Voltage-dependent and Src-mediated regulation of NMDA receptor single channel outward currents in cortical neurons

A voltage-dependent but Ca²⁺-independent regulation of N-methyl-D-aspartate (NMDA) receptor outward activity was studied at the single channel level using outside-out patches of cultured mouse cortical neurons. Unlike the inward activity associated with Ca²⁺ and Na⁺ influx, the NMDA receptor outward K⁺ conductance was unaffected by changes in Ca²⁺ concentration. Following a depolarizing pre-pulse, the single channel open probability (NP _o), amplitude, and open duration of the NMDA inward current decreased, whereas the same pre-depolarization increased those parameters of the NMDA outward current (pre-pulse facilitation). The outward NP _o was increased by the pre-pulse facilitation, disregarding Ca²⁺ changes. The voltage–current relationships of the inward and outward currents were shifted by the pre-depolarization toward opposite directions. The Src family kinase inhibitor, PP1, and the Src kinase antibody, but not the anti-Fyn antibody, blocked the pre-pulse facilitation of the NMDA outward activity. On the other hand, a hyperpolarizing pre-pulse showed no effect on NMDA inward currents but inhibited outward currents (pre-pulse depression). Application of Src kinase, but not Fyn kinase, prevented the pre-pulse depression. We additionally showed that a depolarization pre-pulse potentiated miniature excitatory synaptic currents (mEPSCs). The effect was blocked by application of the NMDA receptor antagonist AP-5 during depolarization. These data suggest a voltage-sensitive regulation of NMDA receptor channels mediated by Src kinase. The selective changes in the NMDA receptor-mediated K⁺ efflux may represent a physiological and pathophysiological plasticity at the receptor level in response to dynamic changes in the membrane potential of central neurons.     

The fertilization potential is not necessary for the block to polyspermy or the activation of development in the medaka egg

The egg of the medaka, Oryzias latipes, was impaled with two microelectrodes so that its membrane potential could be clamped at a constant level during fertilization. Fertilization occurred at all membrane potentials between ?80 and +48 mV. Therefore, there is apparently no electrical block to polyspermy in this egg. In 16 of these eggs the membrane potential was also clamped at a constant level during the 6- to 14-min period after fertilization and the eggs' subsequent development was studied. All of these eggs developed normally up to at least the beating heart stage. Therefore, the fertilization potential is not necessary for further development. When the egg is clamped at levels more negative than ?25 mV, the injected clamping current is usually biphasic just after fertilization with an inward current phase preceding a longer outward phase. The inward current phase corresponds well in time with the membrane depolarization normally triggered by fertilization. The outward current phase was observed in all eggs studied and the more positive the holding potential, the longer was the outward current duration.     

Effects of conditioning polarization on the membrane ionic currents in rat myometrium

Summary Membrane ionic currents were measured in pregnant rat uterine smooth muscle under voltage clamp conditions by utilizing the double sucrose gap method, and the effects of conditioning pre-pulses on these currents were investigated. With depolarizing pulses, the early inward current was followed by a late outward current. Cobalt (1mm) abolished the inward current and did not affect the late outward currentper se, but produced changes in the current pattern, suggesting that the inward current overlaps with the initial part of the late outward current. After correction for this overlap, the inward current reached its maximum at about +10 mV and its reversal potential was estimated to be +62 mV. Tetraethylammonium (TEA) suppressed the outward currents and increased the apparent inward current. The increase in the inward current by TEA thus could be due to a suppression of the outward current. The reversal potential for the outward current was estimated to be –87 mV. Conditioning depolarization and hyperpolarization both produced a decrease in the inward current. Complete depolarization block occurred at a membrane potential of –20 mV. Conditioning hyperpolarization experiments in the presence of cobalt and/or TEA revealed that the decrease in the inward current caused by conditioning hyperpolarization was a result of an increase in the outward current overlapping with the inward current. It appears that a part of the potassium channel population is inactivated at the resting membrane potential and that this inactivation is removed by hyperpolarization.     

