Role of the S4 segment in a voltage-dependent calcium-sensitive potassium (hSlo) channel

We investigated the role of individual charged residues of the S4 region of a MaxiK channel (hSlo) in channel gating. We measured macroscopic currents induced by wild type (WT) and point mutants of hSlo in inside-out membrane patches of Xenopus laevis oocytes. Of all the residues tested, only neutralizations of Arg-210 and Arg-213 were associated with a reduction in the number of gating charges as determined using the limiting slope method. Channel activation in WT and mutant channels was interpreted using an allosteric model. Mutations R207Q, R207E, and R210N facilitated channel opening in the absence of Ca2+; however, this facilitation was not observed in the channels Ca2+-bound state. Mutation R213Q behaved similarly to the WT channel in the absence of Ca2+, but Ca2+ was unable to stabilize the open state to the same extent as it does in the WT. Mutations R207Q, R207E, R210N, and R213Q reduced the coupling between Ca2+ binding and channel opening when compared with the WT. Mutations L204R, L204H, Q216R, E219Q, and E219K in the S4 domain showed a similar phenotype to the WT channel. We conclude that the S4 region in the hSlo channel is part of the voltage sensor and that only two charged amino acid residues in this region (Arg-210 and Arg-213) contribute to the gating valence of the channel.

Cut-open oocyte voltage-clamp technique

In this article, we described the use and limitations of the COVG technique. The advantages of the present method as follows: 1. High-frequency response and low noise recording (24-microseconds time constant and 1.2-nA rms at 5 kHz). This allows the accurate description of small gating currents and fast activation or deactivation of ionic currents to be adequately resolved. Currents up to 20-30 microA can be adequately clamped. 2. Stable recording conditions lasting for several hours. Even though this technique has internal access, rundown is minimized compared to excised patches. 3. Access to the cell interior. The intracellular medium can be exchanged with various solutions. This was of invaluable help in recording K+ gating currents, an experimental condition that required the complete blockade of all ionic currents. This advantage will make the present method suitable for the study of channel modulation by second messengers and drugs. In addition, channel selectivity properties can be defined by ion substitution experiments. 4. Channel localization. A study of the membrane compartmentalization of channels can be easily obtained with this method, since it allows the voltage clamping of different regions of the oocyte surface.

Structures and functions of calcium channel beta subunits

Calcium channel beta subunits have profound effects on how alpha1 subunits perform. In this article we summarize our present knowledge of the primary structures of beta subunits as deduced from cDNAs and illustrate their different properties. Upon co-expression with alpha1 subunits, the effects of beta subunits vary somewhat between L-type and non-L-type channels mostly because the two types of channels have different responses to voltage which are affected by beta subunits, such as long-lasting prepulse facilitation of alpha1C (absent in alpha1E) and inhibition by G protein betagamma dimer of alpha1E, absent in alpha1C. One beta subunit, a brain beta2a splice variant that is palmitoylated, has several effects not seen with any of the others, and these are due to palmitoylation. We also illustrate the finding that functional expression of alpha1 in oocytes requires a beta subunit even if the final channel shows no evidence for its presence. We propose two structural models for Ca2+ channels to account for "alpha1 alone" channels seen in cells with limited beta subunit expression. In one model, beta dissociates from the mature alpha1 after proper folding and membrane insertion. Regulated channels seen upon co-expression of high levels of beta would then have subunit composition alpha1beta. In the other model, the "chaperoning" beta remains associated with the mature channel and "alpha1 alone" channels would in fact be alpha1beta channels. Upon co-expression of high levels of beta the regulated channels would have composition [alpha1beta]beta.

Pore accessibility during C-type inactivation in Shaker K+ channels

Shaker K+ channels inactivate through two distinct molecular mechanisms: N-type, which involves the N-terminal domain and C-type that appears to involve structural modifications at the external mouth of the channel. We have tested pore accessibility of the Shaker K+ channel during C-type inactivation using Ba2+ as a probe. We determined that external Ba2+ binds to C-type inactivated channels forming an extremely stable complex; i.e. there is Ba2+ trapping by C-type inactivated channels. The structural changes Shaker channels undergo during C-type inactivation create high energy barriers that hinder Ba2+ exit to either the extracellular solution or to the intracellular solution.

