides the calcified internal skeleton, the most obvious uniform characteristic of the teleost fish is their tail, with upper and lower halves of about equal size, whereas in cartilaginous fish the tail has two lobes of unequal size. Almost all sport, commercial, and ornamental fish are teleosts.
Devonian Period: From approximately 400 million to 345 million years ago. Sometimes called the Age of the Sea, since more of the Earth was underwater than is now.
Cretaceous Period: The geological period ranging approximately from 146 million years ago to 65 million years ago.
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2. SUPERCOOLED WATER
The following points are made by P.G. Debenedetti and H.E. Stanley (Physics Today 2003 June):
1) One of the four Aristotelian elements, water has played a central role in scientific thought for millennia. To the physical scientist it is a continuing source of fascination because of its many unusual and counterintuitive properties. For example, liquid water, if sufficiently cold, expands and becomes more compressible when cooled, and less viscous when compressed. Water can also exist in at least two distinct glass forms -- a phenomenon known as "polyamorphism".
2) Water is not only fascinating, but it is also one of the most important and ubiquitous substances on Earth. There are 1.3 X 10^(9) km^(3) of water in the oceans, 3.3 x 10^(7) km^(3) in the polar ice caps, 2 X 10^(5) km^(3) in glaciers, 10^(5) km^(3) in lakes, and 1.2 x 10^(3) km^(3) in rivers. In addition, 2.2 x 10^(5) km^(3) of water fall annually as precipitation. Nearly every aspect of our daily lives is influenced or controlled by water. From agriculture to travel, and from public health to commerce, the properties of water shape human activity and define the geography, topography, and environment in which we live. Indeed, life itself cannot exist without water.
3) Water can exist in many different crystalline forms, 13 of which have been identified to date. Of those, nine are stable over some range of temperature and pressure -- for example, at atmospheric pressure, ordinary hexagonal ice is stable between 72 and 273 K -- and the other forms are metastable. Although the stable form of water at sufficiently low temperature is invariably crystalline, liquid water can also exist inside the crystalline domain of stability. When that occurs, water is said to be "supercooled".
4) Supercooled water exists in a state of precarious equilibrium. Minor perturbations such as dissolved or suspended impurities can trigger the sudden appearance of the stable crystalline phase. The largest natural inventory of supercooled water occurs in the form of small droplets in clouds and plays a key role in the processing of solar and terrestrial radiative energy fluxes. Supercooled water is also important for life at subfreezing conditions, for the commercial preservation of proteins and cells, and for the prevention of hydrate formation in natural gas pipelines.
5) If liquid water is cooled fast enough (at rates on the order of 106 K/s), freezing can be avoided altogether, and water then becomes a noncrystalline solid -- that is, a "glass". Glassy water may be the most common form of water in the Universe. It is observed as a frost on interstellar dust, constitutes the bulk of matter in comets, and is thought to play an important role in the phenomena associated with planetary activity. Its formation in the laboratory, however, requires elaborate procedures.
Related Material:
ON WATER STRUCTURE
The following points are made by T. Head-Gordon and G. Hura (Chem. Rev. 2002 102:2651):
1) The fundamental unit of water structure is the hydrogen bond. In ice I a given water molecule is hydrogen bonded to four water neighbors in a tetrahedral structure that gives rise to a crystal made up of connected hexagonal rings. In the case of crystalline materials such as ice I, X-rays and neutrons are scattered by atomic centers at discrete angles represented as sinusoidal (Fourier) components of the electron density and nuclear scattering potential of the specimen, respectively. The scattering angle is determined by the spatial period of the Fourier component that is responsible for the scattering. The spatial period of each Fourier component of the electron density is determined by the lengths of the unit cell vectors of the crystal.
2) Representation of the electron density as a sum of Fourier components is equally applicable to noncrystalline materials, however, such as the water liquid. As a result it is still true that the spatial period of the Fourier component can be calculated from the measured scattering angle. As with crystalline materials, the amplitude of each Fourier component of the electron density is given by the square root of the scattered intensity. Information about the vector direction of the Fourier component is lost in scattering from liquids, however, unlike the case of crystals.
3) In the case of liquid water, the strict adherence to hydrogen-bonded hexagons of the ice crystal gives way to greater translational and rotational motion of waters and a broader distribution of hydrogen-bonded configurations, including a variety of polygons of varying sizes and degrees of puckering or distortion, all of which result in a more compact arrangement of water molecules. The electron density of the liquid is now characterized by the scattering as a diffuse water ring rather than a discrete distribution of Fourier components. Furthermore, the scattering intensity is peaked at a distance that remains larger than the center-to-center distance between individual water molecules, which is typically approximately 0.28 nm. Thus, it is clear that the most prominent Fourier components of the scattering density of pure liquid water have a repeat distance that is larger than the oxygen-oxygen nearest neighbor distance. This tells us that the fundamental scattering unit in liquid water must be something bigger than pairs of hydrogen-bonded water molecules. In fact, it is a measure of the highly associated three-dimensional hydrogen-bonded network of the water liquid. The importance of accurate experimental information and classical and emerging ab initio simulation methodologies is their ability to characterize this fundamental unit of scattering to help us to understand the topology of the hydrogen-bonded network over the full phase diagram.
