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

The C-terminal Residues in the Alpha-interacting Domain (AID) Helix Anchor CaVβ Subunit Interaction and Modulation of CaV2.3 Channels

The alpha-interacting domain (AID) in the I-II linker of high voltage-activated (HVA) Ca(2+) channel alpha1 subunits binds with high affinity to Ca(V)beta auxiliary subunits. The recently solved crystal structures of the AID-Ca(V)beta complex in Ca(V)1.1/1.2 have revealed that this interaction occurs through a set of six mostly invariant residues Glu/Asp(6), Leu(7), Gly(9), Tyr(10), Trp(13), and Ile(14) (where the superscript refers to the position of the residue starting with the QQ signature doublet) distributed among three alpha-helical turns in the proximal section of the I-II linker. We show herein that alanine mutations of N-terminal AID residues Gln(1), Gln(2), Ile(3), Glu(4), Glu(6), Leu(7), and Gly(9) in Ca(V)2.3 did not abolish [(35)S]Ca(V)beta 1b or [(35)S]Ca(V)beta 3 subunit overlay binding to fusion proteins nor did they prevent the typical modulation of whole cell currents by Ca(V)beta 3. Mutations of the invariant Tyr(10) with either hydrophobic (Ala), aromatic (Phe), or positively charged (Arg, Lys) residues yielded Ca(V)beta 3-responsive whole cell currents, whereas mutations with negatively charged residues (Asp, Glu) disrupted Ca(V)beta 3 binding and modulation. In contrast, modulation and binding by Ca(V)beta 3 was significantly weakened in I14A (neutral and hydrophobic) and I14S (neutral and polar) mutants and eradicated in negatively charged I14D and I14E or positively charged I14R and I14K mutants. Ca(V)beta 3-induced modulation was only preserved with the conserved I14L mutation. Molecular replacement analyses carried out using a three-dimensional homology model of the AID helix from Ca(V)2.3 suggests that a high degree of hydrophobicity and a restrained binding pocket could account for the strict structural specificity of the interaction site found at position Ile(14). Altogether these results indicate that the C-terminal residues Trp(13) (1) and Ile(14) anchor Ca(V)beta subunit functional modulation of HVA Ca(2+) channels.

Removal of Ca2+ channel beta3 subunit enhances Ca2+ oscillation frequency and insulin exocytosis

An oscillatory increase in pancreatic beta cell cytoplasmic free Ca2+ concentration, [Ca2+]i, is a key feature in glucose-induced insulin release. The role of the voltage-gated Ca2+ channel beta3 subunit in the molecular regulation of these [Ca2+]i oscillations has now been clarified by using beta3 subunit-deficient beta cells. beta3 knockout mice showed a more efficient glucose homeostasis compared to wild-type mice due to increased glucose-stimulated insulin secretion. This resulted from an increased glucose-induced [Ca2+]i oscillation frequency in beta cells lacking the beta3 subunit, an effect accounted for by enhanced formation of inositol 1,4,5-trisphosphate (InsP3) and increased Ca2+ mobilization from intracellular stores. Hence, the beta3 subunit negatively modulated InsP3-induced Ca2+ release, which is not paralleled by any effect on the voltage-gated L type Ca2+ channel. Since the increase in insulin release was manifested only at high glucose concentrations, blocking the beta3 subunit in the beta cell may constitute the basis for a novel diabetes therapy.

Targeting mechanisms of high voltage-activated Ca2+ channels

Functional voltage-dependent Ca2+ channel complexes are assembled by three to four subunits: alpha1, beta, alpha2delta subunits (C. Leveque et al., 1994, J. Biol Chem. 269, 6306-6312; M. W. McEnery et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88, 11095-11099) and at least in muscle cells also y subunits (B. M. Curtis and W. A. Catterall, 1984, Biochemistry 23, 2113-2118). Ca2+ channels mediate the voltage-dependent Ca2+ influx in subcellular compartments, triggering such diverse processes as neurotransmitter release, dendritic action potentials, excitation-contraction, and excitation-transcription coupling. The targeting of biophysically defined Ca2+ channel complexes to the correct subcellular structures is, thus, critical to proper cell and physiological functioning. Despite their importance, surprisingly little is known about the targeting mechanisms by which Ca2+ channel complexes are transported to their site of function. Here we summarize what we know about the targeting of Ca2+ channel complexes through the cell to the plasma membrane and subcellular structures.

Negatively charged residues in the N-terminal of the AID helix confer slow voltage dependent inactivation gating to CaV1.2

The E462R mutation in the fifth position of the AID (alpha1 subunit interaction domain) region in the I-II linker is known to significantly accelerate voltage-dependent inactivation (VDI) kinetics of the L-type CaV1.2 channel, suggesting that the AID region could participate in a hinged-lid type inactivation mechanism in these channels. The recently solved crystal structures of the AID-CaVbeta regions in L-type CaV1.1 and CaV1.2 channels have shown that in addition to E462, positions occupied by Q458, Q459, E461, K465, L468, D469, and T472 in the rabbit CaV1.2 channel could also potentially contribute to a hinged-lid type mechanism. A mutational analysis of these residues shows that Q458A, Q459A, K465N, L468R, D469A, and T472D did not significantly alter VDI gating. In contrast, mutations of the negatively charged E461, E462, and D463 to neutral or positively charged residues increased VDI gating, suggesting that the cluster of negatively charged residues in the N-terminal end of the AID helix could account for the slower VDI kinetics of CaV1.2. A mutational analysis at position 462 (R, K, A, G, D, N, Q) further confirmed that E462R yielded faster VDI kinetics at +10 mV than any other residue with E462R >> E462K approximately E462A > E462N > wild-type approximately E462Q approximately E462G > E462D (from the fastest to the slowest). E462R was also found to increase the VDI gating of the slow CEEE chimera that includes the I-II linker from CaV1.2 into a CaV2.3 background. The fast VDI kinetics of the CaV1.2 E462R and the CEEE + E462R mutants were abolished by the CaVbeta2a subunit and reinstated when using the nonpalmitoylated form of CaVbeta2a C3S + C4S (CaVbeta2a CS), confirming that CaVbeta2a and E462R modulate VDI through a common pathway, albeit in opposite directions. Altogether, these results highlight the unique role of E461, E462, and D463 in the I-II linker in the VDI gating of high-voltage activated CaV1.2 channels.

Auxiliary subunit regulation of high-voltage activated calcium channels expressed in mammalian cells

The effects of auxiliary calcium channel subunits on the expression and functional properties of high-voltage activated (HVA) calcium channels have been studied extensively in the Xenopus oocyte expression system, but are less completely characterized in a mammalian cellular environment. Here, we provide the first systematic analysis of the effects of calcium channel beta and alpha(2)-delta subunits on expression levels and biophysical properties of three different types (Ca(v)1.2, Ca(v)2.1 and Ca(v)2.3) of HVA calcium channels expressed in tsA-201 cells. Our data show that Ca(v)1.2 and Ca(v)2.3 channels yield significant barium current in the absence of any auxiliary subunits. Although calcium channel beta subunits were in principle capable of increasing whole cell conductance, this effect was dependent on the type of calcium channel alpha(1) subunit, and beta(3) subunits altogether failed to enhance current amplitude irrespective of channel subtype. Moreover, the alpha(2)-delta subunit alone is capable of increasing current amplitude of each channel type examined, and at least for members of the Ca(v)2 channel family, appears to act synergistically with beta subunits. In general agreement with previous studies, channel activation and inactivation gating was regulated both by beta and by alpha(2)-delta subunits. However, whereas pronounced regulation of inactivation characteristics was seen with the majority of the auxiliary subunits, effects on voltage dependence of activation were only small (< 5 mV). Overall, through a systematic approach, we have elucidated a previously underestimated role of the alpha(2)-delta(1) subunit with regard to current enhancement and kinetics. Moreover, the effects of each auxiliary subunit on whole cell conductance and channel gating appear to be specifically tailored to subsets of calcium channel subtypes.

Effects of volatile anesthetics on cardiac ion channels

The focus of the present review is on how interference with various ion channels in the heart may be the molecular basis for cardiac side-effects of gaseous anesthetics. Electrophysiological studies in isolated animal and human cardiomyocytes have identified the L-type Ca(2+) channel as a prominent target of anesthetics. Since this ion channel is of fundamental importance for the plateau phase of the cardiac action potential as well as for Ca(2+)-mediated electromechanical coupling, its inhibition may facilitate arrhythmias by shortening the refractory period and may decrease the contractile force. Effective inhibition of this ion channel has been shown for clinically used concentrations of halothane and, to a lesser extent, of isoflurane and sevoflurane, whereas xenon was without effect. Anesthetics furthermore inhibit several types of voltage-gated K(+) channels. Thereby, they may disturb the repolarization and bear a considerable risk for the induction of ventricular tachycardia in predisposed patients. In future, an advanced understanding of cardiac side-effects of anesthetics will derive from more detailed analyses of how and which channels are affected as well as from a better comprehension of how altered channel function influences heart function.