Appearance and maturation of voltage-dependent conductances in solitary spiking cells during retinal regeneration in the adult newt

Summary Electrical membrane properties of solitary spiking cells during newt (Cynops pyrrhogaster) retinal regeneration were studied with whole-cell patch-clamp methods in comparison with those in the normal retina.The membrane currents of normal spiking cells consisted of 5 components: inward Na⁺ and Ca⁺⁺ currents and 3 outward K⁺ currents of tetraethylammonium (TEA)-sensitive, 4-aminopyridine (4-AP)-sensitive, and Ca⁺⁺-activated varieties. The resting potential was about -40mV. The activation voltage for Na⁺ and Ca⁺⁺ currents was about -30 and -17 mV, respectively. The maximum Na⁺ and Ca⁺⁺ currents were about 1057 and 179 pA, respectively.In regenerating retinae after 19–20 days of surgery, solitary cells with depigmented cytoplasm showed slowrising action potentials of long duration. The ionic dependence of this activity displayed two voltage-dependent components: slow inward Na⁺ and TEA-sensitive outward K⁺ currents. The maximum inward current (about 156 pA) was much smaller than that of the control. There was no indication of an inward Ca⁺⁺ current.During subsequent regeneration, the inward Ca⁺⁺ current appeared in most spiking cells, and the magnitude of the inward Na⁺, Ca⁺⁺, and outward K⁺ currents all increased. By 30 days of regeneration, the electrical activities of spiking cells became identical to those in the normal retina. No significant difference in the resting potential and the activation voltage for Na⁺ and Ca⁺⁺ currents was found during the regenerating period examined.     

On the mechanism of cesium-induced voltage and current tails in single ventricular myocytes

The mechanisms by which different concentrations of cesium modify membrane potentials and currents were investigated in guinea pig single ventricular myocytes. In a dose-dependent manner, cesium reversibly decreases the resting potential and action potential amplitude and duration, and induces a diastolic decaying voltage tail (V_ex), which increases at more negative and reverses at less negative potentials. In voltage-clamped myocytes, Cs⁺ increases the holding current, increases the outward current at plateau levels while decreasing it at potentials closer to resting potential, induces an inward tail current (I_ex) on return to resting potential and causes a negative shift of the threshold for the inward current. During depolarizing ramps, Cs⁺ decreases the outward current negative to inward rectification range, whereas it increases the current past that range. During repolarizing ramps, Cs⁺ shifts the threshold for removal of inward rectification negative slope to less negative values. Cs⁺-induced voltage and current tails are increased by repetitive activity, caffeine (5 mM) and high [Ca²⁺]_o (8.1 mM), and are reduced by low Ca²⁺ (0.45 mM), Cd²⁺ (0.2 mM) and Ni²⁺ (2 mM). Ni²⁺ also abolishes the tail current that follows steps more positive than E_Ca. We conclude that Cs⁺ (1) decreases the resting potential by decreasing the outward current at more negative potentials, (2) shortens the action potential by increasing the outward current at potentials positive to the negative slope of inward rectification, and (3) induces diastolic tails through a Ca²⁺-dependent mechanism, which apparently is an enhanced electrogenic Na-Ca exchange.     

Manipulation of cytokinesis affects polar ionic current around the egg of Lymnaea stagnalis

Summary The fertilized egg of the mollusc Lymnaea stagnalis generates a polarized current pattern as measured with the vibrating probe. Here we investigated the basis of these polar ionic currents. Ionic currents were measured around eggs during the second meiotic division after interference with cytokinesis. Cytokinesis was either displaced by centrifugation or inhibited with cytochalasin or nocodazole. Furthermore, ectopic constrictions were induced with lectin treatment. It appeared that the inward current of the animal pole can be displaced by centrifugation and remains associated with the position of the meiotic apparatus. The influence of the meiotic apparatus on the polar current pattern seems to be directly related to membrane constrictions rather than to karyokinesis. This was demonstrated by a change in current density after induction of an ectopic constriction at the vegetal pole and by the abolishment of currents after cytochalasin treatment. Since the location of the outward current was not sensitive to centrifugation, it may be concluded that the vegetal outward current depends upon properties of the vegetal cortex. On the basis of these results, we conclude that the Lymnaea egg generates two types of ionic currents during the second meiotic division. The first is an inward current activated at the site of membrane constrictions. The second is an outward current associated with the vegetal cortex.     

