Determination of charge, stoichiometry and reaction constants fromI–V curve studies on a K+ transporter inNitella

A simple recipe for setting up the flux equations of cyclic and linear reaction schemes of ion transport with a high number of states: The arrow scheme

The calculation of flux equations or current-voltage relationships in reaction kinetic models with a high number of states can be very cumbersome. Here, a recipe based on an arrow scheme is presented, which yields a straightforward access to the minimum form of the flux equations and the occupation probability of the involved states in cyclic and linear reaction schemes. This is extremely simple for cyclic schemes without branches. If branches are involved, the effort of setting up the equations is a little bit higher. However, also here a straightforward recipe making use of so-called reserve factors is provided for implementing the branches into the cyclic scheme, thus enabling also a simple treatment of such cases.

Effect of cytosolic pH on inward currents reveals structural characteristics of the proton transport cycle in the influenza A protein M2 in cell-free membrane patches of Xenopus oocytes

Transport activity through the mutant D44A of the M2 proton channel from influenza virus A was measured in excised inside-out macro-patches of Xenopus laevis oocytes at cytosolic pH values of 5.5, 7.5 and 8.2. The current-voltage relationships reveal some peculiarities: 1. "Transinhibition", i.e., instead of an increase of unidirectional outward current with increasing cytosolic H(+) concentration, a decrease of unidirectional inward current was found. 2. Strong inward rectification. 3. Exponential rise of current with negative potentials. In order to interpret these findings in molecular terms, different kinetic models have been tested. The transinhibition basically results from a strong binding of H(+) to a site in the pore, presumably His37. This assumption alone already provides inward rectification and exponential rise of the IV curves. However, it results in poor global fits of the IV curves, i.e., good fits were only obtained for cytosolic pH of 8.2, but not for 7.5. Assuming an additional transport step as e.g. caused by a constriction zone at Val27 resulted in a negligible improvement. In contrast, good global fits for cytosolic pH of 7.5 and 8.2 were immediately obtained with a cyclic model. A "recycling step" implies that the protein undergoes conformational changes (assigned to Trp41 and Val27) during transport which have to be reset before the next proton can be transported. The global fit failed at the low currents at pHcyt = 5.5, as expected from the interference of putative transport of other ions besides H(+). Alternatively, a regulatory effect of acidic cytosolic pH may be assumed which strongly modifies the rate constants of the transport cycle.

Saturation and microsecond gating of current indicate depletion-induced instability of the MaxiK selectivity filter

Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed with a high temporal resolution (50-kHz filter) in symmetrical solutions with 50, 150, or 400 mM KCl and 2.5 mM CaCl(2) and 2.5 mM MgCl(2). At membrane potentials >+100 mV, the single-channel current showed a negative slope resistance, concomitantly with a flickery block, which was not influenced by Ca(2+) or Mg(2+). The analysis of the amplitude histograms by beta distributions revealed that current in this voltage range was reduced by two effects: rate limitation at the cytosolic side of the pore and gating with rate constants 10-20-fold higher than the cutoff frequency of the filter (i.e., dwell times in the microsecond range). The data were analyzed in terms of a model that postulates a coupling between both effects; if the voltage over the selectivity filter withdraws ions from the cavity at a higher rate than that of refilling from the cytosol, the selectivity filter becomes instable because of ion depletion, and current is interrupted by the resulting flickering. The fit of the IV curves revealed a characteristic voltage of 35 mV. In contrast, the voltage dependence of the gating factor R, i.e., the ratio between true and apparent single-channel current, could be fitted by exponentials with a characteristic voltage of 60 mV, suggesting that only part of the transmembrane potential is felt by the flux through the selectivity filter.

Oscillations in plant membrane transport: model predictions, experimental validation, and physiological implications