Folding of Active Calcium Channel β1b -Subunit by Size-exclusion Chromatography and Its Role on Channel Function

Voltage-gated calcium channels mediate the influx of Ca(2+) ions into eukaryotic cells in response to membrane depolarization. They are hetero-multimer membrane proteins formed by at least three subunits, the poreforming alpha(1)-subunit and the auxiliary beta- and alpha(2)delta-subunits. The beta-subunit is essential for channel performance because it regulates two distinct features of voltage-gated calcium channels, the surface expression and the channel activity. Four beta-subunit genes have been cloned, beta(1-4), with molecular masses ranging from 52 to 78 kDa, and several splice variants have been identified. The beta(1b)-subunit, expressed at high levels in mammalian brain, has been used extensively to study the interaction between the pore forming alpha(1)- and the regulatory beta-subunit. However, structural characterization has been impaired for its tendency to form aggregates when expressed in bacteria. We applied an on-column refolding procedure based on size exclusion chromatography to fold the beta(1b)-subunit of the voltage gated-calcium channels from Escherichia coli inclusion bodies. The beta(1b)-subunit refolds into monomers, as shown by sucrose gradient analysis, and binds to a glutathione S-transferase protein fused to the known target in the alpha(1)-subunit (the alpha-interaction domain). Using the cut-open oocyte voltage clamp technique, we measured gating and ionic currents in Xenopus oocytes expressing cardiac alpha(1)-subunit (alpha(1C)) co-injected with folded-beta(1b)-protein or beta(1b)-cRNA. We demonstrate that the co-expression of the alpha(1C)-subunit with either folded-beta(1b)-protein or beta(1b)-cRNA increases ionic currents to a similar extent and with no changes in charge movement, indicating that the beta(1b)-subunit primarily modulates channel activity, rather than expression.

Structural Analysis of the Voltage-Dependent Calcium Channel β Subunit Functional Core and Its Complex with the α1 Interaction Domain

Voltage-dependent calcium channels (VDCC) are multiprotein assemblies that regulate the entry of extracellular calcium into electrically excitable cells and serve as signal transduction centers. The alpha1 subunit forms the membrane pore while the intracellular beta subunit is responsible for trafficking of the channel to the plasma membrane and modulation of its electrophysiological properties. Crystallographic analyses of a beta subunit functional core alone and in complex with a alpha1 interaction domain (AID) peptide, the primary binding site of beta to the alpha1 subunit, reveal that beta represents a novel member of the MAGUK protein family. The findings illustrate how the guanylate kinase fold has been fashioned into a protein-protein interaction module by alteration of one of its substrate sites. Combined results indicate that the AID peptide undergoes a helical transition in binding to beta. We outline the mechanistic implications for understanding the beta subunit's broad regulatory role of the VDCC, particularly via the AID.

Membrane-associated guanylate kinase-like properties of beta-subunits required for modulation of voltage-dependent Ca2+ channels

High-voltage-activated Ca2+ channels regulate diverse functions ranging from muscle contraction to synaptic transmission. Association between auxiliary beta- and distinct pore-forming alpha1-subunits is obligatory for forming functional high-voltage-activated Ca2+ channels, yet the structural determinants underlying this interaction remain poorly understood. Recently, homology modeling of Ca(2+)-channel beta1b-subunit identified src homology 3 (SH3) and guanylate kinase (GK) motifs in a tandem arrangement reminiscent of the membrane-associated guanylate kinase (MAGUK) class of scaffolding proteins. However, direct evidence for MAGUK-like properties and their functional implications in beta-subunits is lacking. Here, we show a functional requirement for both SH3 and GK domains in beta2a. Point mutations in either the putative beta2a SH3 or GK domains severely blunted modulation of recombinant L-type channels, showing the importance of both motifs for a functional alpha1-beta interaction. Coexpression of these functionally deficient beta2a-SH3 and GK mutants rescued WT currents, demonstrating trans complementation similar to that observed in MAGUKs. Truncated "hemi-beta2a" subunits, containing either the SH3 or GK domain, were ineffective on their own, but reconstituted WT currents when coexpressed. Moreover, the SH3 and GK domains were found to interact in vitro. These findings reveal MAGUK-like properties in beta-subunits that are critical for alpha1-subunit modulation, revise current models of alpha1-beta association, and predict new physiological dimensions of beta-subunit function.

Cavbeta-subunit displacement is a key step to induce the reluctant state of P/Q calcium channels by direct G protein regulation

P/Q Ca(2+) channel activity is inhibited by G protein-coupled receptor activation. Channel inhibition requires a direct Gbetagamma binding onto the pore-forming subunit, Ca(v)2.1. It is characterized by biophysical changes, including current amplitude reduction, activation kinetic slowing, and an I-V curve shift, which leads to a reluctant mode. Here, we have characterized the contribution of the auxiliary beta(3)-subunit to channel regulation by G proteins. The shift in I-V to a P/Q reluctant mode is exclusively observed in the presence of beta(3). Along with the observation that Gbetagamma has no effect on the I-V curve of Ca(v)2.1 alone, we propose that the reluctant mode promoted by Gbetagamma corresponds to a state in which the beta(3)-subunit has been displaced from its channel-binding site. We validate this hypothesis with a beta(3)-I-II(2.1) loop chimera construct. Gbetagamma binding onto the I-II(2.1) loop portion of the chimera releases the beta(3)-binding domain and makes it available for binding onto the I-II loop of Ca(v)1.2, a G protein-insensitive channel. This finding is extended to the full-length Ca(v)2.1 channel by using fluorescence resonance energy transfer. Gbetagamma injection into Xenopus oocytes displaces a Cy3-labeled beta(3)-subunit from a GFP-tagged Ca(v)2.1 channel. We conclude that beta-subunit dissociation from the channel complex constitutes a key step in P/Q calcium channel regulation by G proteins that underlies the reluctant state and is an important process for modulating neurotransmission through G protein-coupled receptors.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

Specific sites in the Beta Interaction Domain of a schistosome Ca2+ channel beta subunit are key to its role in sensitivity to the anti-schistosomal drug praziquantel

Praziquantel, the drug of choice against schistosomiasis, disrupts calcium (Ca2+) homeostasis in schistosomes via an unknown mechanism. Voltage-gated Ca2+ channels are heteromultimeric transmembrane protein complexes that contribute to impulse propagation and also regulate intracellular Ca2+ levels. Beta subunits modulate the properties of the pore-forming alpha1 subunit of high voltage-activated Ca2+ channels. Unlike other Ca2+ channel beta subunits, which have current stimulatory effects, a beta subunit subtype found in S. mansoni (SmbetaA) and S. japonicum (Sjbeta) dramatically reduces current levels when co-expressed with Ca2+ channel alpha1 subunits in Xenopus oocytes. It also confers praziquantel sensitivity to the mammalian Cav2.3 alpha1 subunit. The Beta Interaction Domains (BIDs) of SmbetaA and Sjbeta lack 2 conserved serines that each constitute a consensus site for protein kinase C (PKC) phosphorylation. Here, we use site-directed mutagenesis of schistosome beta subunits to show that these unique functional properties are correlated with the absence of these consensus PKC sites in the BID. Furthermore, a second schistosome beta subunit subtype contains both serines in the BID, enhances currents through alpha1 subunits, and does not confer praziquantel sensitivity. Thus, phosphorylation sites in the BID may play important roles in defining the modulatory properties and pharmacological sensitivities of schistosome Ca2+ channel beta subunits.