References (abridged):
1. Water, a comprehensive treatise; Franks, F., Ed.; Plenum Press: New York, 1972.
2. Dore, J. C.; Teixeira, J. Hydrogen-bonded liquids: proceedings of the NATO Advanced Study Institute on Hydrogen-bonded Liquids, Institut Scientifique de Cargese, Corsica, April 3-15, 1989; Kluwer Academic Publishers: Dordrecht,; Boston, 1991.
3. Bellissent-Funel, M. C.; Dore, J. C. Hydrogen bond networks; Kluwer Academic Publishers: Dordrecht, Boston, 1994.
4. Franks, F. Water: A Matrix of Life, 2nd ed.; Royal Society of Chemistry: Cambridge, 2000.
5. Stillinger, F. H. Science (USA) 1980, 209, 451-7.
Related Material
LIQUID WATER: CURRENT RESEARCH PROBLEMS
In general, "ab initio" (from first principles) calculations utilize experimental data on atomic systems to facilitate the adjustment of parameters. The excellent performance of ab initio techniques distinguishes them from their predecessors, the "semiempirical" methods, with the quantitative predictions of ab initio techniques usually falling within experimental error when comparisons are made to experimental measurements.
Contemporary molecular dynamics simulations, which are extrapolations of statistical mechanics and which originate in the work of Alder and Wainright in the 1960s, are computer simulations of molecular systems typically involving hundreds or sometimes thousands of idealized particles interacting with physically realistic potentials. Such molecular dynamics simulations can provide time-dependent properties of a liquid, but most commonly they are used to produce a set of configurations and forces which can be averaged to give equilibrium properties of the system.
The following points are made by F.N. Keutsch and R.L. Saykally (Proc. Nat. Acad. Sci. 2001 98:10533):
1) The quest to achieve an accurate description of liquid water has produced major advances in the last two decades, but despite the construction of hundreds of model force fields for use in simulations, the great advances in computational technology, and the development of powerful ab initio molecular dynamics methods, we remain unable to accurately calculate the properties of liquid water (e.g., heat capacity, density, dielectric constant, compressibility) over significant ranges in various conditions. We do not yet have a satisfactory molecular description of how a proton moves in the liquid, we do not fully understand the molecular nature of the surfaces of either ice or liquid water, nor do we understand the origin of the intriguing anomalies and singularities found in the deeply supercooled region.
2) Although it is clear that the hydrogen bond network and its fluctuations and rearrangement dynamics determine the properties of the liquid, no experimental studies exist that reveal detailed information on a molecular level without considerable interpretation. Moreover, the reliability of water models for simulating solvation phenomena and biological processes remains relatively untested.
3) A general obstacle to resolving these issues is that of correctly describing the many-body or cooperative nature of the hydrogen bonding interactions among a collection of water molecules. Theoretical work has demonstrated that the H-bond is dominated by electrostatic interactions, balanced by the repulsive electron exchange, but that dispersion makes an appreciable contribution, whereas induction (polarization) is the dominant many-body effect. It has proven notoriously difficult to accurately parameterize these interactions from ab initio calculations.
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3. JUPITER'S MOONS
The following points are made by Douglas P. Hamilton (Nature 2003 423:235):
1) The preeminence of Jupiter as the planet with the largest number of natural satellites (moons) has been dramatically and decisively re-established by new observations. Fending off strong challenges from Saturn and Uranus, Jupiter, the Solar System's largest planet, now has nearly as many known moons as all of its competitors combined. Nearly two dozen new jovian moons have recently been discovered.
2) The search for planetary satellites has a long history, dating back to 1610 and the discovery by Galileo Galilei (1564-1642) of four star-like objects orbiting Jupiter -- Io, Europa, Ganymede and Callisto. Saturn's splendid ring system and its largest moon, haze-enshrouded Titan, were first seen by Christiaan Huygens (1629-1695) about 50 years later, and, in 1684, the discoveries of icy Dione and Tethys established Saturn as the planet with the most moons, a title it held for 230 years. Jupiter's Sinope, spotted in 1914, evened the score at nine known moons apiece, and two additional findings in 1938 allowed the giant planet to surge into the lead. Saturn staged a surprise comeback in 1980, when seven new satellites were spotted by the Voyager spacecraft and ground-based observers. Then came the great upset of 1999: dark horse Uranus revealed three additional outer satellites and vaulted to the forefront. But the title has since been reclaimed, first by Saturn, with a dozen new discoveries reported in 2000, and now by Jupiter, with the 23 new findings.