Functional coupling between human E-type Ca2+ channels and mu opioid receptors expressed in Xenopus oocytes

Neuronal alpha1E Ca2+ channels were expressed in Xenopus laevis oocytes alone and in combination with the mu opioid receptor. Macroscopic currents were recorded under voltage clamp conditions. The stimulation of the morphine receptor by the synthetic [D-Ala2,N-Me-Phe4,Gly-ol5] enkephalin (DAMGO) produced a 20% reduction in the alpha1E ionic current. This effect was associated with a large change in the decay phase of the Ba2+ current. The effect of 1 microM DAMGO was fully antagonized by the universal mu opioid receptor antagonist naloxone and by the selective antagonist beta-funaltrexamine. The ionic current inhibition induced by DAMGO was partially recovered by preceding strong depolarizations. The injection of the catalytic subunit of pertussis toxin (A-protomer) abolished the effect of DAMGO, suggesting the involvement of a GTP binding protein in the alpha1E modulation. The coexpression of the regulatory beta2a Ca2a channel subunit, together with the alpha1E subunit and the mu opioid receptor, prevented the reduction of the ionic current following the receptor stimulation with DAMGO, whereas the coexpression with the beta3 subunit reduced by approximately 50% the modulatory effect of DAMGO. The effect produced by the stimulation of the opioid receptor could be mimicked by coexpressing the alpha1E channel with the G-protein betagamma subunits.

Fast inactivation in Shaker K+ channels. Properties of ionic and gating currents

Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the installation and recovery of the fast inactivation of ionic current with changes in the kinetics of gating current known as "charge immobilization" (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567-590.). Shaker H4 W434F gating currents are very similar to those of the conducting clone recorded in potassium-free solutions. This mutant channel allows the recording of the total gating charge return, even when returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating currents decay phase at -90 mV return potential changed from a single fast component to at least two components, the slower requiring approximately 200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries are similar at -120 and -90 mV. In contrast, at higher potentials (-70 and -50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot account for all our data, which strongly support the existence of "parallel" inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH2-terminus region.

Modulation of human neuronal alpha 1E-type calcium channel by alpha 2 delta-subunit

Calcium channels are composed of a pore-forming subunit, alpha 1, and at least two auxiliary subunits, beta- and alpha 2 delta-subunits. It is well known that beta-subunits regulate most of the properties of the channel. The function of alpha 2 delta-subunit is less understood. In this study, the effects of the calcium channel alpha 2 delta-subunit on the neuronal alpha 1E voltage-gated calcium channel expressed in Xenopus oocytes was investigated without and with simultaneous coexpression of either the beta 1b- or the beta 2a-subunit. Most aspects of alpha 1E function were affected by alpha 2 delta. Thus alpha 2 delta caused a shift in the current-voltage and conductance-voltage curves toward more positive potentials and accelerated activation, deactivation, and the installation of the inactivation process. In addition, the efficiency with which charge movement is coupled to pore opening assessed by determining ratios of limiting conductance to limiting charge movement was decreased by alpha 2 delta by factors that ranged from 1.6 (P < 0.01) for alpha 1E-channels to 3.0 (P < 0.005) for alpha 1E beta 1b-channels. These results indicate that alpha 2 delta facilitates the expression and the maturation of alpha 1E-channels and converts these channels into molecules responding more rapidly to voltage.

Unique regulatory properties of the type 2a Ca2+ channel beta subunit caused by palmitoylation

Beta subunits of voltage-gated Ca2+ channels are encoded in four genes and display additional molecular diversity because of alternative splicing. At the functional level, all forms are very similar except for beta2a, which differs in that it does not support prepulse facilitation of alpha1C Ca2+ channels, inhibits voltage-induced inactivation of neuronal alpha1E Ca2+ channels, and is more effective in blocking inhibition of alpha1E channels by G protein-coupled receptors. We show that the distinguishing properties of beta2a, rather than interaction with a distinct site of alpha1, are because of the recently described palmitoylation of cysteines in positions three and four, which also occurs in the Xenopus oocyte. Essentially, all of the distinguishing features of beta2a were lost in a mutant that could not be palmitoylated [beta2a(Cys3,4Ser)]. Because protein palmitoylation is a dynamic process, these findings point to the possibility that regulation of palmitoylation may contribute to activity-dependent neuronal and synaptic plasticity. Evidence is presented that there may exist as many as three beta2 splice variants differing only in their N-termini.

