Gating properties of gap junction channels assembled from connexin43 and connexin43 fused with green fluorescent protein.

We used cell lines expressing wild-type connexin43 (Cx43) and Cx43 fused with enhanced green fluorescent protein (Cx43-EGFP) to examine mechanisms of gap junction channel gating. Previously it was suggested that each hemichannel in a cell-cell channel possesses two gates, a fast gate that closes channels to a nonzero conductance or residual state via fast (< approximately 2 ms) transitions and a slow gate that fully closes channels via slow transitions (> approximately 10 ms). Here we demonstrate that transjunctional voltage (V(j)) regulates both gates and that they are operating in series and in a contingent manner in which the state of one gate affects gating of the other. Cx43-EGFP channels lack fast V(j) gating to a residual state but show slow V(j) gating. Both Cx43 and Cx43-EGFP channels exhibit slow gating by chemical uncouplers such as CO(2) and alkanols. Chemical uncouplers do not induce obvious changes in Cx43-EGFP junctional plaques, indicating that uncoupling is not caused by dispersion or internalization of junctional plaques. Similarity of gating transitions during chemical gating and slow V(j) gating suggests that both gating mechanisms share common structural elements. Cx43/Cx43-EGFP heterotypic channels showed asymmetrical V(j) gating with fast transitions between open and residual states only when the Cx43 side was relatively negative. This result indicates that the fast V(j) gate of Cx43 hemichannels closes for relative negativity at its cytoplasmic end.     

Molecular dissection of transjunctional voltage dependence in the connexin-32 and connexin-43 junctions.

Most gap junction channels are sensitive to the voltage difference between the two cellular interiors, termed the transjunctional voltage (V(j)). In several junctions, the conductance transitions induced by V(j) show more than one kinetic component. To elucidate the structural basis of the fast and slow components that characterize the V(j )dependence of connexin-32 (Cx32) and connexin-43 (Cx43) junctions, we created deletions of both connexins, where most of the carboxy-terminal (CT) domain was removed. The wild-type and "tailless" mutants were expressed in paired Xenopus oocytes, and the macroscopic gating properties were analyzed using the dual voltage clamp technique. Truncation of the CT domain of Cx32 and Cx43 abolished the fast mechanism of conductance transitions and induced novel gating properties largely attributable to the slow mechanism of gating. The formation of hybrid junctions comprising wild-type and truncated hemichannels allowed us to infer that the fast and slow components of gating reside in each hemichannel and that both gates close at a negative V(j) on the cytoplasmic side. Thus we conclude that the two kinetic components of V(j)-sensitive conductance are a result of the action of two different gating mechanisms. They constitute separate structures in the Cx32 and Cx43 molecules, the CT domain being an integral part of fast V(j) gating.     

Stochastic 16-state model of voltage gating of gap-junction channels enclosing fast and slow gates

Gap-junction (GJ) channels formed of connexin (Cx) proteins provide a direct pathway for electrical and metabolic cell-cell interaction. Each hemichannel in the GJ channel contains fast and slow gates that are sensitive to transjunctional voltage (Vj). We developed a stochastic 16-state model (S16SM) that details the operation of two fast and two slow gates in series to describe the gating properties of homotypic and heterotypic GJ channels. The operation of each gate depends on the fraction of Vj that falls across the gate (VG), which varies depending on the states of three other gates in series, as well as on parameters of the fast and slow gates characterizing 1), the steepness of each gate's open probability on VG; 2), the voltage at which the open probability of each gate equals 0.5; 3), the gating polarity; and 4), the unitary conductances of the gates and their rectification depending on VG. S16SM allows for the simulation of junctional current dynamics and the dependence of steady-state junctional conductance (gj,ss) on Vj. We combined global coordinate optimization algorithms with S16SM to evaluate the gating parameters of fast and slow gates from experimentally measured gj,ss-Vj dependencies in cells expressing different Cx isoforms and forming homotypic and/or heterotypic GJ channels.     

Chemical gating of gap junction channels; roles of calcium, pH and calmodulin

Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins. Some evidence suggests that low pH(i) affects gating via an increase in [Ca(2+)](i); in turn, Ca(2+) is likely to induce gating by activation of CaM, which may act directly as a gating particle. The effective concentrations of both Ca(2+) and H(+) vary depending on cell type, type of connexin expressed and procedure employed to increase their cytosolic concentrations; however, pH(i) as high as 7.2 and [Ca(2+)](i) as low as 150 nM or lower have been reported to be effective in some cells. Some data suggest that Ca(2+) and H(+) affect gating by acting synergistically, but other data do not support synergism. Chemical gating follows the activation of a slow gate distinct from the fast V(j)-sensitive gate, and there is evidence that the chemical/slow gate is V(j)-sensitive. At the single channel level, the chemical/slow gate closes the channels slowly and completely, whereas the fast V(j) gate closes the channels rapidly and incompletely. At least three molecular models of channel gating have been proposed, but all of them are mostly based on circumstantial evidence.     