3) Currently, the number of known planetary moons stands at 128. More than half of this total has been added since 1997, when B. Gladman and colleagues found the first two distant, or "irregular", satellites of Uranus. The large number of satellite discoveries over the past six years, at an ever quickening pace, is reminiscent of the situation following Jewitt and Luu's 1992 discovery of the first transneptunian (or Kuiper belt) objects. Both population explosions have been fuelled by major improvements in digital-camera technology.
4) Nearly two-thirds of the known moons (including all of the recent discoveries) are irregular satellites, orbiting far from their planets along highly tilted, elliptical paths. These objects are believed to have been captured by their planets from independent orbits around the Sun early in the history of the Solar System. Regular satellites, by contrast, have much smaller, untilted, circular orbits, and were probably formed out of the disks of gas and dust that surrounded the giant planets in their youth. Energy dissipation in these early accretion disks also acted to facilitate the capture of the irregular satellites.
Related Material:
THE MOONS OF SATURN
The following points are made by B. Gladman et al (Nature 2001 412:163):
1) The giant planets in the Solar System each have two groups of satellites. The "regular satellites" move along nearly circular orbits in the planet's orbital plane, revolving about the planet in the same sense as the spin of the planet. In contrast, the so-called "irregular satellites" are generally smaller in size and are characterized by large orbits with significant eccentricity, inclination, or both. The differences in the characteristics of the two groups of satellites suggest that the regular and irregular satellites formed by different mechanisms: the regular satellites are believed to have formed in an accretion disk around the planet, like a miniature Solar System, whereas the irregular satellites are generally thought to be captured planetesimals.
2) The authors report the discovery of 12 irregular satellites of Saturn, along with the determinations of the orbits of these satellites. These orbits, along with the orbits of irregular satellites of Jupiter and Uranus, fall into groups on the basis of their orbital inclinations. The authors interpret this result as indicating that most of the irregular moons are collisional remnants of larger satellites that were fragmented after capture, rather than being captured independently.
Related Material:
ON THE MOONS OF JUPITER
The following points are made by David J. Stevenson (Science 2001 294:72):
1) There are many similarities between the Jovian system (Jupiter plus its moons) and our Solar System. Both systems are extremely regular. Bodies orbit in a nearly common plane -- Jupiter's equatorial plane for the Galilean satellites (the moons discovered by Galileo: Io, Europa, Ganymede, and Callisto), and the Sun's ecliptic plane for the Solar System. In both cases, bodies orbit in a prograde sense (anti-clockwise when viewed from above), with orbits spaced in approximate geometric progression.
2) The total mass of Jupiter's satellites is approximately the same as that of Mars and probably approximately 1 percent of the heavy-element mass (everything except hydrogen and helium) inside Jupiter. This is a similar ratio to the heavy-element distribution in our Solar System, where the Sun contains approximately 10 Jupiter masses and the planetary system tens of Earth masses of heavy elements.
3) Yet there are also striking differences between the Jovian system and the Solar System. The Solar System is spread out relative to the size of the Sun, with even the Sun-hugging Earth over 200 solar radii away, whereas the most distant Galilean satellite, Callisto, is less than 30 Jupiter radii from Jupiter. The compactness of the Jupiter system undoubtedly arises from the limited size of the region in which Jupiter is gravitationally dominant over the Sun.
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4. REPAIR MECHANISMS OF DAMAGED CELL MEMBRANES
The following points are made by Juliet A. Ellis (Nature 2003 423:129):
1) Cell membranes in tissues such as skin, gut and muscle are routinely exposed to mechanical damage, which can produce holes in them. When that damage is not repaired, the consequences can be severe -- often resulting in cell death -- and may contribute to the development of the muscle degenerative diseases termed "muscular dystrophies". From a combination of observations on human muscular dystrophy patients and experiments with mice, Bansal et al (Nature 2003 423:168) have reported that a protein called dysferlin is a component of the mechanism for resealing the holes, and thus healing the muscle membrane.
2) Membrane resealing is generally carried out by a mechanism that resembles the calcium-regulated release of vesicles from a cell (exocytosis). The repair pathway is initiated by an influx of calcium through a wound, resulting in an increase in calcium levels at the site of injury. This, in turn, triggers the accumulation of vesicles, which fuse with one another and then with the plasma membrane, within the injury. A "patch" is thereby added across the wounded area, resealing the plasma membrane. The entire process -- still mysterious -- takes between ten and thirty seconds.