Effect of bay K 8644 (-) and the beta2a subunit on Ca2+-dependent inactivation in alpha1C Ca2+ channels

Ca2+ currents recorded from Xenopus oocytes expressing only the alpha1C pore-forming subunit of the cardiac Ca2+ channel show Ca2+-dependent inactivation with a single exponential decay. This current-dependent inactivation is not detected for inward Ba2+ currents in external Ba2+. Facilitation of pore opening speeds up the Ca2+-dependent inactivation process and makes evident an initial fast rate of decay. Facilitation can be achieved by (a) coexpression of the beta2a subunit with the alpha1C subunit, or (b) addition of saturating Bay K 8644 (-) concentration to alpha1C channels. The addition of Bay K 8644 (-) to alpha1Cbeta2a channels makes both rates of inactivation faster. All these maneuvers do not induce inactivation in Ba2+ currents in our expression system. These results support the hypothesis of a mechanism for the Ca2+-dependent inactivation process that is sensitive to both Ca2+ flux (single channel amplitude) and open probability. We conclude that the Ca2+ site for inactivation is in the alpha1C pore-forming subunit and we propose a kinetic model to account for the main features of alpha1Cbeta2a Ca2+ currents.

Long lasting facilitation of the rabbit cardiac Ca2+ channel: correlation with the coupling efficiency between charge movement and pore opening

Facilitation of Ca2+ entry into cells enhances Ca(2+)-activated events such as transmitter release and stimulation of second messenger systems. We have found a long lasting prepulse facilitation (up to 3-fold) of the cardiac Ca2+ channel alpha1Cbeta1b that lasts for tens of seconds without altering current kinetics. The voltage- and time-dependence of the installation of facilitation was characterized as well as the time- and use-dependence of the decay of facilitation. The degree of facilitation was correlated with the coupling efficiency between the charge movement and pore opening channels that were poorly coupled prior to facilitation exhibited the largest facilitation.

Facilitation by the beta2a subunit of pore openings in cardiac Ca2+ channels

1. Single channel recordings were performed on the cardiac calcium channel (alpha1C) in order to study the effect of coexpression of the accessory beta2a subunit. On-cell patch clamp recordings were performed after expression of these channels in Xenopus oocytes. 2. The alpha1C subunit, when expressed alone, had similar single channel properties to native cardiac channels. Slow transitions between low and high open probability (Po) gating modes were found as well as fast gating transitions between the open and closed states. 3. Coexpression of the beta2a subunit caused changes in the fast gating during high Po mode. In this mode, open time distributions reveal at least three open states and the beta2a subunit favours the occupancy of the longest, 10-15 ms open state. No effect of the beta2a subunit was found when the channel was gating in the low Po mode. 4. Slow gating transitions were also affected by the beta2a subunit. The high Po mode was maintained for the duration of the depolarizing pulse in the presence of the beta2a subunit; while the alpha1C channel when expressed alone, frequently switched into and out of the high Po mode during the course of a sweep. 5. The beta2a subunit also affected mode switching that occurred between sweeps. Runs analysis revealed that the alpha1C subunit has a tendency toward non-random mode switching. The beta2a subunit increased this tendency. A chi2 analysis of contingency tables indicated that the beta2a subunit caused the alpha1C channel to gain 'intrinsic memory', meaning that the mode of a given sweep can be non-independent of the mode of the previous sweep. 6. We conclude that the beta2a subunit causes changes to the alpha1C channel in both its fast and slow gating behaviour. The beta2a subunit alters fast gating by facilitating movement of the channel into an existing open state. Additionally, the beta2a subunit decreases the slow switching between low and high Po modes.