R-type voltage-gated ca2+ channel interacts with synaptic proteins and recruits synaptotagmin to the plasma membrane of xenopus oocytes

It is well established that syntaxin 1A, synaptosomal-associated protein of 25 kDa (SNAP-25) and synaptotagmin either alone or in combination, modulate the kinetic properties of voltage-gated Ca(2+) channels Ca(v)1.2 (Lc-channel) Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type). The interaction interface was found to reside at the cytosolic II-III domain of the alpha1 subunit of the channels. In this study, we demonstrated a functional coupling of human neuronal Ca(v)2.3 (R-type channel) with syntaxin 1A, SNAP-25 and synaptotagmin in BAPTA injected Xenopus oocytes. The kinetic properties of Ca(v)2.3 assembled with syntaxin 1A, SNAP-25 or synaptotagmin individually differed from Ca(v)2.3 associated with binary complexes syntaxin 1A/SNAP-25, syntaxin 1A/synaptotagmin or SNAP-25/synaptotagmin. Co-expression of Ca(v)2.3 with syntaxin 1A, SNAP-25 and synaptotagmin together, produced a channel with distinctive kinetic properties analogous to excitosome multiprotein complex generated by Ca(v)1.2 and Ca(v)2.2. Exchanging the current-carrying ions altered the kinetics of channel/synaptic proteins interaction, indicating a tight crosstalk formed between the permeation pathway of Ca(v)2.3 and the fusion apparatus during membrane depolarization. This putative coupling could predict how the release site might be organized to allow a rapid communication between the channel and the release machinery. In vivo confocal imaging of oocytes revealed GFP-synaptotagmin at the plasma membrane when the channel was present, as opposed to random distribution in its absence, consistent with Ca(2+)-independent molecular link of synaptotagmin and the channel. Synaptotagmin was detected at the membrane also in oocytes co-expressing the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Both imaging studies and protein-protein interactions in Xenopus oocytes show that channel linkage to synaptotagmin precedes Ca(2+) influx. Altogether, the R-type channel appears to associate with synaptic proteins to generate a multiprotein excitosome complex prior to Ca(2+)-entry. We propose that the distinct kinetics of the Ca(2+)-channel acquired by the close association with the vesicle and the t-SNAREs within the excitosome complex may be essential for depolarization evoked transmitter release.

Several Structural Domains Contribute to the Regulation of N-type Calcium Channel Inactivation by the β3 Subunit

Calcium channel beta subunits are essential regulatory elements of the gating properties of high voltage-activated calcium channels. Co-expression with beta(3) subunits typically accelerates inactivation, whereas co-expression with beta(4) subunits results in a slowly inactivating phenotype. Here, we have examined the molecular basis of the differential effect of these two subunits on the inactivation characteristics of Ca(v)2.2 + alpha(2)-delta(1) N-type calcium channels by creating a series of 22 chimeric beta subunits that are based on various combinations of variable and conserved regions of the parent beta subunit isoforms. Our data show that replacement of the N terminus region of beta(4) with a corresponding 14-amino acid stretch of beta(3) sequence accelerates the inactivation kinetics to levels seen with wild type beta(3). A similar kinetic speeding is observed by a concomitant substitution of the second conserved and variable regions, but not when these regions are substituted individually, suggesting that 1) the second variable and conserved regions cooperatively regulate N-type calcium channel inactivation and 2) that there are two redundant mechanisms that allow the beta(3) subunit to accelerate N-type channel inactivation. In contrast with previous reports in Ca(v)2.1 calcium channels, deletion of the C-terminal region of Ca(v)2.2 did not alter the regulation of the channel by wild type and chimeric beta subunits. Hence, the molecular underpinnings of beta subunit regulation of voltage-gated calcium channels appear to vary with calcium channel subtype.

G Protein Modulation of Voltage-Gated Calcium Channels

Calcium influx into any cell requires fine tuning to guarantee the correct balance between activation of calcium-dependent processes, such as muscle contraction and neurotransmitter release, and calcium-induced cell damage. G protein-coupled receptors play a critical role in negative feedback to modulate the activity of the CaV2 subfamily of the voltage-dependent calcium channels, which are largely situated on neuronal and neuro-endocrine cells. The basis for the specificity of the relationships among membrane receptors, G proteins, and effector calcium channels will be discussed, as well as the mechanism by which G protein-mediated inhibition is thought to occur. The inhibition requires free G beta gamma dimers, and the cytoplasmic linker between domains I and II of the CaV2 alpha 1 subunits binds G beta gamma dimers, whereas the intracellular N terminus of CaV2 alpha 1 subunits provides essential determinants for G protein modulation. Evidence suggests a key role for the beta subunits of calcium channels in the process of G protein modulation, and the role of a class of proteins termed "regulators of G protein signaling" will also be described.

Modified cardiovascular L-type channels in mice lacking the voltage-dependent Ca2+ channel beta3 subunit

The beta subunits of voltage-dependent calcium channels are known to modify calcium channel currents through pore-forming alpha1 subunits. Of the four beta subunits reported to date, the beta3 subunit is highly expressed in smooth muscle cells and is thought to consist of L-type calcium channels. To determine the role of the beta3 subunit in the voltage-dependent calcium channels of the cardiovascular system in situ, we performed a series of experiments in beta3-null mice. Western blot analysis indicated a significant reduction in expression of the alpha1 subunit in the plasma membrane of beta3-null mice. Dihydropyridine binding experiments also revealed a significant decrease in the calcium channel population in the aorta. Electrophysiological analyses indicated a 30% reduction in Ca2+ channel current density, a slower inactivation rate, and a decreased dihydropyridine-sensitive current in beta3-null mice. The reductions in the peak current density and inactivation rate were reproduced in vitro by co-expression of the calcium channel subunits in Chinese hamster ovary cells. Despite the reduced channel population, beta3-null mice showed normal blood pressure, whereas a significant reduction in dihydropyridine responsiveness was observed. A high salt diet significantly elevated blood pressure only in the beta3-null mice and resulted in hypertrophic changes in the aortic smooth muscle layer and cardiac enlargement. In conclusion, this study demonstrates the involvement and importance of the beta3 subunit of voltage-dependent calcium channels in the cardiovascular system and in regulating channel populations and channel properties in vascular smooth muscle cells.

The voltage-dependent calcium channel beta subunit contains two stable interacting domains

Voltage-dependent calcium channels selectively enable Ca2+ ion movement through cellular membranes. These multiprotein complexes are involved in a wide spectrum of biological processes such as signal transduction and cellular homeostasis. alpha1 is the membrane pore-forming subunit, whereas beta is an intracellular subunit that binds to alpha1, facilitating and modulating channel function. We have expressed, purified, and characterized recombinant beta3 and beta2a using both biochemical and biophysical methods, including electrophysiology, to better understand the beta family's protein structural and functional correlates. Our results indicate that the beta protein is composed of two distinct domains that associate with one another in a stable manner. The data also suggest that the polypeptide regions outside these domains are not structured when beta is not in complex with the channel. In addition, the beta structural core, comprised of just these two domains without other sequences, binds tightly to the alpha interaction domain (AID) motif, a sequence derived from the alpha1 subunit and the principal anchor site of beta. Domain II is responsible for this binding, but domain I enhances it.

Creation by mutagenesis of a mammalian Ca2+ channel β subunit that confers praziquantel sensitivity to a mammalian Ca2+ channel

Voltage-gated Ca(2+) channel beta subunits are important modulators of the pore-forming alpha(1) subunit. We have cloned two schistosome beta subunits that confer sensitivity to the antischistosomal drug praziquantel (PZQ) to an otherwise insensitive mammalian alpha(1) subunit. The primary site of beta subunit interaction with alpha(1) subunits is the beta interaction domain (BID). The BID contains two conserved serines (225, 235 in rat beta2a) that constitute consensus sites for protein kinase C phosphorylation. However, these serines are absent in these schistosome beta subunits. Here we show that the capability to confer PZQ sensitivity can be created in the rat beta2a subunit by eliminating both serines in the BID. These results are consistent with, and should help our understanding of, the selective toxicity of PZQ.

Competitive and synergistic interactions of G protein beta(2) and Ca(2+) channel beta(1b) subunits with Ca(v)2.1 channels, revealed by mammalian two-hybrid and fluorescence resonance energy transfer measurements

Presynaptic Ca2+ channels are inhibited by metabotropic receptors. A possible mechanism for this inhibition is that G protein betagamma subunits modulate the binding of the Ca2+ channel beta subunit on the Ca2+ channel complex and induce a conformational state from which channel opening is more reluctant. To test this hypothesis, we analyzed the binding of Ca2+ channel beta and G protein beta subunits on the two separate binding sites, i.e. the loopI-II and the C terminus, and on the full-length P/Q-type alpha12.1 subunit by using a modified mammalian two-hybrid system and fluorescence resonance energy transfer (FRET) measurements. Analysis of the interactions on the isolated bindings sites revealed that the Ca2+ channel beta1b subunit induces a strong fluorescent signal when interacting with the loopI-II but not with the C terminus. In contrast, the G protein beta subunit induces FRET signals on both the C terminus and loopI-II. Analysis of the interactions on the full-length channel indicates that Ca2+ channel beta1b and G protein beta subunits bind to the alpha1 subunit at the same time. Coexpression of the G protein increases the FRET signal between alpha1/beta1b FRET pairs but not for alpha1/beta1b FRET pairs where the C terminus was deleted from the alpha1 subunit. The results suggest that the G protein alters the orientation and/or association between the Ca2+ channel beta and alpha12.1 subunits, which involves the C terminus of the alpha1 subunit and may corresponds to a new conformational state of the channel.