Inward and outward currents of native and cloned K(ATP) channels (Kir6.2/SUR1) share single-channel kinetic properties

BackgroundThe ATP-sensitive K⁺ (K(ATP)) channel is found in a variety of tissues extending from the heart and vascular smooth muscles to the endocrine pancreas and brain. Common to all K(ATP) channels is the pore-forming subunit Kir6.x, a member of the family of small inwardly rectifying K⁺ channels, and the regulatory subunit sulfonylurea receptor (SURx). In insulin secreting β-cells in the endocrine part of the pancreas, where the channel is best studied, the K(ATP) channel consists of Kir6.2 and SUR1. Under physiological conditions, the K(ATP) channel current flow is outward at membrane potentials more positive than the K⁺ equilibrium potential around ?80 mV. However, K(ATP) channel kinetics have been extensively investigated for inward currents and the single-channel kinetic model is based on this type of recording, whereas only a limited amount of work has focused on outward current kinetics.MethodsWe have estimated the kinetic properties of both native and cloned K(ATP) channels under varying ionic gradients and membrane potentials using the patch-clamp technique.ResultsAnalyses of outward currents in K(ATP) and cloned Kir6.2ΔC26 channels, alone or co-expressed with SUR1, show openings that are not grouped in bursts as seen for inward currents. Burst duration for inward current corresponds well to open time for outward current.ConclusionsOutward K(ATP) channel currents are not grouped in bursts regardless of membrane potential, and channel open time for outward currents corresponds to burst duration for inward currents.     

"Creep currents" in single frog atrial cells may be generated by electrogenic Na/Ca exchange

The objective of these experiments was to test the hypothesis that the "creep currents" induced by Na loading of single frog atrial cells (Hume, J. R., and A. Uehara. 1986. Journal of General Physiology. 87:833) may be generated by an electrogenic Na/Ca exchanger. Creep currents induced by Na loading were examined over a wide range of membrane potentials. During depolarizing voltage-clamp pulses, outward creep currents were observed, followed by inward creep currents upon the return to the holding potential. During hyperpolarizing voltage-clamp pulses, creep currents of the opposite polarity were observed: inward creep currents were observed during the pulses, followed by outward creep currents upon the return to the holding potential. The current-voltage relations for inward and outward creep currents in response to depolarizing or hyperpolarizing voltage displacements away from the holding potential all intersect the voltage axis at a common potential, which indicates that inward and outward creep currents may have a common reversal potential under equilibrium conditions and may therefore be generated by a common mechanism. Measurements of inward creep currents confirm that voltage displacements away from the holding potential rapidly alter equilibrium conditions. Current-voltage relationships of inward creep currents after depolarizing voltage-clamp pulses are extremely labile and depend critically upon the amplitude and duration of outward creep currents elicited during preceding voltage-clamp pulses. An optical monitor of mechanical activity in single cells revealed (a) a similar voltage dependence for the outward creep currents induced by Na loading and tonic contraction, and (b) a close correlation between the time course of the decay of the inward creep current and the time course of mechanical relaxation. A mathematical model of electrogenic Na/Ca exchange (Mullins, L.J. 1979. Federation Proceedings. 35:2583; Noble, D. 1986. Cardiac Muscle. 171-200) can adequately account for many of the properties of creep currents. It is concluded that creep currents in single frog atrial cells may be attributed to the operation of an electrogenic Na/Ca exchange mechanism.     

Effect of sotalol on transmembrane ionic currents responsible for repolarization in cardiac ventricular myocytes from rabbit and guinea pig

The effects of sotalol, a β-adrenoceptor blocker and class III antiarrhythmic agent, on transmembrane ionic currents were examined in single rabbit and guinea pig ventricular myocytes using whole-cell voltage-clamp techniques. In neither of these species did 60 μM sotalol appreciably effect the inward rectifier, the transient outward or the inward calcium currents. In addition, sotalol did not elicit a slowly inactivating component of the sodium current as did 1 μg/ml veratrine. In guinea pig ventricular myocytes, sotalol also significantly depressed the outward delayed rectifier current. An outward delayed rectifier current was not observed in rabbit ventricular myocytes examined at room temperature; and, under these conditions sotalol did not lengthen action potential duration. Sotalol induced lengthening of cardiac action potential duration can, therefore, be explained by depression the outward delayed rectifier current.     