Ionic currents underlying HTRP3 mediated agonist-dependent Ca2+ influx in stably transfected HEK293 cells

hTrp3 is a human homologue of the Drosophila gene responsible for a transient receptor potential (trp) mutation. When stably expressed in HEK293 cells, hTrp3 formed ion channels that were active under resting conditions but could be further stimulated by carbachol or ATP via endogenous muscarinic or purinergic receptors, respectively. Agonist evoked currents reversed polarity near 0 mV in physiological ionic conditions and were associated with a significant increase in the current variance. These results suggest the involvement of a non-selective cation channel with relatively large unitary amplitude. Consistent with this, resolved unitary events had a conductance of approximately 60 pS in the negative voltage range and an extrapolated reversal potential near 0 mV.

Voltage-dependent proton transport by the voltage sensor of the Shaker K+ channel

In voltage-dependent ion channels, pore opening is initiated by electrically driven movements of charged residues, and this movement generates a gating current. To examine structural rearrangements in the Shaker K+ channel, basic residues R365 and R368 in the S4 segment were replaced with histidine, and gating currents were recorded. Changes in gating charge displacement with solvent pH reveal voltage-dependent changes in exposure of the histidine to solvent protons. This technique directly monitors accessibility changes during gating, probes the environment even in confined locations, and introduces minimal interference of gating charge motion. The results indicate that charges 365 and 368 traverse the entire electric field during gating. The remarkable implication of the successive exposure of histidine to each side of the membrane is that in a pH gradient, the voltage sensor transports protons.

Cloning and expression of a novel mammalian homolog of Drosophila transient receptor potential (Trp) involved in calcium entry secondary to activation of receptors coupled by the Gq class of G protein

Hormonal stimulation of Gq-protein coupled receptors triggers Ca2+ mobilization from internal stores. This is followed by a Ca2+ entry through the plasma membrane. Drosophila Trp and Trpl proteins have been implicated in Ca2+ entry and three mammalian homologues of Drosophila Trp/Trpl, hTrp1, hTrp3 and bTrp4 (also bCCE) have been cloned and expressed. Using mouse brain RNA as template, we report here the polymerase chain reaction-based cloning and functional expression of a novel Trp, mTrp6. The cDNA encodes a protein of 930 amino acids, the sequence of which is 36.8, 36.3, 43.1, 38.6, and 74. 1% identical to Drosophila Trp and Trpl, bovine Trp4, and human Trp1 and Trp3, respectively. Transient expression of mTrp6 in COS.M6 cells by transfection of the full-length mTrp6 cDNA increases Ca2+ entry induced by stimulation of co-transfected M5 muscarinic acetylcholine receptor with carbachol (CCh), as seen by dual wavelength fura 2 fluorescence ratio measurements. The mTrp6-mediated increase in Ca2+ entry activity was blocked by SKF-96365 and La3+. Ca2+ entry activity induced by thapsigargin was similar in COS cells transfected with or without the mTrp6 cDNA. The thapsigargin-stimulated Ca2+ entry could not be further stimulated by CCh in control cells but was markedly increased in mTrp6-transfected cells. Records of whole cell transmembrane currents developed in response to voltage ramps from -80 to +40 mV in control HEK cells and HEK cells stably expressing mTrp6 revealed the presence of a muscarinic receptor responsive non-selective cation conductance in Trp6 cells that was absent in control cells. Our data support the hypothesis that mTrp6 encodes an ion channel subunit that mediates Ca2+ entry stimulated by a G-protein coupled receptor, but not Ca2+ entry stimulated by intracellular Ca2+ store depletion.

Correlation between charge movement and ionic current during slow inactivation in Shaker K+ channels

