The amino terminus of a calcium channel beta subunit sets rates of channel inactivation independently of the subunit's effect on activation

Interaction via a key tryptophan in the I-II linker of N-type calcium channels is required for beta1 but not for palmitoylated beta2, implicating an additional binding site in the regulation of channel voltage-dependent properties

The CaVbeta subunits of voltage-gated calcium channels regulate these channels in several ways. Here we investigate the role of these auxiliary subunits in the expression of functional N-type channels at the plasma membrane and in the modulation by G-protein-coupled receptors of this neuronal channel. To do so, we mutated tryptophan 391 to an alanine within the alpha-interacting domain (AID) in the I-II linker of CaV2.2. We showed that the mutation W391 virtually abolishes the binding of CaVbeta1b and CaVbeta2a to the CaV2.2 I-II linker and strongly reduced current density and cell surface expression of both CaV2.2/alpha2delta-2/beta1b and/beta2a channels. When associated with CaVbeta1b, the W391A mutation also prevented the CaVbeta1b-mediated hyperpolarization of CaV2.2 channel activation and steady-state inactivation. However, the mutated CaV2.2W391A/beta1b channels were still inhibited to a similar extent by activation of the D2 dopamine receptor with the agonist quinpirole. Nevertheless, key hallmarks of G-protein modulation of N-type currents, such as slowed activation kinetics and prepulse facilitation, were not observed for the mutated channel. In contrast, CaVbeta2a was still able to completely modulate the biophysical properties of CaV2.2W391A channel and allow voltage-dependent G-protein modulation of CaV2.2W391A. Additional data suggest that the concentration of CaVbeta2a in the proximity of the channel is enhanced independently of its binding to the AID by its palmitoylation. This is essentially sufficient for all of the functional effects of CaVbeta2a, which may occur via a second lower-affinity binding site, except trafficking the channel to the plasma membrane, which requires interaction with the AID region.

Modulation of CaV2.1 channels by the neuronal calcium-binding protein visinin-like protein-2

CaV2.1 channels conduct P/Q-type Ca2+ currents that are modulated by calmodulin (CaM) and the structurally related Ca2+-binding protein 1 (CaBP1). Visinin-like protein-2 (VILIP-2) is a CaM-related Ca2+-binding protein expressed in the neocortex and hippocampus. Coexpression of CaV2.1 and VILIP-2 in tsA-201 cells resulted in Ca2+ channel modulation distinct from CaM and CaBP1. CaV2.1 channels with beta2a subunits undergo Ca2+-dependent facilitation and inactivation attributable to association of endogenous Ca2+/CaM. VILIP-2 coexpression does not alter facilitation measured in paired-pulse experiments but slows the rate of inactivation to that seen without Ca2+/CaM binding and reduces inactivation of Ca2+ currents during trains of repetitive depolarizations. CaV2.1 channels with beta1b subunits have rapid voltage-dependent inactivation, and VILIP-2 has no effect on the rate of inactivation or facilitation of the Ca2+ current. In contrast, when Ba2+ replaces Ca2+ as the charge carrier, VILIP-2 slows inactivation. The effects of VILIP-2 are prevented by deletion of the CaM-binding domain (CBD) in the C terminus of CaV2.1 channels. However, both the CBD and an upstream IQ-like domain must be deleted to prevent VILIP-2 binding. Our results indicate that VILIP-2 binds to the CBD and IQ-like domains of CaV2.1 channels like CaM but slows inactivation, which enhances facilitation of CaV2.1 channels during extended trains of stimuli. Comparison of VILIP-2 effects with those of CaBP1 indicates striking differences in modulation of both facilitation and inactivation. Differential regulation of CaV2.1 channels by CaM, VILIP-2, CaBP1, and other neurospecific Ca2+-binding proteins is a potentially important determinant of Ca2+ entry in neurotransmission.