Whole-cell K+ current activation in response to voltages and carbachol in gastric parietal cells isolated from guinea pig

Summary Patch-clamp studies of whole-cell ionic currents were carried out in parietal cells obtained by collagenase digestion of the gastric fundus of the guinea pig stomach. Applications of positive command pulses induced outward currents. The conductance became progressively augmented with increasing command voltages, exhibiting an outwardly rectifying current-voltage relation. The current displayed a slow time course for activation. In contrast, inward currents were activated upon hyperpolarizing voltage applications at more negative potentials than the equilibrium potential to K⁺ (E _K). The inward currents showed time-dependent inactivation and an inwardly rectifying current-voltage relation. Tail currents elicited by voltage steps which had activated either outward or inward currents reversed at nearE _K, indicating that both time-dependent and voltagegated currents were due to K⁺ conductances. Both outward and inward K⁺ currents were suppressed by extracellular application of Ba²⁺, but little affected by quinine. Tetraethylammonium inhibited the outward current without impairing the inward current, whereas Cs⁺ blocked the inward current but not the outward current. The conductance of inward K⁺ currents, but not outward K⁺ currents, became larger with increasing extracellular K⁺ concentration. A Ca²⁺-mobilizing acid secretagogue, carbachol, and a Ca²⁺ ionophore, ionomycin, brought about activation of another type of outward K⁺ currents and voltage-independent cation currents. Both currents were abolished by cytosolic Ca²⁺ chelation. Quinine preferentially inhibited this K⁺ current. It is concluded that resting parietal cells of the guinea pig have two distinct types of voltage-dependent K⁺ channels, inward rectifier and outward rectifier, and that the cells have Ca²⁺-activated K⁺ channels which might be involved in acid secretion under stimulation by Ca²⁺-mobilizing secretagogues.     

Role of Na-Ca exchange in the action potential changes caused by drive in cardiac myocytes exposed to different Ca2+ loads.

The role of Na-Ca exchange in the membrane potential changes caused by repetitive activity ("drive") was studied in guinea pig single ventricular myocytes exposed to different [Ca2+]o. The following results were obtained. (i) In 5.4 mM [Ca2+]o, the action potentials (APs) gradually shortened during drive, and the outward current during a train of depolarizing voltage clamp steps gradually increased. (ii) The APs shortened more and were followed by a decaying voltage tail during drive in the presence of 5 mM caffeine; the outward current became larger and there was an inward tail current on repolarization during a train of depolarizing steps. (iii) These effects outlasted drive so that immediately after a train of APs, currents were already bigger and, after a train of steps, APs were already shorter. (iv) In 0.54 mM [Ca2+]o, the above effects were much smaller. (v) In high [Ca2+]o APs were shorter and outward currents larger than in low [Ca2+]o. (vi) In 10.8 mM [Ca2+]o, both outward and inward currents during long steps were exaggerated by prior drive, even with steps (+80 and +120 mV) at which there was no apparent inward current identifiable as I(Ca). (vii) In 0.54 mM [Ca2+]o, the time-dependent outward current was small and prior drive slightly increased it. (viii) During long steps, caffeine markedly increased outward and inward tail currents, and these effects were greatly decreased by low [Ca2+]o. (ix) After drive in the presence of caffeine, Ni2+ decreased the outward and inward tail currents. It is concluded that in the presence of high [Ca2+]o drive activates outward and inward Na-Ca exchange currents. During drive, the outward current participates in the plateau shortening and the inward tail current in the voltage tail after the action potential.     

Patch-clamp studies in human macrophages: Single-channel and whole-cell characterization of two K+ conductances

Summary Human peripheral blood monocytes cultured for varying periods of time were studied using whole-cell and single-channel patch-clamp recording techniques. Whole-cell recordings revealed both an outward K current activating at potentials >20 mV and an inwardly rectifying K current present at potentials negative to –60 mV. Tail currents elicited by voltage steps that activated outward current reversed nearE _K, indicating that the outward current was due to a K conductance. TheI–V curve for the macroscopic outward current was similar to the mean single-channelI–V curve for the large conductance (240 pS in symmetrical K) calcium-activated K channel present in these cells. TEA and charybdotoxin blocked the whole-cell outward current and the single-channel current. Excised and cell-attached single-channel data showed that calcium-activated K channels were absent in freshly isolated monocytes but were present in >85% of patches from macrophages cultured for >7 days. Only 35% of the human macrophages cultured for >7 days exhibited whole-cell inward currents. The inward current was blocked by external barium and increased when [K] _o increased. Inward-rectifying single-channel currents with a conductance of 28 pS were present in cells exhibiting inward whole-cell currents. These single-channel currents are similar to those described in detail in J774.1 cells (L.C. McKinney & E.K. Gallin,J. Membrane Biol. 103:41–53, 1988).     