Prolonged depolarization induces a slow inactivation process in some K+ channels. We have studied ionic and gating currents during long depolarizations in the mutant Shaker H4-Delta(6-46) K+ channel and in the nonconducting mutant (Shaker H4-Delta(6-46)-W434F). These channels lack the amino terminus that confers the fast (N-type) inactivation (Hoshi, T., W.N. Zagotta, and R.W. Aldrich. 1991. Neuron. 7:547-556). Channels were expressed in oocytes and currents were measured with the cut-open-oocyte and patch-clamp techniques. In both clones, the curves describing the voltage dependence of the charge movement were shifted toward more negative potentials when the holding potential was maintained at depolarized potentials. The evidences that this new voltage dependence of the charge movement in the depolarized condition is associated with the process of slow inactivation are the following: (a) the installation of both the slow inactivation of the ionic current and the inactivation of the charge in response to a sustained 1-min depolarization to 0 mV followed the same time course; and (b) the recovery from inactivation of both ionic and gating currents (induced by repolarizations to -90 mV after a 1-min inactivating pulse at 0 mV) also followed a similar time course. Although prolonged depolarizations induce inactivation of the majority of the channels, a small fraction remains non-slow inactivated. The voltage dependence of this fraction of channels remained unaltered, suggesting that their activation pathway was unmodified by prolonged depolarization. The data could be fitted to a sequential model for Shaker K+ channels (Bezanilla, F., E. Perozo, and E. Stefani. 1994. Biophys. J. 66:1011-1021), with the addition of a series of parallel nonconducting (inactivated) states that become populated during prolonged depolarization. The data suggest that prolonged depolarization modifies the conformation of the voltage sensor and that this change can be associated with the process of slow inactivation.

Direct interaction of gbetagamma with a C-terminal gbetagamma-binding domain of the Ca2+ channel alpha1 subunit is responsible for channel inhibition by G protein-coupled receptors

Several classes of voltage-gated Ca2+ channels (VGCCs) are inhibited by G proteins activated by receptors for neurotransmitters and neuromodulatory peptides. Evidence has accumulated to indicate that for non-L-type Ca2+ channels the executing arm of the activated G protein is its betagamma dimer (Gbetagamma). We report below the existence of two Gbetagamma-binding sites on the A-, B-, and E-type alpha1 subunits that form non-L-type Ca2+ channels. One, reported previously, is in loop 1 connecting transmembrane domains I and II. The second is located approximately in the middle of the ca. 600-aa-long C-terminal tails. Both Gbetagamma-binding regions also bind the Ca2+ channel beta subunit (CCbeta), which, when overexpressed, interferes with inhibition by activated G proteins. Replacement in alpha1E of loop 1 with that of the G protein-insensitive and Gbetagamma-binding-negative loop 1 of alpha1C did not abolish inhibition by G proteins, but the exchange of the alpha1E C terminus with that of alpha1C did. This and properties of alpha1E C-terminal truncations indicated that the Gbetagamma-binding site mediating the inhibition of Ca2+ channel activity is the one in the C terminus. Binding of Gbetagamma to this site was inhibited by an alpha1-binding domain of CCbeta, thus providing an explanation for the functional antagonism existing between CCbeta and G protein inhibition. The data do not support proposals that Gbetagamma inhibits alpha1 function by interacting with the site located in the loop I-II linker. These results define the molecular mechanism by which presynaptic G protein-coupled receptors inhibit neurotransmission.

Voltage-controlled gating in a large conductance Ca2+-sensitive K+channel (hslo)

Large conductance calcium- and voltage-sensitive K+ (MaxiK) channels share properties of voltage- and ligand-gated ion channels. In voltage-gated channels, membrane depolarization promotes the displacement of charged residues contained in the voltage sensor (S4 region) inducing gating currents and pore opening. In MaxiK channels, both voltage and micromolar internal Ca2+ favor pore opening. We demonstrate the presence of voltage sensor rearrangements with voltage (gating currents) whose movement and associated pore opening is triggered by voltage and facilitated by micromolar internal Ca2+ concentration. In contrast to other voltage-gated channels, in MaxiK channels there is charge movement at potentials where the pore is open and the total charge per channel is 4-5 elementary charges.

Feedback inhibition of Ca2+ channels by Ca2+ depends on a short sequence of the C terminus that does not include the Ca2+ -binding function of a motif with similarity to Ca2+ -binding domains

alpha(1C)- and alpha(1E)-based Ca2+ channels differ in that the former are inhibited by Ca2+ entering through its pore, while the latter are not. It has been proposed on the basis of analysis of alpha(1E)/alpha(1C) chimeras that the molecular determinants responsible for Ca2+ inhibition involve both a conserved Ca2+-binding motif (EF hand) plus additional sequences located C-terminal to the EF hand. Through construction of similar alpha(1E)/alpha(1C) chimeras, we transferred Ca2+ inhibition from alpha(1C) to alpha(1E) by replacing a 134-aa segment of alpha(1E) with the homologous 142-aa segment of alpha(1C). This segment is located immediately after the proposed Ca2+ -binding EF hand motif. Replacement of the alpha(1C) EF hand with the corresponding EF hand of alpha(1E) did not interfere with inhibition of alpha(1C) by Ca2+, and a triple mutant of alpha(1C), alpha(1C)[D1535A,E1537A,D1546A], that disrupts the potential Ca2+-coordinating ability of the EF hand continued to be inhibited by Ca2+. These results indicate that a small portion of the alpha(1C) C terminus is essential for inhibition by Ca2+ and place the Ca2+ -binding site anywhere in alpha(1C), with the exception of its EF hand-like motif.