Differential modulation of cardiac Ca2+ channel gating by beta-subunits

To investigate the mechanisms that increase ionic currents when Ca(2+) channels' alpha(1) subunits are co-expressed with the beta-subunits, we compared channel activity of Ca(V)1.2 (alpha(1C)) co-expressed with beta(1a) and beta(2a) in Xenopus oocytes. Normalized by charge movement, ionic currents were near threefold larger with beta(2a) than with beta(1a). At the single-channel level, the open probability (P(o)) was over threefold larger with beta(2a), and traces with high P(o) were more frequent. Among traces with P(o) > 0.1, the mean duration of burst of openings (MBD) were nearly twice as long for alpha(1C)beta(2a) (15.1 +/- 0.7 ms) than for alpha(1C)beta(1a) (8.4 +/- 0.5 ms). Contribution of endogenous beta(3xo) was ruled out by comparing MBDs with alpha(1C)-cRNA alone (4.7 +/- 0.1 ms) with beta(3xo) (14.3 +/- 1.1 ms), and with beta(1b) (8.2 +/- 0.5 ms). Open-channel current amplitude distributions were indistinguishable for alpha(1C)beta(1a) and alpha(1C)beta(2a), indicating that opening and closing kinetics are similar with both subunits. Simulations with constant opening and closing rates reproduced the microscopic kinetics accurately, and therefore we conclude that the conformational change-limiting MBD is differentially regulated by the beta-subunits and contributes to the larger ionic currents associated with beta(2a), whereas closing and opening rates do not change, which should reflect the activity of a separate gate.

Ca2+ current and charge movements in skeletal myotubes promoted by the beta-subunit of the dihydropyridine receptor in the absence of ryanodine receptor type 1

The beta-subunit of the dihydropyridine receptor (DHPR) enhances the Ca(2+) channel and voltage-sensing functions of the DHPR. In skeletal myotubes, there is additional modulation of DHPR functions imposed by the presence of ryanodine receptor type-1 (RyR1). Here, we examined the participation of the beta-subunit in the expression of L-type Ca(2+) current and charge movements in RyR1 knock-out (KO), beta1 KO, and double beta1/RyR1 KO myotubes generated by mating heterozygous beta1 KO and RyR1 KO mice. Primary myotube cultures of each genotype were transfected with various beta-isoforms and then whole-cell voltage-clamped for measurements of Ca(2+) and gating currents. Overexpression of the endogenous skeletal beta1a isoform resulted in a low-density Ca(2+) current either in RyR1 KO (36 +/- 9 pS/pF) or in beta1/RyR1 KO (34 +/- 7 pS/pF) myotubes. However, the heterologous beta2a variant with a double cysteine motif in the N-terminus (C3, C4), recovered a Ca(2+) current that was entirely wild-type in density in RyR1 KO (195 +/- 16 pS/pF) and was significantly enhanced in double beta1/RyR1 KO (115 +/- 18 pS/pF) myotubes. Other variants tested from the four beta gene families (beta1a, beta1b, beta1c, beta3, and beta4) were unable to enhance Ca(2+) current expression in RyR1 KO myotubes. In contrast, intramembrane charge movements in beta2a-expressing beta1a/RyR1 KO myotubes were significantly lower than in beta1a-expressing beta1a/RyR1 KO myotubes, and the same tendency was observed in the RyR1 KO myotube. Thus, beta2a had a preferential ability to recover Ca(2+) current, whereas beta1a had a preferential ability to rescue charge movements. Elimination of the double cysteine motif (beta2a C3,4S) eliminated the RyR1-independent Ca(2+) current expression. Furthermore, Ca(2+) current enhancement was observed with a beta2a variant lacking the double cysteine motif and fused to the surface membrane glycoprotein CD8. Thus, tethering the beta2a variant to the myotube surface activated the DHPR Ca(2+) current and bypassed the requirement for RyR1. The data suggest that the Ca(2+) current expressed by the native skeletal DHPR complex has an inherently low density due to inhibitory interactions within the DHPR and that the beta1a-subunit is critically involved in process.

The beta1a subunit regulates the functional properties of adult frog and mouse L-type Ca2+ channels of skeletal muscle

The beta(1a) subunit, one of the auxiliary subunits of Ca(V)1.1 channels, was expressed in COS-1 cells, purified by electroelution and electrodialysis techniques and identified by Western blot using monoclonal antibodies. The purified beta(1a) subunit strongly interacted in vitro with the alpha interaction domain (AID) of Ca(V)1.1 channels. The actions of the purified beta(1a) subunit on Ca(V)1.1 channel currents were assessed in whole cell voltage clamp experiments performed in vesicles derived from frog and mouse adult skeletal muscle plasma membranes. L-type inward currents were recorded in solutions containing Ba(2+) (I(Ba)). Values of peak I(Ba) were doubled by the beta(1a) subunit in frog and mouse muscle vesicles and the amplitude of the slow component of tail currents was greatly increased. The actions of the beta(1a) subunit on Ca(V)1.1 channel currents reached a steady state within 20 min. The beta(1a) subunit had no effect on the time courses of activation or inactivation of I(Ba) or shifted the current-voltage relation. Non-linear capacitive currents were recorded in solutions that contained mostly impermeant ions. Charge movement depended on voltage with average Boltzmann parameters: Q(max) = 28.0 +/- 6.6 nC microF(-1), V = -58.0 +/- 2.0 mV and k = 15.3 +/- 1.1 mV (n = 24). In the presence of the beta(1a) subunit, these parameters remained unchanged: Q(max) = 29.8 +/- 3.5 nC microF(-1), V = -54.5 +/- 2.2 mV and k = 16.4 +/- 1.3 mV (n = 21). Overall, the work describes a novel preparation to explore in situ the role of the beta(1a) subunit on the function of adult Ca(V)1.1 channels.

A specific tryptophan in the I-II linker is a key determinant of beta-subunit binding and modulation in Ca(V)2.3 calcium channels

The ancillary beta subunits modulate the activation and inactivation properties of high-voltage activated (HVA) Ca(2+) channels in an isoform-specific manner. The beta subunits bind to a high-affinity interaction site, alpha-interaction domain (AID), located in the I-II linker of HVA alpha1 subunits. Nine residues in the AID motif are absolutely conserved in all HVA channels (QQxExxLxGYxxWIxxxE), but their contribution to beta-subunit binding and modulation remains to be established in Ca(V)2.3. Mutations of W386 to either A, G, Q, R, E, F, or Y in Ca(V)2.3 disrupted [(35)S]beta3-subunit overlay binding to glutathione S-transferase fusion proteins containing the mutated I-II linker, whereas mutations (single or multiple) of nonconserved residues did not affect the protein-protein interaction with beta3. The tryptophan residue at position 386 appears to be an essential determinant as substitutions with hydrophobic (A and G), hydrophilic (Q, R, and E), or aromatic (F and Y) residues yielded the same results. beta-Subunit modulation of W386 (A, G, Q, R, E, F, and Y) and Y383 (A and S) mutants was investigated after heterologous expression in Xenopus oocytes. All mutant channels expressed large inward Ba(2+) currents with typical current-voltage properties. Nonetheless, the typical hallmarks of beta-subunit modulation, namely the increase in peak currents, the hyperpolarization of peak voltages, and the modulation of the kinetics and voltage dependence of inactivation, were eliminated in all W386 mutants, although they were preserved in part in Y383 (A and S) mutants. Altogether these results suggest that W386 is critical for beta-subunit binding and modulation of HVA Ca(2+) channels.