Vascular calcium channels and high blood pressure: pathophysiology and therapeutic implications

Long-lasting Ca(2+) (Ca(L)) channels of the Ca(v)1.2 gene family are heteromultimeric structures that are minimally composed of a pore-forming alpha(1C) subunit and regulatory beta and alpha(2)delta subunits in vascular smooth muscle cells. The Ca(L) channels are the primary pathways for voltage-gated Ca(2+) influx that trigger excitation-contraction coupling in small resistance vessels. Notably, vascular smooth muscle cells of hypertensive rats show an increased expression of Ca(L) channel alpha(1C) subunits, which is associated with elevated Ca(2+) influx and the development of abnormal arterial tone. Indeed, blood pressure per se appears to promote Ca(L) channel expression in small arteries, and even short-term rises in pressure may alter channel expression. Membrane depolarization has been shown to be one stimulus associated with elevated blood pressure that promotes Ca(L) channel expression at the plasma membrane. Future studies to define the molecular processes that regulate Ca(L) channel expression in vascular smooth muscle cells will provide a rational basis for designing antihypertensive therapies to normalize Ca(L) channel expression and the development of anomalous vascular tone in hypertensive pathologies.

Unique modulation of L-type Ca2+channels by short auxiliary β1dsubunit present in cardiac muscle

Recent studies have identified a growing diversity of splice variants of auxiliary Ca2+channel Cavβ subunits. The Cavβ1disoform encodes a putative protein composed of the amino-terminal half of the full-length Cavβ1isoform and thus lacks the known high-affinity binding site that recognizes the Ca2+channel α1-subunit, the α-binding pocket. The present study investigated whether the Cavβ1dsubunit is expressed at the protein level in heart, and whether it exhibits any of the functional properties typical of full-length Cavβ subunits. On Western blots, an antibody directed against the unique carboxyl terminus of Cavβ1didentified a protein of the predicted molecular mass of 23 kDa from canine and human hearts. Immunocytochemistry and surface-membrane biotinylation experiments in transfected HEK-293 cells revealed that the full-length Cavβ1bsubunit promoted membrane trafficking of the pore-forming α1C(Cav1.2)-subunit to the surface membrane, whereas the Cavβ1dsubunit did not. Whole cell patch-clamp analysis of transfected HEK-293 cells demonstrated no effect of coexpression of the Cavβ1dwith the α1C-subunit compared with the 15-fold larger currents and leftward shift in voltage-dependent activation induced by full-length Cavβ1bcoexpression. In contrast, cell-attached patch single-channel studies demonstrated that coexpression of either Cavβ1bor Cavβ1dsignificantly increased mean open probability four- to fivefold relative to the α1C-channels alone, but only Cavβ1bcoexpression increased the number of channels observed per patch. In conclusion, the Cavβ1disoform is expressed in heart and can modulate the gating of L-type Ca2+channels, but it does not promote membrane trafficking of the channel complex.

Syntaxin 1A regulation of weakly inactivating N-type Ca2+ channels

N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they interact with proteins of the release apparatus, including syntaxin 1A and SNAP-25. In previous studies on N- or P/Q-type Ca2+ channels, syntaxin 1A co-expression reduced current amplitudes, increased voltage-dependent inactivation and/or enhanced G-protein inhibition. However, these studies were conducted in Ca2+ channels that exhibited significant voltage-dependent inactivation. We previously reported that N-type current in bovine chromaffin cells exhibits very little voltage-dependent inactivation and we identified the Ca2+ channel subunits involved. This study was undertaken to determine the effect of syntaxin 1A on this weakly inactivating Ca2+ channel. Co-expression of syntaxin 1A with the weakly inactivating bovine N-type Ca2+ channels in Xenopus oocytes did not appear to alter inactivation but dramatically reduced current amplitudes, without changing cell surface expression. To further understand the mechanisms of syntaxin 1A regulation of this weakly inactivating channel, we examined mutants of the alpha1B subunit, beta2a subunit and syntaxin 1A. We determined that the synprint site of alpha1B and the C-terminal third of syntaxin 1A were necessary for the reduced current amplitude. In addition we show that enhanced G-protein-dependent modulation of the Ca2+ current by syntaxin 1A cannot explain the large suppression of Ca2+ current observed. Of most significance, syntaxin 1A increased voltage-dependent inactivation in channels containing mutant beta2a subunits that cannot be palmitoylated. Our data suggest that changes in inactivation can not explain the reduction in current amplitude produced by co-expressing syntaxin and a weakly inactivating Ca2+ channel.