A Xenopus oocyte beta subunit: evidence for a role in the assembly/expression of voltage-gated calcium channels that is separate from its role as a regulatory subunit

Two closely related beta subunit mRNAs (xo28 and xo32) were identified in Xenopus oocytes by molecular cloning. One or both appear to be expressed as active proteins, because: (i) injection of Xenopus beta antisense oligonucleotides, but not of sense or unrelated oligonucleotides, significantly reduced endogenous oocyte voltage-gated Ca2+ channel (VGCC) currents and obliterated VGCC currents that arise after injection of mammalian alpha1 cRNAs (alpha(1C) and alpha(1E)); (ii) coinjection of a Xenopus beta antisense oligonucleotide and excess rat beta cRNA rescued expression of alpha1 Ca2+ channel currents; and (iii) coinjection of mammalian alpha1 cRNA with cRNA encoding either of the two Xenopus beta subunits facilitated both activation and inactivation of Ca2+ channel currents by voltage, as happens with most mammalian beta subunits. The Xenopus beta subunit cDNAs (beta3xo cDNAs) predict proteins of 484 aa that differ in only 22 aa and resemble most closely the sequence of the mammalian type 3 beta subunit. We propose that "alpha1 alone" channels are in fact tightly associated alpha1beta3xo channels, and that effects of exogenous beta subunits are due to formation of higher-order [alpha1beta]beta(n) complexes with an unknown contribution of beta3xo. It is thus possible that functional mammalian VGCCs, rather than having subunit composition alpha1beta, are [alpha1beta]beta(n) complexes that associate with alpha2delta and, as appropriate, other tissue-specific accessory proteins. In support of this hypothesis, we discovered that the last 277-aa of alpha(1E) have a beta subunit binding domain. This beta binding domain is distinct from the previously known interaction domain located between repeats I and II of calcium channel alpha1 subunits.

Fast inactivation of Shaker K+ channels is highly temperature dependent

The energy profile of the interaction between the NH2-terminal inactivation domain and the internal mouth of the Shaker H4 K+ channel has been investigated. Macroscopic currents from channels normally inactivating (Shaker H4) and with the inactivation removed (Shaker H4-IR) were recorded at different temperatures using the cut-open oocyte technique. Changes in temperature had a dramatic effect on the inactivation phase. The following parameters were obtained in Shaker H4, lowering the temperature from 20 degrees C to 5 degrees C: (1) the peak amplitude decreased with the temperature coefficient Q10 equal to 1.51; (2) the activation time constant increased with a Q10 equal to 3.14; (3) the decay time constant increased with a Q10 of 7.20, while the recovery from inactivation was less temperature-dependent (Q10=1.57) than the installation of the inactivation phase. At 0 mV, the ratio between the steady state level and the peak amplitude of the current increased with a Q10 of 2.95. These findings indicate that the installation of a fast inactivation process has a strong temperature dependence, while the recovery phase from inactivation is less temperature dependent. These observations support the idea of an NH2-terminal blocking mechanism for inactivation and flexible conformation of the blocking particle.