Conductance and permeability of the residual state of connexin43 gap junction channels

We used cell lines expressing wild-type connexin43 and connexin43 fused with the enhanced green fluorescent protein (Cx43-EGFP) to examine conductance and perm-selectivity of the residual state of Cx43 homotypic and Cx43/Cx43-EGFP heterotypic gap junction channels. Each hemichannel in Cx43 cell-cell channel possesses two gates: a fast gate that closes channels to the residual state and a slow gate that fully closes channels; the transjunctional voltage (V(j)) closes the fast gate in the hemichannel that is on the relatively negative side. Here, we demonstrate macroscopically and at the single-channel level that the I-V relationship of the residual state rectifies, exhibiting higher conductance at higher V(j)s that are negative on the side of gated hemichannel. The degree of rectification increases when Cl(-) is replaced by Asp(-) and decreases when K(+) is replaced by TEA(+). These data are consistent with an increased anionic selectivity of the residual state. The V(j)-gated channel is not permeable to monovalent positively and negatively charged dyes, which are readily permeable through the fully open channel. These data indicate that a narrowing of the channel pore accompanies gating to the residual state. We suggest that the fast gate operates through a conformational change that introduces positive charge at the cytoplasmic vestibule of the gated hemichannel, thereby producing current rectification, increased anionic selectivity, and a narrowing of channel pore that is largely responsible for reducing channel conductance and restricting dye transfer. Consequently, the fast V(j)-sensitive gating mechanism can serve as a selectivity filter, which allows electrical coupling but limits metabolic communication.     

Voltage-dependent gating of single gap junction channels in an insect cell line.

De novo formation of cell pairs was used to examine the gating properties of single gap junction channels. Two separate cells of an insect cell line (clone C6/36, derived from the mosquito Aedes albopictus) were pushed against each other to provoke formation of gap junction channels. A dual voltage-clamp method was used to control the voltage gradient between the cells (Vj) and measure the intercellular current (Ij). The first sign of channel activity was apparent 4.7 min after cell contact. Steady-state coupling reached after 30 min revealed a conductance of 8.7 nS. Channel formation involved no leak between the intra- and extracellular space. The first opening of a newly formed channel was slow (25-28 ms). Each preparation passed through a phase with only one operational gap junction channel. This period was exploited to examine the single channel properties. We found that single channels exhibit several conductance states with different conductances gamma j; a fully open state (gamma j(main state)), several substates (gamma j(substates)), a residual state (gamma j(residual)) and a closed state (gamma j(closed)). The gamma j(main state) was 375 pS, and gamma j(residual) ranged from 30 to 90 pS. The transitions between adjacent substates were 1/7-1/4 of gamma j(main state). Vj had no effect on gamma j(main state), but slightly affected gamma j (residual). The lj transitions involving gamma j(closed) were slow (15-60 ms), whereas those not involving gamma j(closed) were fast (< 2 ms). An increase in Vj led to a decrease in open channel probability. Depolarization of the membrane potential (Vm) increased the incidence of slow transitions leading to gamma j(closed). We conclude that insect gap junctions possess two gates, a fast gate controlled by Vj and giving rise to gamma j(substates) and gamma j(residual), and a slow gate sensitive to Vm and able to close the channel completely.     

Correlative studies of gating in Cx46 and Cx50 hemichannels and gap junction channels