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.

Calcium channel function regulated by the SH3-GK module in beta subunits

beta subunits of voltage-gated calcium channels (VGCCs) regulate channel trafficking and function, thereby shaping the intensity and duration of intracellular changes in calcium. beta subunits share limited sequence homology with the Src homology 3-guanylate kinase (SH3-GK) module of membrane-associated guanylate kinases (MAGUKs). Here, we show biochemical similarities between beta subunits and MAGUKs, revealing important aspects of beta subunit structure and function. Similar to MAGUKs, an SH3-GK interaction within beta subunits can occur both intramolecularly and intermolecularly. Mutations that disrupt the SH3-GK interaction in beta subunits alter channel inactivation and can inhibit binding between the alpha(1) and beta subunits. Coexpression of beta subunits with complementary mutations in their SH3 and GK domains rescues these deficits through intermolecular beta subunit assembly. In MAGUKs, the SH3-GK module controls protein scaffolding. In beta subunits, this module regulates the inactivation of VGCCs and provides an additional mechanism for tuning calcium responsiveness.

The CACNA1F gene encodes an L-type calcium channel with unique biophysical properties and tissue distribution

Glutamate release from rod photoreceptors is dependent on a sustained calcium influx through L-type calcium channels. Missense mutations in the CACNA1F gene in patients with incomplete X-linked congenital stationary night blindness implicate the Ca(v)1.4 calcium channel subtype. Here, we describe the functional and pharmacological properties of transiently expressed human Ca(v)1.4 calcium channels. Ca(v)1.4 is shown to encode a dihydropyridine-sensitive calcium channel with unusually slow inactivation kinetics that are not affected by either calcium ions or by coexpression of ancillary calcium channel beta subunits. Additionally, the channel supports a large window current and activates near -40 mV in 2 mM external calcium, making Ca(v)1.4 ideally suited for tonic calcium influx at typical photoreceptor resting potentials. Introduction of base pair changes associated with four incomplete X-linked congenital night blindness mutations showed that only the G369D alteration affected channel activation properties. Immunohistochemical analyses show that, in contrast with previous reports, Ca(v)1.4 is widely distributed outside the retina, including in the immune system, thus suggesting a broader role in human physiology.

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.

Molecular heterogeneity of calcium channel beta-subunits in canine and human heart: evidence for differential subcellular localization

Multiple Ca2+ channel beta-subunit (Ca(v)beta) isoforms are known to differentially regulate the functional properties and membrane trafficking of high-voltage-activated Ca2+ channels, but the precise isoform expression pattern of Ca(v)beta subunits in ventricular muscle has not been fully characterized. Using sequence data from the Human Genome Project to define the intron/exon structure of the four known Ca(v)beta genes, we designed a systematic RT-PCR strategy to screen human and canine left ventricular myocardial samples for all known Ca(v)beta isoforms. A total of 18 different Ca(v)beta isoforms were detected in both canine and human ventricles including splice variants from all four Ca(v)beta genes. Six of these isoforms have not previously been described. Western blots of ventricular membrane fractions and immunocytochemistry demonstrated that all four Ca(v)beta subunit genes are expressed at the protein level, and the Ca(v)beta subunits show differential subcellular localization with Ca(v)beta1b, Ca(v)beta2, and Ca(v)beta3 predominantly localized to the T-tubule sarcolemma, whereas Ca(v)beta1a and Ca(v)beta4 are more prevalent in the surface sarcolemma. Coexpression of the novel Ca(v)beta2c subunits (Ca(v)beta(2cN1), Ca(v)beta(2cN2), Ca(v)beta(2cN4)) with the pore-forming alpha1C (Ca(v)1.2) and Ca(v)alpha2delta subunits in HEK 293 cells resulted in a marked increase in ionic current and Ca(v)beta2c isoform-specific modulation of voltage-dependent activation. These results demonstrate a previously unappreciated heterogeneity of Ca(v)beta subunit isoforms in ventricular myocytes and suggest the presence of different subcellular populations of Ca2+ channels with distinct functional properties.