Positive inotropic action of NMDA receptor antagonist (+)-MK801 in rat heart

(+)-MK801, a noncompetitive NMDA receptor antagonist, was reported to exhibit anticonvulsive and neuroprotective activities during the postischemic period. Intravenous administration of (+)-MK801 produced tachycardia in rats, but bradycardia in pigs. We examined the mechanical and electrophysiological effects of (+)-MK801 on rat cardiac tissues. (+)-MK801 dose-dependently increased (3–100 µM) twitch tension in rat atria and ventricular strips. The spontaneous beating rate in rat right atria, however, was dose-dependently decreased by (+)-MK801. The inotropic effect of (+)-MK801 was affected neither by ₁-antagonist (1 µM prazosin) nor by ₁-adrenoceptor antagonist (3 µM atenolol), but significantly by a transient outward K⁺ channel blocker (3 mM 4-aminopyridine). (+)-MK801 did not cause any significant change of intracellular cAMP content. Electrophysiological study in rat ventricular cells revealed that (+)-MK801 concentration-dependently prolonged the action potential duration with a concomitant decrease in the maximum rate of the action potential upstroke (V_max) and an increase in the recovery time constant of V_max. Voltage clamp study showed that (+)-MK801 (3 µM) reduced inward Na⁺ current (I_Na), along with a slowing of its recovery from inactivation and a slight negative shift of its voltage-dependent steady-state inactivation curves. At a much higher concentration (30 µM), (+)-MK801 slightly reduced the amplitude of L-type calcium inward current (I_Ca), although the voltage dependence of its steady-state inactivation was unaffected. For the potassium currents in rat ventricular cells, 3 µM of (+)-MK801 reduced the peak transient outward current (I_to), steady-state outward current (I_ss) and inward current through K₁ channels. The inhibition of I_to was associated with a prominent negative shift in the voltage dependence of its steady-state inactivation curve. The outward current through K₁ channels was unaffected. These results indicate that (+)-MK801 may be a strong I_Na and I_to blocker with some I_Ca blocking activity. The inhibition of I_to and other K⁺ efflux would prolong action potential duration, produce positive inotropic action and contribute to the negative chronotropic effect of (+)-MK801.     

The protoplasmic flow in the myxomycete plasmodium as revealed by a volumetric analysis

Summary The manner of the locomotion of the slime mold,Physarum polycephalum, was shown graphically using a double-chamber volumeter developed by the author. It enabled him to represent in undulating curves every detail of the way in which the slime mold moves on little by little by availing itself of the difference in transport-volume of the endoplasm produced at each repetition of the back and forth streaming.The curve showing the locomotion of the organism pointed out that more than 4 mm³ of protoplasm is sometimes shifted in a direction in one streaming duration. No close relationship is found between the streaming duration and the transport-volume of protoplasm. The intensity of the flow, which may be defined as the volume of protoplasm transported per unit time, can be obtained from the transport-volume curve through its graphical differentiation.     

Postsynaptic mechanisms initiating bursting activity in theHelix pomatia RPal neuron under the influence of an interneuron

Postsynaptic mechanisms of the connection between the interneuron in the visceral ganglion initiating bursting activity in RPal and B7 neurons and these neurons themselves were investigated in the snail (Helix pomatia). Using voltage clamping at the membrane of these cells, stimulation of the interneuron gave rise to a slow inward current with a 2 sec latency; it rose in amplitude as stimulation increased in duration. Reducing the temperature from 25 to 5°C diminished the rise and decay rate of this current with a temperature coefficient of about 10. The current-voltage relationship of the slow inward current was nonlinear, with a maximum of –65 mV. Reducing the concentration of sodium ions in the extracellular fluid increased the amplitude of the current. While hyperpolarization of the burster neuron membrane produced a burst of inward current prior to stimulation, this same hyperpolarization induced a pulse of outward current at the peak of the slow inward current. Stimulating the interneuron is thus thought to activate at least two types of ionic channel in the cell body of the burster neurons: a steady sodium and a voltage- and time-dependent channel for outward current. This process could well be mediated by a biochemical cytoplasmic chain reaction.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 1, pp. 28–36, January–February, 1987.     