Transjunctional voltage (V(j)) gating of gap junction (GJ) channels formed of connexins has been proposed to occur by gating of the component hemichannels. We took advantage of the ability of Cx46 and Cx50 to function as unapposed hemichannels to identify gating properties intrinsic to hemichannels and how they contribute to gating of GJ channels. We show that Cx46 and Cx50 hemichannels contain two distinct gating mechanisms that generate reductions in conductance for both membrane polarities. At positive voltages, gating is similar in Cx46 and Cx50 hemichannels, primarily showing increased transitioning to long-lived substates. At negative voltages, Cx46 currents deactivate completely and the underlying single hemichannels exhibit transitions to a fully closed state. In contrast, Cx50 currents do not deactivate completely at negative voltages and the underlying single hemichannels predominantly exhibit transitions to various substates. Transitions to a fully closed state occur, but are infrequent. In the respective GJ channels, both forms of gating contribute to the reduction in conductance by V(j). However, examination of gating of mutant hemichannels and GJ channels in which the Asp at position 3 was replaced with Asn (D3N) showed that the positive hemichannel gate predominantly closes Cx50 GJs, whereas the negative hemichannel gate predominantly closes Cx46 GJs in response to V(j). We also report, for the first time, single Cx50 hemichannels in oocytes to be inwardly rectifying, high conductance channels (gamma = 470 pS). The antimalarial drug mefloquine, which selectively blocks Cx50 and not Cx46 GJs, shows the same selectivity in Cx50 and Cx46 hemichannels indicating that the actions of such uncoupling agents, like voltage gating, are intrinsic hemichannel properties.     

Stochastic Model of Gap Junctions Exhibiting Rectification and Multiple Closed States of Slow Gates

Gap-junction (GJ) channels formed from connexin (Cx) proteins provide direct pathways for electrical and metabolic cell-cell communication. Earlier, we developed a stochastic 16-state model (S16SM) of voltage gating of the GJ channel containing two pairs of fast and slow gates, each operating between open (o) and closed (c) states. However, experimental data suggest that gates may in fact contain two or more closed states. We developed a model in which the slow gate operates according to a linear reaction scheme, o↔c₁↔c₂, where c₁ and c₂ are initial-closed and deep-closed states that both close the channel fully, whereas the fast gate operates between the open state and the closed state and exhibits a residual conductance. Thus, we developed a stochastic 36-state model (S36SM) of GJ channel gating that is sensitive to transjunctional voltage (V_j). To accelerate simulation and eliminate noise in simulated junctional conductance (g_j) records, we transformed an S36SM into a Markov chain 36-state model (MC36SM) of GJ channel gating. This model provides an explanation for well-established experimental data, such as delayed g_j recovery after V_j gating, hysteresis of g_j-V_j dependence, and the low ratio of functional channels to the total number of GJ channels clustered in junctional plaques, and it has the potential to describe chemically mediated gating, which cannot be reflected using an S16SM. The MC36SM, when combined with global optimization algorithms, can be used for automated estimation of gating parameters including probabilities of c₁↔c₂ transitions from experimental g_j-time and g_j-V_j dependencies.     

Gating properties of heterotypic gap junction channels formed of connexins 40, 43, and 45

Connexins (Cxs) 40, 43, and 45 are expressed in many different tissues, but most abundantly in the heart, blood vessels, and the nervous system. We examined formation and gating properties of heterotypic gap junction (GJ) channels assembled between cells expressing wild-type Cx40, Cx43, or Cx45 and their fusion forms tagged with color variants of green fluorescent protein. We show that these Cxs, with exception of Cxs 40 and 43, are compatible to form functional heterotypic GJ channels. Cx40 and Cx43 hemichannels are unable or effectively impaired in their ability to dock and/or assemble into junctional plaques. When cells expressing Cx45 contacted those expressing Cx40 or Cx43 they readily formed junctional plaques with cell-cell coupling characterized by asymmetric junctional conductance dependence on transjunctional voltage, V(j). Cx40/Cx45 heterotypic GJ channels preferentially exhibit V(j)-dependent gating transitions between open and residual states with a conductance of approximately 42 pS; transitions between fully open and closed states with conductance of approximately 52 pS in magnitude occur at substantially lower ( approximately 10-fold) frequency. Cx40/Cx45 junctions demonstrate electrical signal transfer asymmetry that can be modulated between unidirectional and bidirectional by small changes in the difference between holding potentials of the coupled cells. Furthermore, both fast and slow gating mechanisms of Cx40 exhibit a negative gating polarity.     

Calcium-dependent gating of MthK, a prokaryotic potassium channel

MthK is a calcium-gated, inwardly rectifying, prokaryotic potassium channel. Although little functional information is available for MthK, its high-resolution structure is used as a model for eukaryotic Ca(2+)-dependent potassium channels. Here we characterize in detail the main gating characteristics of MthK at the single-channel level with special focus on the mechanism of Ca(2+) activation. MthK has two distinct gating modes: slow gating affected mainly by Ca(2+) and fast gating affected by voltage. Millimolar Ca(2+) increases MthK open probability over 100-fold by mainly increasing the frequency of channel opening while leaving the opening durations unchanged. The Ca(2+) dose-response curve displays an unusually high Hill coefficient (n = approximately 8), suggesting strong coupling between Ca(2+) binding and channel opening. Depolarization affects both the fast gate by dramatically reducing the fast flickers, and to a lesser extent, the slow gate, by increasing MthK open probability. We were able to capture the mechanistic features of MthK with a modified MWC model.     