Calcium Current in Rat Cardiomyocytes Is Modulated by the Carboxyl-terminal Ahnak Domain

Ahnak, a protein of 5643 amino acids, interacts with the regulatory beta-subunit of cardiac calcium channels and with F-actin. Recently, we defined the binding sites among the protein partners in the carboxyl-terminal domain of ahnak. Here we further narrowed down the beta(2)-interaction sites to the carboxyl-terminal 188 amino acids of ahnak by the recombinant ahnak protein fragments P3 (amino acids 5456-5556) and P4 (amino acids 5556-5643). The effects of these P3 and P4 fragments on the calcium current were investigated under whole-cell patch clamp conditions on rat ventricular cardiomyocytes. P4 but not P3 increased significantly the current amplitude by 22.7 +/- 5% without affecting its voltage dependence. The slow component of calcium current inactivation was slowed down by both P3 and P4, whereas only P3 slowed significantly the fast one. The composite recombinant protein fragment P3-P4 induced similar modifications to the ones induced by each of the ahnak fragments. In the presence of carboxyl-terminal ahnak protein fragments, isoprenaline induced a similar relative increase in current amplitude and shift in current kinetics. The actin-stabilizing agents, phalloidin and jasplakinolide, did not modify the effects of these ahnak protein fragments on calcium current in control conditions nor in the presence of isoprenaline. Hence, our results suggest that the functional effects of P3, P4, and P3-P4 on calcium current are mediated by targeting the ahnak-beta(2)-subunit interaction rather than by targeting the ahnak-F-actin interaction. More specifically they suggest that binding of the beta(2)-subunit to the endogenous subsarcolemmal giant ahnak protein re-primes the alpha(1C)/beta(2)-subunit interaction and that the ahnak-derived proteins relieve the beta(2)-subunit from this inhibition.

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.

Functional roles of cytoplasmic loops and pore lining transmembrane helices in the voltage-dependent inactivation of HVA calcium channels

Voltage-dependent inactivation of calcium channels is a key mechanism for regulating intracellular calcium levels and neuronal excitability. In sodium and potassium channels, the molecular determinants that govern fast inactivation involve pore block by a cytoplasmic gating particle. As we discuss here, there is an increasing body of evidence that is consistent with a qualitatively similar inactivation mechanism in high-voltage-activated calcium channels. Work from a number of laboratories has implicated both cytoplasmic regions and the pore-lining S6 transmembrane helices in the inactivation process. Together with our recent findings, this leads us to propose a model in which the intracellular domain I-II linker region acts as a 'hinged lid' that physically occludes the pore by docking to the cytoplasmic ends of the S6 segments. We further propose that the ancillary calcium channel Beta subunits differentially modulate inactivation kinetics by binding to and thereby regulating the mobility of the putative inactivation gate. Indeed, additional evidence suggests that the carboxy terminus, amino terminus and domain III-IV linker regions of the channel modulate inactivation rates through interactions with the I-II linker per se, or indirectly via the ancillary Beta subunits. Taken together, the fast voltage-dependent inactivation of calcium channels appears reminiscent of that of sodium channels, but appears to show a more complex regulation through intramolecular interactions between the putative inactivation gate and other cytoplasmic regions.

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.

Genetic characterization of a new splice variant of the beta2 subunit of the voltage-dependent calcium channel

This study reports a novel splice variant form of the voltage-dependent calcium channel beta2 subunit (beta2g). This variant is composed of the conserved amino-terminal sequences of the beta2a subunit, but lacks the beta-subunit interaction domain (BID), which is thought essential for interactions with the alpha1 subunit. Gene structure analysis revealed that this gene was composed of 13 translated exons spread over 107 kb of the genome. The gene structure of the beta2 subunit was similar in exon-intron organization to the murine beta3 and human beta4 subunits. Electrophysiological evaluation revealed that beta2a and 2g affected channel properties in different ways. The beta2a subunit increased the peak amplitude, but failed to increase channel inactivation, while beta2g had no significant effects on either the peak current amplitude or channel inactivation. Other beta3 subunits, such as beta3 and beta4, significantly increased the peak current and accelerated current inactivation.

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.

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.