External barium influences the gating charge movement of Shaker potassium channels

External Ba2+ speeds the OFF gating currents (IgOFF) of Shaker K+ channels but only upon repolarization from potentials that are expected to open the channel pore. To study this effect we used a nonconducting and noninactivating mutant of the Shaker K+ channel, ShH4-IR (W434F). External Ba2+ slightly decreases the quantity of ON gating charge (QON) upon depolarization to potentials near -30 mV but has little effect on the quantity of charge upon stepping to more hyperpolarized or depolarized potentials. More strikingly, Ba2+ significantly increases the decay rate of IgOFF upon repolarization to -90 mV from potentials positive to approximately -55 mV. For Ba2+ to have this effect, the depolarizing command must be maintained for a duration that is dependent on the depolarizing potential (> 4 ms at -30 mV and > 1 ms at 0 mV). The actions of Ba2+ on the gating current are dose-dependent (EC50 approximately 0.2 mM) and are not produced by either Ca2+ or Mg2+ (2 mM). The results suggest that Ba2+ binds to a specific site on the Shaker K+ channel that destabilizes the open conformation and thus facilitates the return of gating charge upon repolarization.

On the molecular basis and regulation of cellular capacitative calcium entry: roles for Trp proteins

During the last 2 years, our laboratory has worked on the elucidation of the molecular basis of capacitative calcium entry (CCE) into cells. Specifically, we tested the hypothesis that CCE channels are formed of subunits encoded in genes related to the Drosophila trp gene. The first step in this pursuit was to search for mammalian trp genes. We found not one but six mammalian genes and cloned several of their cDNAs, some in their full length. As assayed in mammalian cells, overexpression of some mammalian Trps increases CCE, while expression of partial trp cDNAs in antisense orientation can interfere with endogenous CCE. These findings provided a firm connection between CCE and mammalian Trps. This article reviews the known forms of CCE and highlights unanswered questions in our understanding of intracellular Ca2+ homeostasis and the physiological roles of CCE.

Coupling between charge movement and pore opening in vertebrate neuronal alpha 1E calcium channels

1. Neuronal alpha 1E Ca2+ channels were expressed alone and in combination with the beta 2a subunit in Xenopus laevis oocytes. 2. The properties of ionic and gating currents of alpha 1E were investigated: ionic currents were measured in 10 mM external Ba2+; gating currents were isolated in 2 mM external Co2+. 3. Charge movement preceded channel opening. The charge movement voltage curve (Q(V)) preceded the ionic conductance voltage dependence (G(V)) by approximately 20 mV. 4. Coexpression of alpha 1E with the beta 2a subunit did not modify the voltage dependence of charge movement but shifted the G(V) curve to more negative potentials. The voltage gap between Q(V) and G(V) curves was reduced by the beta 2a subunit and both curves overlapped at potentials near 0 mV. 5. The coupling efficiency between the charge movement and pore opening was estimated by the ration between limiting conductance and maximum charge movement (Gmax/Qmax). Coexpression of the beta 2a subunit increased the Gmax/Qmax ratio from 9.2 x 10(5) +/- 1.4 x 10(5) to 21.9 x 10(5) +/- 2.8 X 10(5) S C-1 for alpha 1E and alpha 1E + beta 2a, respectively. 6. We conclude that in the neuronal alpha 1E the charge movement is tightly coupled with the pore opening and that the beta 2a subunit coexpression further improves this coupling.

Identification of a second region of the beta-subunit involved in regulation of calcium channel inactivation

Previous studies have shown that NH2 termini of the type 1 and 2 beta-subunits modulate the rate at which the neuronal alpha 1E calcium channel inactivates in response to voltage and that they do so independently of their common effect to stimulate activation by voltage (R. Olcese, N. Qin, T. Schneider, A. Neely, X. Wei, E. Stefani, and L. Birnbaumer, Neuron 13: 1433-1438, 1994). By constructing NH2-terminal deletions of several splice variants of beta-subunits, we have now found differences in the way they affect the rate of alpha 1E inactivation that lead us to identify a second domain that also regulates the rate of voltage-induced inactivation of the Ca2+ channel. This second domain, named segment 3, lies between two regions of high-sequence identity between all known beta-subunits and exists in two lengths (long and short), each encoded in a separate exon. Beta-Subunits with the longer 45- to 53-amino acid version cause the channel to inactivate more slowly than subunits with the shorter 7-amino acid version. As is the case for the NH2 terminus, the segment 3 does not affect the regulation of channel activation by the beta-subunit. In addition, the effect of the NH2-terminal segment prevails over that of the internal segment. This raises the possibility that phosphorylation, other types of posttranslational modification, or interaction with other auxiliary calcium channel subunits may be necessary to unmask the regulatory effect of the internal segment.