Two distinct gating mechanisms in gap junction channels: CO2-sensitive and voltage-sensitive.

The chemical gating of single-gap junction channels was studied by the dual whole-cell voltage-clamp method in HeLa cells transfected with connexin43 (HeLa43) and in fibroblasts from sciatic nerves. Junctional current (Ij), single-channel conductance, and Ij kinetics were studied in cell pairs during CO2 uncoupling and recoupling at small transjunctional voltages (Vj < 35 mV: Vj gating absent) and at high Vj (Vj > 40 mV: Vj gating strongly activated). In the absence of Vj gating, CO2 exclusively caused Ij slow transitions from open to closed channel states (mean transition time: approximately 10 ms), corresponding to a single-channel conductance of approximately 120 pS. At Vj > 40 mV, Vj gating induced fast Ij flickering between open, gamma j(main state), and residual, gamma j(residual), states (transition time: approximately 2 ms). The ratio gamma j(main state)/gamma j(residual) was approximately 4-5. No obvious correlation between Ij fast flickering and CO2 treatment was noticed. At high Vj, in addition to slow Ij transitions between open and closed states, CO2 induced slow transitions between residual and closed states. During recoupling, each channel reopened by a slow transition (mean transition time: approximately 10 ms) from closed to open state (rarely from closed to residual state). Fast Ij flickering between open and residual states followed. The data are in agreement with the hypothesis that gap junction channels possess two gating mechanisms, and indicate that CO2 induces channel gating exclusively by the slow gating mechanism.     

Conformational change opening the CFTR chloride channel pore coupled to ATP-dependent gating

Opening and closing of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel are controlled by ATP binding and hydrolysis by its nucleotide binding domains (NBDs). This is presumed to control opening of a single "gate" within the permeation pathway, however, the location of such a gate has not been described. We used patch clamp recording to monitor access of cytosolic cysteine reactive reagents to cysteines introduced into different transmembrane (TM) regions in a cysteine-less form of CFTR. The rate of modification of Q98C (TM1) and I344C (TM6) by both [2-sulfonatoethyl] methanethiosulfonate (MTSES) and permeant Au(CN)(2)(-) ions was reduced when ATP concentration was reduced from 1mM to 10μM, and modification by MTSES was accelerated when 2mM pyrophosphate was applied to prevent channel closure. Modification of K95C (TM1) and V345C (TM6) was not affected by these manoeuvres. We also manipulated gating by introducing the mutations K464A (in NBD1) and E1371Q (in NBD2). The rate of modification of Q98C and I344C by both MTSES and Au(CN)(2)(-) was decreased by K464A and increased by E1371Q, whereas modification of K95C and V345C was not affected. These results suggest that access from the cytoplasm to K95 and V345 is similar in open and closed channels. In contrast, modifying ATP-dependent channel gating alters access to Q98 and I344, located further into the pore. We propose that ATP-dependent gating of CFTR is associated with the opening and closing of a gate within the permeation pathway at the level of these pore-lining amino acids.     

The voltage gates of connexin channels are sensitive to CO(2)

Cx45 channel sensitivity to CO(2), transjunctional voltage (V(j)) and inhibition of calmodulin (CaM) expression was tested in oocytes by dual voltage-clamp. Cx45 channels are very sensitive to V(j) and close preferentially by the slow gate, likely the same as the chemical gate. With CO(2)-induced drop in junctional conductance (G(j)), the speed of V(j)-dependent inactivation of junctional current (I(j)) and V(j) sensitivity increased. With 40 mV V(j), the tau of single exponential I(j) decay reversibly decreased by approximately 40% with CO(2), and G(j steady state)/G(j peak) decreased multiphasically, indicating that kinetics and V(j) sensitivity of chemical/slow-V(j) gating are altered by changes in [H(+)](i) and/or [Ca(2+)](i). With 15 min exposure to CO(2), G(j) dropped to 0% in controls and by approximately 17% following CaM expression inhibition; similarly, V(j) sensitivity decreased significantly. This indicates that the speed and sensitivity of V(j)-dependent inactivation of Cx45 channels are increased by CO(2), and that CaM plays a role in gating. Cx32 channels behaved similarly, but the drop in both G(j steady state)/G(j peak) and tau with CO(2) matched more closely that of G(j peak). In contrast, sensitivity and speed of V(j) gating of Cx40 and Cx26 channels decreased, rather than increased, with CO(2) application.     