Distinctive modulatory effects of five human auxiliary beta2 subunit splice variants on L-type calcium channel gating

Sequence analysis of the human genome permitted cloning of five Ca(2+)-channel beta(2) splice variants (beta(2a)-beta(2e)) that differed only in their proximal amino-termini. The functional consequences of such beta(2)-subunit diversity were explored in recombinant L-type channels reconstituted in HEK 293 cells. Beta(2a) and beta(2e) targeted autonomously to the plasma membrane, whereas beta(2b)-beta(2d) localized to the cytosol when expressed in HEK 293 cells. The pattern of modulation of L-type channel voltage-dependent inactivation gating correlated with the subcellular localization of the component beta(2) variant-membrane-bound beta(2a) and beta(2e) subunits conferred slow(er) channel inactivation kinetics and displayed a smaller fraction of channels recovering from inactivation with fast kinetics, compared to beta(2b)-beta(2d) channels. The varying effects of beta(2) subunits on inactivation gating were accounted for by a quantitative model in which L-type channels reversibly distributed between fast and slow forms of voltage-dependent inactivation-membrane-bound beta(2) subunits substantially decreased the steady-state fraction of fast inactivating channels. Finally, the beta(2) variants also had distinctive effects on L-type channel steady-state activation gating, as revealed by differences in the waveforms of tail-activation (G-V) curves, and conferred differing degrees of prepulse facilitation to the channel. Our results predict important physiological consequences arising from subtle changes in Ca(2+)-channel beta(2)-subunit structure due to alternative splicing and emphasize the utility of splice variants in probing structure-function mechanisms.

Cloning and functional characterization of squid voltage-dependent Ca2+ channel beta subunits: involvement of N-terminal sequences in differential modulation of the current

cDNAs that encode beta subunits of voltage-dependent Ca(2+) channel were cloned from the optic lobe of the squid Loligo bleekeri. The subunits, LoCa(v)beta(1a) and LoCa(v)beta(1b) are 96% identical in amino acid sequence. The sole sequence differences are in the N-terminal region and in a five amino acid insertion in the central region of LoCa(v)beta(1b). RT-PCR revealed that LoCa(v)beta(1a) and LoCa(v)beta(1b) transcripts were expressed mainly in the optic lobe and stellate ganglion, and more weakly in mantle muscle, systemic heart, gill, branchial heart, stomach and liver. Coexpression of LoCa(v)beta(1a) or LoCa(v)beta(1b) with mammalian Ca(v)2.3 and alpha(2)/delta subunits in the Xenopus oocyte resulted in high-voltage-activated currents, and showed slow current inactivation and moderate steady-state inactivation. Comparison of the squid subunits with four mammalian beta subunits, beta(1b), beta(2a), beta(3) and beta(4), demonstrated that the modulatory effects of the beta subunits on steady-state inactivation kinetics were beta(3)<beta(4) approximately beta(1b)<LoCa(v)beta(1a) approximately LoCa(v)beta(1b)<beta(2a). LoCa(v)beta(1a)-induced current amplitude was about two to four times higher than that of LoCa(v)beta(1b). Experiments with point mutants and chimeras suggest that potential PKC and CK2 phosphorylation sites in the N-terminal region of LoCa(v)beta(1b) affect the current amplitude reciprocally, and may be responsible for regulating current amplitude.

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.

Truncation of the carboxyl terminus of the dihydropyridine receptor beta1a subunit promotes Ca2+ dependent excitation-contraction coupling in skeletal myotubes