The presence of Ca2+ channel beta subunit is required for mitogen-activated protein kinase (MAPK)-dependent modulation of alpha1B Ca2+ channels in COS-7 cells

In rat sensory neurones, voltage-dependent calcium channels (VDCCs), including the N-type, are tonically up-regulated via Ras/mitogen-activated protein kinase (MAPK) signalling. To determine whether VDCC beta subunit is involved in this process, the role of the four neuronal betas (beta1b, beta2a, beta3, beta4) in MAPK-dependent modulation of alpha1B (Ca(v)2.2, N-type) Ca(2+) channels has been examined in COS-7 cells. MAPK is exclusively activated by MAPK kinase (MEK), and here, acute application of a MEK-specific inhibitor UO126, significantly inhibited peak alpha1B Ca2+ channel current (I(max)) within a period of 5-10 min, regardless of which beta subunit was co-expressed (25-50 %, P < 0.01). With beta2a however, the percentage inhibition of I(max) was less than that observed with any other beta (ANOVA: F(3,34) = 6.48, P < 0.01). UO126 also caused a hyperpolarising shift (6 +/- 1 mV, P < 0.001) in the voltage dependence of beta2a current activation, such that inhibition occurred only at depolarised potentials (> +5 mV) whereas at more negative potentials the current amplitude was enhanced. A marked change in beta2a current kinetics, perceived either as decreased activation or increased inactivation, was also associated with UO126 application. A similar effect of UO126 on beta4 current kinetics was also observed. The beta2a-specific effects of UO126 on current inhibition and voltage dependence of activation were abolished when alpha1B was co-expressed with de-palmitoylated beta2a(C3,4S), in which amino terminal cysteines 3 and 4 had been mutated to serines. In the absence of beta subunit, UO126 had no effect on alpha1B Ca2+ channel current. Together, these data suggest an absolute requirement for beta in MAPK-dependent modulation of these channels. Since beta subunits vary both in their temporal expression and localisation within neurones, beta subunit-dependent modulation of N-type Ca2+ channels via MAPK could provide an important new mechanism by which to fine-tune neurotransmitter release.

A mutation in the beta interaction domain of the Ca(2+) channel alpha(1C) subunit reduces the affinity of the (+)-[(3)H]isradipine binding site

The molecular mechanisms of how alpha(1) and beta subunits of voltage-gated Ca(2+) channels interact with one another are still controversial. Here we show that despite a mutation in the beta interaction domain that has previously been shown to disrupt binding, alpha(1C)Y467S and beta(1a-myc) still formed immunoprecipitable complexes when coexpressed in tsA201 cells. However, the alpha(1C)Y467S-beta(1a-myc) complexes had a decreased affinity to (+)-[(3)H]isradipine. This indicates that the beta interaction domain in the I-II loop of the alpha(1) subunit is not merely an anchor required for the functional interaction of the two Ca(2+) channel subunits but is itself part of the effector pathway for beta-induced channel modulation.

Alternative splicing of the beta 4 subunit has alpha1 subunit subtype-specific effects on Ca2+ channel gating

Ca2+ channel beta subunits are important molecular determinants of the kinetics and voltage dependence of Ca2+ channel gating. Through direct interactions with channel-forming alpha1 subunits, beta subunits enhance expression levels, accelerate activation, and have variable effects on inactivation. Four distinct beta subunit genes each encode five homologous sequence domains (D1-5), three of which (D1, D3, and D5) undergo alternative splicing. We have isolated from human spinal cord a novel alternatively spliced beta4 subunit containing a short form of domain D1 (beta4a) that is highly homologous to N termini of Xenopus and rat beta3 subunits. The purpose of this study was to compare the gating properties of various alpha1 subunit complexes containing beta4a with those of complexes containing a beta4 subunit with a longer form of domain D1, beta4b. Expression in Xenopus oocytes revealed that, relative to alpha1A and alpha1B complexes containing beta4b, the voltage dependence of activation and inactivation of complexes containing beta4a were shifted to more depolarized potentials. Moreover, alpha1A and alpha1B complexes containing beta4a inactivated at a faster rate. Interestingly, beta4 subunit alternative splicing did not influence the gating properties of alpha1C and alpha1E subunits. Experiments with beta4 deletion mutants revealed that both the N and C termini of the beta4 subunit play critical roles in setting voltage-dependent gating parameters and that their effects are alpha1 subunit specific. Our data are best explained by a model in which distinct modes of activation and inactivation result from beta-subunit splice variant-specific interactions with an alpha1 subunit gating structure.

Alternative splicing of the beta 4 subunit has alpha1 subunit subtype-specific effects on Ca2+ channel gating

Ca2+ channel beta subunits are important molecular determinants of the kinetics and voltage dependence of Ca2+ channel gating. Through direct interactions with channel-forming alpha1 subunits, beta subunits enhance expression levels, accelerate activation, and have variable effects on inactivation. Four distinct beta subunit genes each encode five homologous sequence domains (D1-5), three of which (D1, D3, and D5) undergo alternative splicing. We have isolated from human spinal cord a novel alternatively spliced beta4 subunit containing a short form of domain D1 (beta4a) that is highly homologous to N termini of Xenopus and rat beta3 subunits. The purpose of this study was to compare the gating properties of various alpha1 subunit complexes containing beta4a with those of complexes containing a beta4 subunit with a longer form of domain D1, beta4b. Expression in Xenopus oocytes revealed that, relative to alpha1A and alpha1B complexes containing beta4b, the voltage dependence of activation and inactivation of complexes containing beta4a were shifted to more depolarized potentials. Moreover, alpha1A and alpha1B complexes containing beta4a inactivated at a faster rate. Interestingly, beta4 subunit alternative splicing did not influence the gating properties of alpha1C and alpha1E subunits. Experiments with beta4 deletion mutants revealed that both the N and C termini of the beta4 subunit play critical roles in setting voltage-dependent gating parameters and that their effects are alpha1 subunit specific. Our data are best explained by a model in which distinct modes of activation and inactivation result from beta-subunit splice variant-specific interactions with an alpha1 subunit gating structure.

Functional analysis of a frame-shift mutant of the dihydropyridine receptor pore subunit (alpha1S) expressing two complementary protein fragments

BACKGROUND

The L-type Ca2+ channel formed by the dihydropyridine receptor (DHPR) of skeletal muscle senses the membrane voltage and opens the ryanodine receptor (RyR1). This channel-to-channel coupling is essential for Ca2+ signaling but poorly understood. We characterized a single-base frame-shift mutant of alpha1S, the pore subunit of the DHPR, that has the unusual ability to function voltage sensor for excitation-contraction (EC) coupling by virtue of expressing two complementary hemi-Ca2+ channel fragments.

RESULTS

Functional analysis of cDNA transfected dysgenic myotubes lacking alpha1S were carried out using voltage-clamp, confocal Ca2+ indicator fluoresence, epitope immunofluorescence and immunoblots of expressed proteins. The frame-shift mutant (fs-alpha1S) expressed the N-terminal half of alpha1S (M1 to L670) and the C-terminal half starting at M701 separately. The C-terminal fragment was generated by an unexpected restart of translation of the fs-alpha1S message at M701 and was eliminated by a M701I mutation. Protein-protein complementation between the two fragments produced recovery of skeletal-type EC coupling but not L-type Ca2+ current.

DISCUSSION

A premature stop codon in the II-III loop may not necessarily cause a loss of DHPR function due to a restart of translation within the II-III loop, presumably by a mechanism involving leaky ribosomal scanning. In these cases, function is recovered by expression of complementary protein fragments from the same cDNA. DHPR-RyR1 interactions can be achieved via protein-protein complementation between hemi-Ca2+ channel proteins, hence an intact II-III loop is not essential for coupling the DHPR voltage sensor to the opening of RyR1 channel.

Calcium channel beta subunits differentially regulate the inhibition of N-type channels by individual Gbeta isoforms

The direct inhibition of N- and P/Q-type calcium channels by G protein betagamma subunits is considered a key mechanism for regulating presynaptic calcium levels. We have recently reported that a number of features associated with this G protein inhibition are dependent on the G protein beta subunit isoform (Arnot, M. I., Stotz, S. C., Jarvis, S. E., Zamponi, G. W. (2000) J. Physiol. (Lond.) 527, 203-212; Cooper, C. B., Arnot, M. I., Feng, Z.-P., Jarvis, S. E., Hamid, J., Zamponi, G. W. (2000) J. Biol. Chem. 275, 40777-40781). Here, we have examined the abilities of different types of ancillary calcium channel beta subunits to modulate the inhibition of alpha(1B) N-type calcium channels by the five known different Gbeta subunit subtypes. Our data reveal that the degree of inhibition by a particular Gbeta subunit is strongly dependent on the specific calcium channel beta subunit, with N-type channels containing the beta(4) subunit being less susceptible to Gbetagamma-induced inhibition. The calcium channel beta(2a) subunit uniquely slows the kinetics of recovery from G protein inhibition, in addition to mediating a dramatic enhancement of the G protein-induced kinetic slowing. For Gbeta(3)-mediated inhibition, the latter effect is reduced following site-directed mutagenesis of two palmitoylation sites in the beta(2a) N-terminal region, suggesting that the unique membrane tethering of this subunit serves to modulate G protein inhibition of N-type calcium channels. Taken together, our data suggest that the nature of the calcium channel beta subunit present is an important determinant of G protein inhibition of N-type channels, thereby providing a possible mechanism by which the cellular/subcellular expression pattern of the four calcium channel beta subunits may regulate the G protein sensitivity of N-type channels expressed at different loci throughout the brain and possibly within a neuron.