3) Specific participants in the process include members of the SNARE and SNAP family of proteins, which are associated with vesicle fusion in nerve transmission. Among them is a protein called synaptotagmin, which is thought to act as a calcium sensor through its possession of two C2 domains. This feature means that it can bind phospholipids -- which are the main components of membranes -- in a calcium-dependent manner. The protein investigated by Bansal et al, dysferlin, is found in the muscle plasma membrane (sarcolemma) and in cytoplasmic vesicles, and its participation in membrane repair is all the more thought-provoking given its association with muscle degeneration.
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5. PLANT STEM CELLS
The following points are made by Peter Doerner (Current Biology 2003 13:R368):
1) Growth and organ formation in plants occur post-embryonically, mediated by meristems located on the tips of growth axes in shoots and roots. Meristems are unique structures made up of pluripotent stem cells, a transitory population of indeterminate cells and determinate organ primordia formed at the periphery. Secondary growth, which increases the girth of stems, is mediated by cambial cells, which continue to add vascular cells to the circumference of the central, vascular cylinder of the plant.
2) Shoot and root meristems are generated during embryogenesis, but do not contribute to the construction of the embryo and are not activated until the seedling germinates. Following germination, the plant passes through several developmental phases that culminate in flowering and reproduction. In the course of these phase changes, shoot meristems change their identity. This is most conspicuous in the lateral structures made on the flanks of the shoot apical meristem. In Arabidopsis, these structures are leaves during the initial vegetative growth, leaves and axillary meristems during the transition to flowering, and floral meristems and bracts by the inflorescence meristem during reproductive growth. In contrast, root meristems do not apparently change their identity during development.
3) Roots and shoots also make lateral structures differently. Lateral appendages of the shoot are initiated on the flanks of the apical meristems. The regular temporal sequence of organ initiation gives rise to the characteristic spiral phyllotaxy of leaves, and to a concentric organization of floral organs into whorls in flowers. Production of leaves in a regular, phyllotactic sequence is a good indication of a functioning apical meristem, and distinguishes these from adventitious structures transiently capable of leaf production. Lateral roots, in contrast are formed only at a distance from the root apex, and appear in stochastic patterns with no regular spatial relationship to each other.
4) In summary: Recent studies have provided significant new insights into the gene actions that specify and maintain stem cells in plant shoots and roots. New layers of genetic control and potential signalling pathways and effector mechanisms have emerged from these new studies. These new findings refine the current model in which stem cells in plant meristems are regulated by negative feedback loops and uncover a fundamental mechanism for stem cell maintenance that might be common to shoots and roots.
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6. CLC CHLORIDE ION CHANNELS
The following points are made by R. Dutzler et al (Science 2003 300:108):
1) Ion channels carry electric current across the membrane of cells in the form of diffusing ions. The two key properties of ion channels are selective ion conduction and gating. Selective conduction refers to a channel's ability to select one ionic species among those present in the cellular environment and catalyze its rapid flow through the pore; gating refers to opening and closing the pore, the process by which ion conduction is turned on or off.
2) In some channels, the functions of selective conduction and gating are mediated by quite separate structural elements. Potassium channels, for example, have a selectivity filter near the extracellular side of the pore and a gate near the intracellular side. Separation of the filter and gate allows ligand-binding domains or voltage sensor domains to open and close the pore through large conformational changes without affecting the selectivity filter, whose structure
must be maintained in order to discriminate among ions on the basis of their small differences in radius.
3) Years of electrophysiological study suggest that the condition of a structurally independent selectivity filter and gate will probably not apply to ClC channels, a large Cl- channel family whose members are found from bacteria to animals. In ClC channels, selective conduction and a certain form of gating referred to as "fast gating" seem to be intimately coupled to each other. Chloride ions conduct rapidly through the pore, and at the same time they affect the probability that the fast gate will be open. Certain ClC channels have even been called "(Cl-)-activated Cl- channels" because extracellular Cl- causes the gate to open. Membrane voltage can influence the open probability as well, but even this property depends on Cl- ions.
4) In summary: ClC channels conduct chloride (Cl-) ions across cell membranes and thereby govern the electrical activity of muscle cells and certain neurons, the transport of fluid and electrolytes across epithelia, and the acidification of intracellular vesicles. The authors report a study of the structural basis of ClC channel gating. Crystal structures of wild-type and mutant Escherichia coli ClC channels bound to a monoclonal Fab fragment reveal three Cl- binding sites within the 15-angstrom neck of an hourglass-shaped pore. The Cl- binding site nearest the extracellular solution can be occupied either by a Cl- ion or by a glutamate carboxyl group. Mutations of this glutamate residue in Torpedo ray ClC channels alter gating in electrophysiological assays. The authors suggest these findings reveal a form of gating in which the glutamate carboxyl group closes the pore by mimicking a Cl- ion.
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