Although oscillations in membrane-transport activity are ubiquitous in plants, the ionic mechanisms of ultradian oscillations in plant cells remain largely unknown, despite much phenomenological data. The physiological role of such oscillations is also the subject of much speculation. Over the last decade, much experimental evidence showing oscillations in net ion fluxes across the plasma membrane of plant cells has been accumulated using the non-invasive MIFE technique. In this study, a recently proposed feedback-controlled oscillatory model was used. The model adequately describes the observed ion flux oscillations within the minute range of periods and predicts: (i) strong dependence of the period of oscillations on the rate constants for the H+ pump; (ii) a substantial phase shift between oscillations in net H+ and K+ fluxes; (iii) cessation of oscillations when H+ pump activity is suppressed; (iv) the existence of some 'window' of external temperatures and ionic concentrations, where non-damped oscillations are observed: outside this range, even small changes in external parameters lead to progressive damping and aperiodic behaviour; (v) frequency encoding of environmental information by oscillatory patterns; and (vi) strong dependence of oscillatory characteristics on cell size. All these predictions were successfully confirmed by direct experimental observations, when net ion fluxes were measured from root and leaf tissues of various plant species, or from single cells. Because oscillatory behaviour is inherent in feedback control systems having phase shifts, it is argued from this model that suitable conditions will allow oscillations in any cell or tissue. The possible physiological role of such oscillations is discussed in the context of plant adaptive responses to salinity, temperature, osmotic, hypoxia, and pH stresses.

Patch-clamp studies on the anomalous mole fraction effect of the K+ channel in cytoplasmic droplets of Nitella: an attempt to distinguish between a multi-ion single-file pore and an enzyme kinetic model with lazy state

Patch-clamp studies have been employed in order to check whether the assumption of a multi-ion single-file pore is necessary for the explanation of the anomalous mole fraction effect or whether this effect can also be explained by a single-barrier enzyme kinetic model. Experiments in the cell-attached configuration were done on the tonoplast membrane of cytoplasmic droplets of Nitella in solutions containing 150 mol m-3 of K+ plus Tl+ with seven different K+/Tl+ ratios. At first sight, the results seem to support the multi-ion single-file pore, because apparent open channel conductivity displays the anomalous mole fraction effect, whereas open-probability has not been found to be dependent on the K+/Tl+ ratio. Changes in open probability would be expected for a single-barrier enzyme kinetic model with a lazy state. On the other hand, the lazy-state model is more successful in explaining the measured I-V curves. The entire slope of the apparent open channel current-voltage curves rotates with changing K+/Tl+ ratios in the whole voltage range between -100 and +80 mV. Numerical calculations on the basis of multi-ion single-file pores could create the anomalous mole fraction effect only in a limited voltage range with intersecting I-V curves. The apparent absence of an effect on open probability which is postulated by the lazy-state model can be explained if switching into and out of the lazy state is faster than can be resolved by the temporal resolution of 1 msec.

Effects of microtubule agents on the spatial and electrical properties of the plasma membrane inChara corallina

The freshwater algaChara corallina Klein ex Willd., em. R.D.W. (=C. australis R.Br.) develops alternating outward (acid) and inward (alkaline) current areas on its surface when illuminated. Exposure of cells to vinblastine, colchicine, or oryzalin caused a reduction in and a shifting of this extracellular current pattern. Removal of these agents from the bathing media resulted in regeneration of the initial current profile. Because these agents all affect tubulin, microtubules may be responsible for orchestrating the transmembrane currents responsible for the acid and alkaline banding phenomenon. Analysis of the membrane potential showed a fast depolarization after vinblastine exposure; however, analysis of the current-voltage curve did not show a change in membrane conductance. A 30-min colchicine treatment decreased the conductance of the plasma membrane with either an hyperor a depolarization in the membrane potential. In contrast, although a 9-h exposure to oryzalin caused a major reduction in the extra-cellular current pattern, only minor changes were observed in the membrane potential and conductance. However, in the presence of oryzalin, the time constants in the light response of the membrane potential increased over this 9-h period. Collectively, these results implicate an involvement of microtubules in spatial control of plasma-membrane transport events inC. corallina.

Potassium ion channels in the plasmalemma

The potassium ion is an indispensible cytosolic component of living cells and a key osmolyte of plant cells, crossing the plasmalemma to drive physiological processes like cell growth and motor cell activity. K(+) transport across the plasmalemma may be passive through channels, driven by the electrochemical gradient, K(+) equilibrium potential (E(K) ) - membrane potential (V(m) ), or secondary active by coupling through a carrier to the inward driving force of H(+) or Na(+) . Known K(+) channels are permeable to monovalent cations, a permeability order being K(+) > Rb(+) > NH(4) (+) > Na(+) ≥ Li(+) > Cs(+) . The macroscopic K(+) currents across a cell or protoplast surface commonly show rectification, i.e. a V(m) -dependent conductance which in turn, may be controlled by the cytosolic activity of Ca(2+) , of K(+) , of H(+) , or by the K(+) driving force. Analysis by the patch clamp technique reveals that plant K(+) channels are similar to animal channels in their single channel conductance (4 to 100 pS), but different in that a given channel population slowly activates and may not inactivate at all. Single-channel kinetics reveal a broad range of open times (ms to s) and closed times (up to 100 s). Further progress in elucidating plant K(+) channels will critically depend on molecular cloning, and the availability of channel-specific (phyto)toxins.