Calcium mediates dorsoventral patterning of mesoderm in Xenopus

Calcium signals participate in the differentiation of electrically excitable and nonexcitable cells; one example of this differentiation is the acquisition of mature neuronal phenotypes. For example, transient elevations of the intracellular calcium concentration have been recorded in the ectoderm of early embryos, and this elevation has been proposed to participate in neural induction. Here, we present molecular evidence indicating that voltage-sensitive calcium channels (VSCC) are involved in early developmental processes leading to the establishment of the dorsoventral (D-V) patterning of a vertebrate embryo. We report that alpha1S VSCC are expressed selectively in the dorsal marginal zone at the early gastrula stage. The expression of the VSCC correlates with elevated intracellular calcium levels, as evaluated by the fluorescence of the intracellular calcium indicator Fluo-3. Misexpression of VSCC leads to a strong dorsalization of the ventral marginal zone and induction of the secondary axis but no direct neuralization of the ectoderm. Moreover, specific inhibition of VSCC by the use of calcicludine results in ventralization of the dorsal mesoderm. Together, these results indicate that calcium channels regulate mesodermal patterning by specificating the D-V identity of the mesodermal cells. The D-V patterning of the mesoderm has been shown to depend on a gradient of BMPs activity. We discuss the possibility that VSCC affect or act downstream of BMPs activity.

Schistosome calcium channel beta subunits. Unusual modulatory effects and potential role in the action of the antischistosomal drug praziquantel

Schistosomes are parasitic flatworms that cause schistosomiasis, a major tropical disease. The current drug of choice against schistosomiasis is praziquantel (PZQ), which has minimal side effects and is potent against all schistosome species. The mode of action of PZQ is unknown, though the drug clearly affects Ca(2+) homeostasis in worms, and there is indirect evidence for interaction of PZQ with schistosome voltage-gated Ca(2+) channels. We have cloned and expressed two Ca(2+) channel beta subunits, one from Schistosoma mansoni and one from Schistosoma japonicum. These two subunits (SmCa(v)beta A and SjCa(v)beta) have structural motifs that differ from those found in other known beta subunits. Surprisingly, coexpression of either SmCa(v)beta A or SjCa(v)beta with a cnidarian (CyCa(v)1) or mammalian (Ca(v)2.3) Ca(2+) channel alpha(1) subunit results in a striking reduction in current amplitude. In the case of Ca(v)2.3, this current reduction can be partially reversed by addition of 100 nm PZQ, which results in a significant increase in current amplitude. Thus, these unusual schistosome beta subunits can confer PZQ sensitivity to an otherwise PZQ-insensitive mammalian Ca(2+) channel, indicating that a possible target for PZQ action is the interaction between beta subunits and pore-forming alpha(1) subunits in schistosomes.

Role of Repeat I in the fast inactivation kinetics of the Ca(V)2.3 channel

The molecular basis for inactivation in Ca(V)2.3 (alpha 1E) channels was studied after expression of alpha 1E/alpha 1C (Ca(V)2.3/Ca(V)1.2) chimeras in Xenopus oocytes. In the presence of 10 mM Ba(2+), the CEEE chimera (Repeat I+part of the I-II linker from Ca(V)1.2) displayed inactivation properties similar to Ca(V)1.2 despite being more than 90% homologous to Ca(V)2.3. The transmembrane segments of Repeat I did not appear to be crucial as inactivation of EC(IS1-6)EEE was not significantly different than Ca(V)2.3. In contrast, EC(AID)EEE, with the beta-subunit binding domain from Ca(V)1.2, tended to behave like Ca(V)1.2 in terms of inactivation kinetics and voltage dependence. A detailed kinetic analysis revealed nonetheless that CEEE and EC(AID)EEE retained the fast inactivation time constant (tau(fast) approximately equal to 20-30 ms) that is a distinctive feature of Ca(V)2.3. Altogether, these data suggest that the region surrounding the AID binding site plays a pivotal albeit not exclusive role in determining the inactivation properties of Ca(V)2.3.

Evidence for two concentration-dependent processes for beta-subunit effects on alpha1B calcium channels

beta-Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha1B (Ca(V)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (approximately 17 nM). This may represent the interaction of the beta-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1B without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gbetagamma dissociation from the beta-subunit-bound alpha1B channels. The data are discussed in terms of two hypotheses, either binding of two beta-subunits to the alpha1B channel or a state-dependent alteration in affinity of the channel for the beta-subunit.

Ca(2+) channel inactivation heterogeneity reveals physiological unbinding of auxiliary beta subunits

Voltage gated Ca(2+) channel (VGCC) auxiliary beta subunits increase membrane expression of the main pore-forming alpha(1) subunits and finely tune channel activation and inactivation properties. In expression studies, co-expression of beta subunits also reduced neuronal Ca(2+) channel regulation by heterotrimeric G protein. Biochemical studies suggest that VGCC beta subunits and G protein betagamma can compete for overlapping interaction sites on VGCC alpha(1) subunits, suggesting a dynamic association of these subunits with alpha(1). In this work we have analyzed the stability of the alpha(1)/beta association under physiological conditions. Regulation of the alpha(1A) Ca(2+) channel inactivation properties by beta(1b) and beta(2a) subunits had two major effects: a shift in voltage-dependent inactivation (E(in)), and an increase of the non-inactivating current (R(in)). Unexpectedly, large variations in magnitude of the effects were recorded on E(in), when beta(1b) was expressed, and R(in), when beta(2a) was expressed. These variations were not proportional to the current amplitude, and occurred at similar levels of beta subunit expression. beta(2a)-induced variations of R(in) were, however, inversely proportional to the magnitude of G protein block. These data underline the two different mechanisms used by beta(1b) and beta(2a) to regulate channel inactivation, and suggest that the VGCC beta subunit can unbind the alpha1 subunit in physiological situations.

The influence exerted by the beta(3) subunit on MVIIA omega-conotoxin binding to neuronal N-type calcium channels

In the present study, two-electrode voltage-clamp techniques have been used to assess the interaction between the MVIIA omega-conotoxin and an isoform of the N-type Ca(2+) channel alpha subunit (alpha(1B-d)). Cloned alpha(1B-d) Ca(2+) channels were expressed in Xenopus laevis oocytes in the presence and absence of the beta(3) subunit. Coexpression of the beta(3) subunit significantly shifted the IC(50) value for MVIIA inhibition of central N-type Ca(2+) channel current. Analysis of the peak conductance vs. depolarising voltage dependence suggested that the beta(3) subunit has no apparent effect on the gating charge which accompanies the closed-open transition of the channels. Instead, coexpression of the beta(3) subunit led to an approx. 10 mV shift to more hyperpolarised potentials in the voltage-dependent activation of N-type Ca(2+) channels. We conclude that MVIIA alters the surface charge on the N-type Ca(2+) channels and might induce allosteric changes on the structure of the channel, leading to an increase in the dissociation constant of MVIIA binding.

Molecular determinants of inactivation within the I-II linker of alpha1E (CaV2.3) calcium channels

Voltage-dependent inactivation of CaV2.3 channels was investigated using point mutations in the beta-subunit-binding site (AID) of the I-II linker. The quintuple mutant alpha1E N381K + R384L + A385D + D388T + K389Q (NRADK-KLDTQ) inactivated like the wild-type alpha1E. In contrast, mutations of alpha1E at position R378 (position 5 of AID) into negatively charged residues Glu (E) or Asp (D) significantly slowed inactivation kinetics and shifted the voltage dependence of inactivation to more positive voltages. When co-injected with beta3, R378E inactivated with tau(inact) = 538 +/- 54 ms (n = 14) as compared with 74 +/- 4 ms (n = 21) for alpha1E (p < 0.001) with a mid-potential of inactivation E(0.5) = -44 +/- 2 mV (n = 10) for R378E as compared with E(0.5) = -64 +/- 3 mV (n = 9) for alpha1E. A series of mutations at position R378 suggest that positively charged residues could promote voltage-dependent inactivation. R378K behaved like the wild-type alpha1E whereas R378Q displayed intermediate inactivation kinetics. The reverse mutation E462R in the L-type alpha1C (CaV1.2) produced channels with inactivation properties comparable to alpha1E R378E. Hence, position 5 of the AID motif in the I-II linker could play a significant role in the inactivation of Ca(V)1.2 and CaV2.3 channels.