Voltage Clamp Studies on the Slow Inward Current during the Excitation of Nitellopsis obtusa

The ionic currents across the plasmalemma of Nitellopsis obtusawere measured in voltage clamp experiments. Depolarization ofthe cell by 30–100 mV from the level of the resting potentialresulted in (1) a rapid inward current, (2) a subsequent slowinward current, and (3) a stationary outward current. The firstcurrent component changed sign at –20 to –30 mV.The second component decreased to a minimum at this clampedlevel. With increasing depolarizing steps some slow transientcurrent component reappeared without changing sign. This transientinward current occurred also when the potential was clampedeither at large depolarizing (+80 mV) or at large hyperpolarizing(–300 mV) potentials. In cases when the slow inward currentcomponent was evident cessation of protoplasmic streaming wasobserved. The ATPase inhibitor dicyclohexylcarbodiimide (DCCD)at a concentration of 2 x 10^–5 M in the external mediuminhibited the slow transient inward current without affectingthe first rapid current component. It is suggested that theirreversible slow transient current component reflects the onsetof some active ion-transport system in the plasmalemma duringcell excitation.     

Calcium currents and calcium-activated potassium currents at the frog motor nerve endings

Intracellular recordings were made of evoked electrical response of the nerve endings during experiments on the frog cutaneous pectoral muscle. A delayed inward current was discovered when superfusing the neuromuscular preparation with a calcium-free solution containing tubocurarine in the response evoked at the nerve endings, using CaCl₂-filled electrodes. This was replaced by the opposite (outward) type of current when 4-aminopyridine was added to the external solution. The outward current was dependent on the calcium concentration at the electrode, decreased after local increase on potassium concentration at the electrode, and disappeared under the effects of cobalt. Local iontophoretic application of tetraethylammonium led to the disappearance of the outward current and the appearance of a powerful and protracted inward current. Similar readings of inward and outward currents were obtained when recording electrical signals using electrodes filled with SrCl₂, BaCl₂, and MgCl₂. It was deduced that the late inward current is carried through voltage-dependent calcium channels and outward delayed current through calcium-activated potassium channels at the nerve terminal. The part played by these currents in transmitter secretion from the motor nerve ending is discussed, together with the relationship between them.S. V. Kurashov Medical Institute, Ministry of Health of the RSFSR. V. I. Ul'yanov-Lenin State University, Kazan'. Translated from Neirofiziologiya, Vol. 19, No. 4, 1987, pp. 467–473, July–August, 1987.     

Ionic currents in avian granulosa cells

Transmembrane ionic currents have been recorded in single granulosa cells from the laying hen using the whole-cell patch-clamp technique. Under voltage-clamp conditions, depolarizing voltage steps evoked currents composed of a fast inactivating inward component and a delayed outward component. The former was activated at voltages more positive than -50 mV and was fully inactivated within 500 ms. It was blocked by D600 (methoxyverapamil) and by cobalt, suggesting that it is a calcium current. The latter displayed inward rectification and did not inactivate during long duration pulses. It was blocked by tetraethylammonium indicating that it is a potassium current. This is the first evidence of the existence of potassium and calcium transmembrane currents in granulosa cells.     

Conserved movement of TMS11 between occluded conformations of LacY and XylE of the major facilitator superfamily suggests a similar hinge‐like mechanism

The Δ‐distance maps can detect local remodeling that is difficult to accurately determine using superimpositions. Transmembrane segments (TMSs) 11 in both LacY and XylE of the major facilitator superfamily uniquely contribute the greatest amount of mobile surface area in the outward‐occluded state and undergo analogous movements. The intracellular part of TMS11 moves away from the C‐terminal domain and into the substrate cavity during the conformational change from the outward‐occluded to the inward‐occluded state. A difference was noted between LacY and XylE when they assumed the inward open state after releasing a substrate to the inside in which TMS11 of LacY moved further into the substrate release space, whereas in XylE, TMS11 slightly retracted into the C‐terminal domain. Independent movement of the N‐terminal half of TMS11 suggests that it is flexible in the middle. Repeat‐swapped homology modeling was used to discover that a loop connecting TMSs 10 and 11 in LacY probably moves during the transition between the unavailable outward‐open state and the outward‐occluded state. TMSs 11 and the other elements displaying a notable domain‐independent movement colocalize with the interdomain linker, suggesting that these elements could drive the alternating access movement between the domain halves. Preliminary evidence indicates that analogous movements occur in other members of the major facilitator superfamily. Proteins 2015; 83:735–745. © 2015 Wiley Periodicals, Inc.