Modal gating of human CaV2.1 (P/Q-type) calcium channels: I. The slow and the fast gating modes and their modulation by beta subunits

The single channel gating properties of human CaV2.1 (P/Q-type) calcium channels and their modulation by the auxiliary beta1b, beta2e, beta3a, and beta4a subunits were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing human CaV2.1 channels. These calcium channels showed a complex modal gating, which is described in this and the following paper (Fellin, T., S. Luvisetto, M. Spagnolo, and D. Pietrobon. 2004. J. Gen. Physiol. 124:463-474). Here, we report the characterization of two modes of gating of human CaV2.1 channels, the slow mode and the fast mode. A channel in the two gating modes differs in mean closed times and latency to first opening (both longer in the slow mode), in voltage dependence of the open probability (larger depolarizations are necessary to open the channel in the slow mode), in kinetics of inactivation (slower in the slow mode), and voltage dependence of steady-state inactivation (occurring at less negative voltages in the slow mode). CaV2.1 channels containing any of the four beta subtypes can gate in either the slow or the fast mode, with only minor differences in the rate constants of the transitions between closed and open states within each mode. In both modes, CaV2.1 channels display different rates of inactivation and different steady-state inactivation depending on the beta subtype. The type of beta subunit also modulates the relative occurrence of the slow and the fast gating mode of CaV2.1 channels; beta3a promotes the fast mode, whereas beta4a promotes the slow mode. The prevailing mode of gating of CaV2.1 channels lacking a beta subunit is a gating mode in which the channel shows shorter mean open times, longer mean closed times, longer first latency, a much larger fraction of nulls, and activates at more positive voltages than in either the fast or slow mode.     

Molecular Analysis of Potential Hinge Residues in the Inactivation Gate of Brain Type IIA Na+ Channels

During inactivation of Na⁺ channels, the intracellular loop connecting domains III and IV is thought to fold into the channel protein and occlude the pore through interaction of the hydrophobic motif isoleucine-phenylalanine-methionine (IFM) with a receptor site. We have searched for amino acid residues flanking the IFM motif which may contribute to formation of molecular hinges that allow this motion of the inactivation gate. Site-directed mutagenesis of proline and glycine residues, which often are components of molecular hinges in proteins, revealed that G1484, G1485, P1512, P1514, and P1516 are required for normal fast inactivation. Mutations of these residues slow the time course of macroscopic inactivation. Single channel analysis of mutations G1484A, G1485A, and P1512A showed that the slowing of macroscopic inactivation is produced by increases in open duration and latency to first opening. These mutant channels also show a higher probability of entering a slow gating mode in which their inactivation is further impaired. The effects on gating transitions in the pathway to open Na⁺ channels indicate conformational coupling of activation to transitions in the inactivation gate. The results are consistent with the hypothesis that these glycine and proline residues contribute to hinge regions which allow movement of the inactivation gate during the inactivation process of Na⁺ channels.     

The cooperative voltage sensor motion that gates a potassium channel

The four arginine-rich S4 helices of a voltage-gated channel move outward through the membrane in response to depolarization, opening and closing gates to generate a transient ionic current. Coupling of voltage sensing to gating was originally thought to operate with the S4s moving independently from an inward/resting to an outward/activated conformation, so that when all four S4s are activated, the gates are driven to open or closed. However, S4 has also been found to influence the cooperative opening step (Smith-Maxwell et al., 1998a), suggesting a more complex mechanism of coupling. Using fluorescence to monitor structural rearrangements in a Shaker channel mutant, the ILT channel (Ledwell and Aldrich, 1999), that energetically isolates the steps of activation from the cooperative opening step, we find that opening is accompanied by a previously unknown and cooperative movement of S4. This gating motion of S4 appears to be coupled to the internal S6 gate and to two forms of slow inactivation. Our results suggest that S4 plays a direct role in gating. While large transmembrane rearrangements of S4 may be required to unlock the gating machinery, as proposed before, it appears to be the gating motion of S4 that drives the gates to open and close.     