Novel omega-conotoxins from Conus catus discriminate among neuronal calcium channel subtypes

omega-Conotoxins selective for N-type calcium channels are useful in the management of severe pain. In an attempt to expand the therapeutic potential of this class, four new omega-conotoxins (CVIA-D) have been discovered in the venom of the piscivorous cone snail, Conus catus, using assay-guided fractionation and gene cloning. Compared with other omega-conotoxins, CVID has a novel loop 4 sequence and the highest selectivity for N-type over P/Q-type calcium channels in radioligand binding assays. CVIA-D also inhibited contractions of electrically stimulated rat vas deferens. In electrophysiological studies, omega-conotoxins CVID and MVIIA had similar potencies to inhibit current through central (alpha(1B-d)) and peripheral (alpha(1B-b)) splice variants of the rat N-type calcium channels when coexpressed with rat beta(3) in Xenopus oocytes. However, the potency of CVID and MVIIA increased when alpha(1B-d) and alpha(1B-b) were expressed in the absence of rat beta(3), an effect most pronounced for CVID at alpha(1B-d) (up to 540-fold) and least pronounced for MVIIA at alpha(1B-d) (3-fold). The novel selectivity of CVID may have therapeutic implications. (1)H NMR studies reveal that CVID possesses a combination of unique structural features, including two hydrogen bonds that stabilize loop 2 and place loop 2 proximal to loop 4, creating a globular surface that is rigid and well defined.

Calcium channel beta subunits differentially modulate recovery of the channel from inactivation

We examined the effects of calcium channel beta subunits upon the recovery from inactivation of alpha(1) subunits expressed in Xenopus oocytes. Recovery of the current carried by the L-type alpha(1) subunit (cyCa(v)1) from the jellyfish Cyanea capillata was accelerated by coexpression of any beta subunit, but the degree of potentiation differed according to which beta isoform was coexpressed. The Cyanea beta subunit was most effective, followed by the mammalian b(3), b(4), and beta(2a) subtypes. Recovery of the human Ca(v)2.3 subunit was also modulated by beta subunits, but was slowed instead. beta(3) was the most potent subunit tested, followed by beta(4), then beta(2a), which had virtually no effect. These results demonstrate that different beta subunit isoforms can affect recovery of the channel to varying degrees, and provide an additional mechanism by which beta subunits can differentially regulate alpha(1) subunits.

Interaction between G proteins and accessory subunits in the regulation of 1B calcium channels in Xenopus oocytes

The accessory beta subunits of voltage-dependent Ca2+ channels (VDCCs) have been shown to regulate their biophysical properties and have also been suggested to antagonise the G protein inhibition of N-type (alpha1B), P/Q-type (alpha1A) and alpha1E channels. Here we have examined the voltage-dependent involvement of the four neuronal isoforms (beta1b, beta2a, beta3 and beta4) in the process of G protein modulation of alpha1B Ca2+ channels. All beta subunits hyperpolarized alpha1B current activation, and all antagonised the G protein-mediated depolarisation of current activation. However, except in the case of beta2a, there was no generalised reduction by beta subunits in the maximal extent of receptor-mediated inhibition of alpha1B current. In addition, all VDCC beta subunits enhanced the rate of current facilitation at +100 mV, for both receptor-mediated and tonic modulation. The rank order for enhancement of facilitation rate was beta3 > beta4 > beta1b > beta2a. In contrast, the amount of voltage-dependent facilitation during tonic modulation was reduced by beta subunit co-expression, despite the fact that the apparent Gbetagamma dissociation rate at +100 mV was enhanced by beta subunits to a similar level as for agonist-induced modulation. Our data provide evidence that G protein activation antagonises Ca2+-channel beta subunit-induced hyperpolarisation of current activation. Conversely, co-expression of all beta subunits increases the apparent Gbetagamma dimer dissociation rate during a depolarising prepulse. This latter feature suggests the co-existence of bound Ca2+-channel beta subunits and Gbetagamma dimers on the alpha1B subunits. Future work will determine how the interaction between Gbetagamma dimers and Ca2+-channel beta subunits with alpha1B results in a functional antagonism at the molecular level.

Calcium channel beta subunit promotes voltage-dependent modulation of alpha 1 B by G beta gamma

Voltage-dependent calcium channels (VDCCs) are heteromultimers composed of a pore-forming alpha1 subunit and auxiliary subunits, including the intracellular beta subunit, which has a strong influence on the channel properties. Voltage-dependent inhibitory modulation of neuronal VDCCs occurs primarily by activation of G-proteins and elevation of the free G beta gamma dimer concentration. Here we have examined the interaction between the regulation of N-type (alpha 1 B) channels by their beta subunits and by G beta gamma dimers, heterologously expressed in COS-7 cells. In contrast to previous studies suggesting antagonism of G protein inhibition by the VDCC beta subunit, we found a significantly larger G beta gamma-dependent inhibition of alpha 1 B channel activation when the VDCC alpha 1 B and beta subunits were coexpressed. In the absence of coexpressed VDCC beta subunit, the G beta gamma dimers, either expressed tonically or elevated via receptor activation, did not produce the expected features of voltage-dependent G protein modulation of N-type channels, including slowed activation and prepulse facilitation, while VDCC beta subunit coexpression restored all of the hallmarks of G beta gamma modulation. These results suggest that the VDCC beta subunit must be present for G beta gamma to induce voltage-dependent modulation of N-type calcium channels.

Alternative splicing in the cytoplasmic II-III loop of the N-type Ca channel alpha 1B subunit: functional differences are beta subunit-specific

Structural diversity of voltage-gated Ca channels underlies much of the functional diversity in Ca signaling in neurons. Alternative splicing is an important mechanism for generating structural variants within a single gene family. In this paper, we show the expression pattern of an alternatively spliced 21 amino acid encoding exon in the II-III cytoplasmic loop region of the N-type Ca channel alpha(1B) subunit and assess its functional impact. Exon-containing alpha(1B) mRNA dominated in sympathetic ganglia and was present in approximately 50% of alpha(1B) mRNA in spinal cord and caudal regions of the brain and in the minority of alpha(1B) mRNA in neocortex, hippocampus, and cerebellum (<20%). The II-III loop exon affected voltage-dependent inactivation of the N-type Ca channel. Steady-state inactivation curves were shifted to more depolarized potentials without affects on either the rate or voltage dependence of channel opening. Differences in voltage-dependent inactivation between alpha(1B) splice variants were most clearly manifested in the presence of Ca channel beta(1b) or beta(4), rather than beta(2a) or beta(3), subunits. Our results suggest that exon-lacking alpha(1B) splice variants that associate with beta(1b) and beta(4) subunits will be susceptible to voltage-dependent inactivation at voltages in the range of neuronal resting membrane potentials (-60 to -80 mV). In contrast, alpha(1B) splice variants that associate with either beta(2a) or beta(3) subunits will be relatively resistant to inactivation at these voltages. The potential to mix and match multiple alpha(1B) splice variants and beta subunits probably represents a mechanism for controlling the plasticity of excitation-secretion coupling at different synapses.

Alternative splicing in the cytoplasmic II-III loop of the N-type Ca channel alpha 1B subunit: functional differences are beta subunit-specific

Structural diversity of voltage-gated Ca channels underlies much of the functional diversity in Ca signaling in neurons. Alternative splicing is an important mechanism for generating structural variants within a single gene family. In this paper, we show the expression pattern of an alternatively spliced 21 amino acid encoding exon in the II-III cytoplasmic loop region of the N-type Ca channel alpha(1B) subunit and assess its functional impact. Exon-containing alpha(1B) mRNA dominated in sympathetic ganglia and was present in approximately 50% of alpha(1B) mRNA in spinal cord and caudal regions of the brain and in the minority of alpha(1B) mRNA in neocortex, hippocampus, and cerebellum (<20%). The II-III loop exon affected voltage-dependent inactivation of the N-type Ca channel. Steady-state inactivation curves were shifted to more depolarized potentials without affects on either the rate or voltage dependence of channel opening. Differences in voltage-dependent inactivation between alpha(1B) splice variants were most clearly manifested in the presence of Ca channel beta(1b) or beta(4), rather than beta(2a) or beta(3), subunits. Our results suggest that exon-lacking alpha(1B) splice variants that associate with beta(1b) and beta(4) subunits will be susceptible to voltage-dependent inactivation at voltages in the range of neuronal resting membrane potentials (-60 to -80 mV). In contrast, alpha(1B) splice variants that associate with either beta(2a) or beta(3) subunits will be relatively resistant to inactivation at these voltages. The potential to mix and match multiple alpha(1B) splice variants and beta subunits probably represents a mechanism for controlling the plasticity of excitation-secretion coupling at different synapses.