Characterization of Cl- transport in vacuolar membrane vesicles using a Cl(-)-sensitive fluorescent probe: reaction kinetic models for voltage- and concentration-dependence of Cl- flux

The effects of Cl- concentration and membrane potential (delta psi) on Cl- influx in isolated vesicles of vacuolar membrane (tonoplast) from red beet (Beta vulgaris L.) storage tissue have been characterized using the Cl(-)-sensitive fluorescent probe, 6-methoxy-1-(3-sulfonatopropyl)quinolinium (SPQ). The initial rate of Cl- transport into the vesicles was enhanced both by the imposition of a positive delta psi and by increases in extravesicular Cl- concentration. The kinetic mechanism underlying these responses was investigated by examining the accuracy with which the data could be described by several transport models. A model based on constant field theory yielded a poor description of the data, but satisfactory fits were generated by pseudo-two-state reaction kinetic models based on classical carrier schemes. Fits were equally good when it was assumed that charge translocation accompanied Cl- entry, or when charge was carried by the unloaded transport system, as long as only a single charge is translocated in each carrier cycle. Expansion of the models to three states enabled description of the Cl- concentration dependence of transport by changes in a single, voltage insensitive rate constant which is tentatively identified with Cl- binding at the external surface of the membrane. The derived value of the dissociation constant between Cl- and the transport system is estimated at between 30 and 52 mM.

Transport of potassium in Chara australis: II. Kinetics of a symport with sodium

An electrogenic K(+)-Na+ symport with a high affinity for K+ has been found in Chara (Smith & Walker, 1989). Under voltage-clamp conditions, the symport shows up as a change in membrane current upon adding either K+ or Na+ to the bathing medium in the presence of the other. Estimation of kinetic parameters for this transport has been difficult when using intact cells, since K(+)-Na+ current changes show a rapid falling off with time at K+ concentrations above 50 microM. Cytoplasm-enriched cell fragments are used to overcome this difficulty, since they do not show the rapid falling off of current change seen with intact cells. Current-voltage curves for the membrane in the absence or presence of either K+ or Na+ are obtained, yielding difference current-voltage curves which isolate the symport currents from other transport processes. The kinetic parameters describing this transport are found to be voltage dependent, with Km for K+ ranging from 30 down to 2 microM as membrane potential varies from -140 to -400 mV, and Km for Na+ ranging between 470 and 700 microM over a membrane potential range of -140 to -310 mV. Two different models for this transport system have been investigated. One of these involves the simultaneous transport of both the driver and substrate ions across the membrane, while the other allows for the possibility of the two ions being transported consecutively in two distinct reaction steps. The experimental results are shown to be consistent with either of these cotransport models, but they do suggest that binding of K+ occurs before that of Na+, and that movement of charge across the membrane (the voltage-dependent step) occurs when the transport protein has neither K+ nor Na+ bound to it.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Electrical tolerance (breakdown) of the Chara corallina plasmalemma: II. Inductive property of membrane and effects of pHo and impermeable monovalent cations on breakdown phenomenon

Changes in the chord conductance G and the membrane electromotive force Em in the so-called breakdown region of large negative potential of the Chara plasmalemma were analyzed in more detail. In addition to the increase in G, the voltage sensitivity of the change in G increased, which was the cause of marked inductive current in the breakdown region. The break-down potential, defined as a critical potential at which both low and high slope conductances of the I-Vm relationship cross, almost coincided with the potential at which an inductive current began to appear. This breakdown potential level changed with pHo in a range between 5 and 9. The Chara plasmalemma was electrically most tolerant around pHo 7. In some cells Em shifted to a positive level as large as +50 approximately +70 mV during the breakdown phenomenon. Such a large positive shift of Em is caused mainly by the increase in conductance of Cl- and partly Ca2+ and K+.