Slow Inactivation in Shaker K Channels Is Delayed by Intracellular Tetraethylammonium

After removal of the fast N-type inactivation gate, voltage-sensitive Shaker (Shaker IR) K channels are still able to inactivate, albeit slowly, upon sustained depolarization. The classical mechanism proposed for the slow inactivation observed in cell-free membrane patches—the so called C inactivation—is a constriction of the external mouth of the channel pore that prevents K⁺ ion conduction. This constriction is antagonized by the external application of the pore blocker tetraethylammonium (TEA). In contrast to C inactivation, here we show that, when recorded in whole Xenopus oocytes, slow inactivation kinetics in Shaker IR K channels is poorly dependent on external TEA but severely delayed by internal TEA. Based on the antagonism with internally or externally added TEA, we used a two-pulse protocol to show that half of the channels inactivate by way of a gate sensitive to internal TEA. Such gate had a recovery time course in the tens of milliseconds range when the interpulse voltage was −90 mV, whereas C-inactivated channels took several seconds to recover. Internal TEA also reduced gating charge conversion associated to slow inactivation, suggesting that the closing of the internal TEA-sensitive inactivation gate could be associated with a significant amount of charge exchange of this type. We interpreted our data assuming that binding of internal TEA antagonized with U-type inactivation (Klemic, K.G., G.E. Kirsch, and S.W. Jones. 2001. Biophys. J. 81:814–826). Our results are consistent with a direct steric interference of internal TEA with an internally located slow inactivation gate as a “foot in the door” mechanism, implying a significant functional overlap between the gate of the internal TEA-sensitive slow inactivation and the primary activation gate. But, because U-type inactivation is reduced by channel opening, trapping the channel in the open conformation by TEA would also yield to an allosteric delay of slow inactivation. These results provide a framework to explain why constitutively C-inactivated channels exhibit gating charge conversion, and why mutations at the internal exit of the pore, such as those associated to episodic ataxia type I in hKv1.1, cause severe changes in inactivation kinetics.     

Electrostatic control and chloride regulation of the fast gating of ClC-0 chloride channels

The opening and closing of chloride (Cl-) channels in the ClC family are thought to tightly couple to ion permeation through the channel pore. In the prototype channel of the family, the ClC-0 channel from the Torpedo electric organ, the opening-closing of the pore in the millisecond time range known as "fast gating" is regulated by both external and internal Cl- ions. Although the external Cl- effect on the fast-gate opening has been extensively studied at a quantitative level, the internal Cl- regulation remains to be characterized. In this study, we examine the internal Cl- effects and the electrostatic controls of the fast-gating mechanism. While having little effect on the opening rate, raising [Cl-]i reduces the closing rate (or increases the open time) of the fast gate, with an apparent affinity of >1 M, a value very different from the one observed in the external Cl- regulation on the opening rate. Mutating charged residues in the pore also changes the fast-gating properties-the effects are more prominent on the closing rate than on the opening rate, a phenomenon similar to the effect of [Cl-]i on the fast gating. Thus, the alteration of fast-gate closing by charge mutations may come from a combination of two effects: a direct electrostatic interaction between the manipulated charge and the negatively charged glutamate gate and a repulsive force on the gate mediated by the permeant ion. Likewise, the regulations of internal Cl- on the fast gating may also be due to the competition of Cl- with the glutamate gate as well as the overall more negative potential brought to the pore by the binding of Cl-. In contrast, the opening rate of the fast gate is only minimally affected by manipulations of [Cl-]i and charges in the inner pore region. The very different nature of external and internal Cl- regulations on the fast gating thus may suggest that the opening and the closing of the fast gate are not microscopically reversible processes, but form a nonequilibrium cycle in the ClC-0 fast-gating mechanism.     

Structural determinants of lateral gate opening in the protein translocon

The heterotrimeric SecY/Sec61 complex is a protein-conducting channel that provides a passage for proteins across the membrane as well as a means to integrate nascent proteins into the membrane. While the first function is common among membrane protein channels and transporters, the latter is unique. Insertion of nascent membrane proteins, one transmembrane segment at a time, by SecY likely occurs through a lateral gate in the channel. Molecular dynamics simulations have been used to investigate the mechanism of gate opening. Opening and closing the gate under different conditions allowed us to identify structural elements that resist opening as well as those that aid closure. SecE, considered to act as a clamp keeping the lateral gate closed, was found to play no such role. Loosening of the plug by lateral gate opening, a potential step in channel gating, was also observed. The simulations revealed that lipids on time scales of up to 1 micros do not flood channels with an open lateral gate.