The alpha1B Ca2+ channel amino terminus contributes determinants for beta subunit-mediated voltage-dependent inactivation properties

Co-expression of auxiliary beta subunits with the alpha1B Ca2+ channel subunit in COS-7 cells resulted in an increase in current density and a hyperpolarising shift in the mid-point of activation. Amongst the beta subunits, beta2a in particular, but also beta4 and beta1b caused a significant retardation of the voltage-dependent inactivation compared to currents with alpha1B alone, whilst no significant changes in inactivation properties were seen for the beta3 subunit in this system. Prevention of beta2a palmitoylation, by introducing cysteine to serine mutations (beta2a(C3,4S)), greatly reduced the ability of beta2a to retard voltage-dependent inactivation. Deletion of the proximal half of the alpha1B cytoplasmic amino terminus (alpha1BDelta1-55) differentially affected beta subunit-mediated voltage-dependent inactivation properties. These effects were prominent with the beta2a subunit and, to a lesser extent, with beta1b. For beta2a, the major effects of this deletion were a partial reversal of beta2a-mediated retardation of inactivation and the introduction of a fast component of inactivation, not seen with full-length alpha1B. Deletion of the amino terminus had no other major effects on the measured biophysical properties of alpha1B when co-expressed with beta subunits. Transfer of the whole alpha1B amino terminus into alpha1C (alpha1bCCCC) conferred a similar retardation of inactivation on alpha1C when co-expressed with beta2a to that seen in parental alpha1B. Individual (alpha1B(Q47A) and alpha1B(R52A)) and double (alpha1B(R52,54A)) point mutations within the amino terminus of alpha1B also opposed the beta2a-mediated retardation of alpha1B inactivation kinetics. These results indicate that the alpha1B amino terminus contains determinants for beta subunit-mediated voltage-dependent inactivation properties. Furthermore, effects were beta subunit selective. As deletion of the alpha1B amino terminus only partially opposed beta subunit-mediated changes in inactivation properties, the amino terminus is likely to contribute to a complex site necessary for complete beta subunit function.

Coding and noncoding variation of the human calcium-channel beta4-subunit gene CACNB4 in patients with idiopathic generalized epilepsy and episodic ataxia

Inactivation of the beta4 subunit of the calcium channel in the mouse neurological mutant lethargic results in a complex neurological disorder that includes absence epilepsy and ataxia. To determine the role of the calcium-channel beta4-subunit gene CACNB4 on chromosome 2q22-23 in related human disorders, we screened for mutations in small pedigrees with familial epilepsy and ataxia. The premature-termination mutation R482X was identified in a patient with juvenile myoclonic epilepsy. The R482X protein lacks the 38 C-terminal amino acids containing part of an interaction domain for the alpha1 subunit. The missense mutation C104F was identified both in a German family with generalized epilepsy and praxis-induced seizures and in a French Canadian family with episodic ataxia. These coding mutations were not detected in 255 unaffected control individuals (510 chromosomes), and they may be considered candidate disease mutations. The results of functional tests of the truncated protein R482X in Xenopus laevis oocytes demonstrated a small decrease in the fast time constant for inactivation of the cotransfected alpha1 subunit. Further studies will be required to evaluate the in vivo consequences of these mutations. We also describe eight noncoding single-nucleotide substitutions, two of which are present at polymorphic frequency, and a previously unrecognized first intron of CACNB4 that interrupts exon 1 at codon 21.

Coexpression of cloned alpha(1B), beta(2a), and alpha(2)/delta subunits produces non-inactivating calcium currents similar to those found in bovine chromaffin cells

Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by omega-conotoxin GVIA (omega-CgTx GVIA). In contrast to neuronal N-type calcium currents that inactivate during long depolarizations and that require negative holding potentials to remove inactivation, many chromaffin cells exhibit N-type calcium channel currents that show little inactivation during maintained depolarizations and that exhibit no decrease in channel availability at depolarized holding potentials. N-type calcium channels are thought to be produced by combination of the pore-forming alpha(1B) subunit and accessory beta and alpha(2)/delta subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the alpha(1B) and accessory beta (beta(1b), beta(1c,) beta(2a), beta(2b), and beta(3a)) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopus oocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by omega-CgTx GVIA. Coexpression of bovine alpha(1B) with beta(1b), beta(1c), beta(2b), or beta(3a) produced currents that were holding potential dependent. In contrast, coexpression of bovine alpha(1B) with beta(2a) produced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine alpha(1B), alpha(2)/delta, and beta(2a).

The molecular biology of invertebrate voltage-gated Ca(2+) channels

The importance of voltage-gated Ca(2+) channels in cellular function is illustrated by the many distinct types of Ca(2+) currents found in vertebrate tissues, a variety that is generated in part by numerous genes encoding Ca(2+) channel subunits. The degree to which this genetic diversity is shared by invertebrates has only recently become apparent. Cloning of Ca(2+) channel subunits from various invertebrate species, combined with the wealth of information from the Caenorhabditis elegans genome, has clarified the organization and evolution of metazoan Ca(2+) channel genes. Functional studies have employed novel structural information gained from invertebrate Ca(2+) channels to complement ongoing research on mammalian Ca(2+) currents, while demonstrating that the strict correspondence between pharmacological and molecular classes of vertebrate Ca(2+) channels does not fully extend to invertebrate tissues. Molecular structures can now be combined with physiological data to develop a more cogent system of categorizing invertebrate channel subtypes. In this review, we examine recent progress in the characterization of invertebrate Ca(2+) channel genes and its relevance to the diversity of invertebrate Ca(2+) currents.

Acidic motif responsible for plasma membrane association of the voltage-dependent calcium channel beta1b subunit

Voltage-dependent calcium channels consist of a pore-forming transmembrane alpha1-subunit, which is known to associate with a number of accessory subunits, including alpha2-delta- and beta-subunits. The beta-subunits, of which four have been identified (beta1-4), are intracellular proteins that have marked effects on calcium channel trafficking and function. In a previous study, we observed that the beta1b-subunit showed selective plasma membrane association when expressed alone in COS7 cells, whereas beta3 and beta4 did not. In this study, we have examined the basis for this, and have identified, by making chimeric beta-subunits, that the C-terminal region, which shows most diversity between beta-subunits, of beta1b is responsible for its plasma membrane association. Furthermore we have identified, by deletion mutations, an 11-amino acid motif present in the C terminus of beta1b but not in beta3 (amino acids 547-556 of beta1b, WEEEEDYEEE), which when deleted, reduces membrane association of beta1b. Future research aims to identify what is binding to this sequence in beta1b to promote membrane association of this calcium channel subunit. It is possible that such membrane association is important for the selective localization or clustering of particular calcium channels with which beta1b is associated.

Coexpression of cloned alpha(1B), beta(2a), and alpha(2)/delta subunits produces non-inactivating calcium currents similar to those found in bovine chromaffin cells

Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by omega-conotoxin GVIA (omega-CgTx GVIA). In contrast to neuronal N-type calcium currents that inactivate during long depolarizations and that require negative holding potentials to remove inactivation, many chromaffin cells exhibit N-type calcium channel currents that show little inactivation during maintained depolarizations and that exhibit no decrease in channel availability at depolarized holding potentials. N-type calcium channels are thought to be produced by combination of the pore-forming alpha(1B) subunit and accessory beta and alpha(2)/delta subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the alpha(1B) and accessory beta (beta(1b), beta(1c,) beta(2a), beta(2b), and beta(3a)) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopus oocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by omega-CgTx GVIA. Coexpression of bovine alpha(1B) with beta(1b), beta(1c), beta(2b), or beta(3a) produced currents that were holding potential dependent. In contrast, coexpression of bovine alpha(1B) with beta(2a) produced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine alpha(1B), alpha(2)/delta, and beta(2a).