We investigated the contribution of the carboxyl terminus region of the beta1a subunit of the skeletal dihydropyridine receptor (DHPR) to the mechanism of excitation-contraction (EC) coupling. cDNA-transfected beta1 KO myotubes were voltage clamped, and Ca(2+) transients were analyzed by confocal fluo-4 fluorescence. A chimera with an amino terminus half of beta2a and a carboxyl terminus half of beta1a (beta2a 1-287/beta1a 325-524) recapitulates skeletal-type EC coupling quantitatively and was used to generate truncated variants lacking 7 to 60 residues from the beta1a-specific carboxyl terminus (Delta7, Delta21, Delta29, Delta35, and Delta60). Ca(2+) transients recovered by the control chimera have a sigmoidal Ca(2+) fluorescence (DeltaF/F) versus voltage curve with saturation at potentials more positive than +30 mV, independent of external Ca(2+) and stimulus duration. In contrast, the amplitude of Ca(2+) transients expressed by the truncated variants varied with the duration of the pulse, and for Delta29, Delta35, and Delta60, also varied with external Ca(2+) concentration. For Delta7 and Delta21, a 50-ms depolarization produced a sigmoidal DeltaF/F versus voltage curve with a lower than control maximum fluorescence. Moreover, for Delta29, Delta35, and Delta60, a 200-ms depolarization increased the maximum fluorescence and changed the shape of the DeltaF/F versus voltage curve, from sigmoidal to bell-shaped, with a maximum at approximately +30 mV. The change in voltage dependence, together with the external Ca(2+) dependence and additional controls with ryanodine, indicated a loss of skeletal-type EC coupling and the emergence of an EC coupling component triggered by the Ca(2+) current. Analyses of d(DeltaF/F)/dt showed that the rate of cytosolic Ca(2+) increase during the Ca(2+) transient was fivefold faster for the control chimera than for the severely truncated variants (Delta29, Delta35, and Delta60) and was consistent with the kinetics of the DHPR Ca(2+) current. In summary, absence of the beta1a-specific carboxyl terminus (last 29 to 60 residues of the control chimera) results in a loss of the fast component of the Ca(2+) transient, bending of the DeltaF/F versus voltage curve, and emergence of EC coupling triggered by the Ca(2+) current. The studies underscore the essential role of the carboxyl terminus region of the DHPR beta1a subunit in fast voltage dependent EC coupling in skeletal myotubes.

Voltage-gated Ca2+ channel Ca(V)1.3 subunit expressed in the hair cell epithelium of the sacculus of the trout Oncorhynchus mykiss: cloning and comparison across vertebrate classes

Full-length sequence (>6.5 kb) has been determined for the Ca(V)1.3 pore-forming subunit of the voltage-gated Ca(2+) channel from the saccular hair cells of the rainbow trout (Oncorhynchus mykiss). Primary structure was obtained from overlapping PCR and cloned fragments, amplified by primers based on teleost, avian, and mammalian sources. Trout saccular Ca(V)1.3 was localized to hair cells, as evidenced by its isolation from an epithelial layer in which the hair cell is the only intact cell type. The predicted amino acid sequence of the trout hair cell Ca(V)1.3 is approximately 70% identical to the sequences of avian and mammalian Ca(V)1.3 subunits and shows L-type characteristics. The trout hair cell Ca(V)1.3 expresses a 26-aa insert in the I-II cytoplasmic loop (exon 9a) and a 10-aa insert in the IVS2-IVS3 cytoplasmic loop (exon 30a), neither of which is appreciably represented in trout brain. The exon 9a insert also occurs in hair cell organs of chick and rat, and appears as an exon in human genomic Ca(V)1.3 sequence (but not in the Ca(V)1.3 coding sequence expressed in human brain or pancreas). The exon 30a insert, although expressed in hair cells of chick as well as trout, does not appear in comparable rat or human tissues. Further, the IIIS2 region shows a splice choice (exon 22a) that is associated with the hair cell organs of trout, chick, and rat, but is not found in human genomic sequence. The elucidation of the primary structure of the voltage-gated Ca(2+) channel Ca(V)1.3 subunit from hair cells of the teleost, representing the lowest of the vertebrate classes, suggests a generality of sensory mechanism for Ca(V)1.3 across hair cell systems. In particular, the exon 9a insert of this channel appears to be the molecular feature most consistently associated with hair cells from fish to mammal, consonant with the hypothesis that the latter region may be a signature for the hair cell.

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.

Novel functional properties of Ca(2+) channel beta subunits revealed by their expression in adult rat heart cells

Recombinant adenoviruses were used to overexpress green fluorescent protein (GFP)-fused auxiliary Ca(2+) channel beta subunits (beta(1)-beta(4)) in cultured adult rat heart cells, to explore new dimensions of beta subunit functions in vivo. Distinct beta-GFP subunits distributed differentially between the surface sarcolemma, transverse elements, and nucleus in single heart cells. All beta-GFP subunits increased the native cardiac whole-cell L-type Ca(2+) channel current density, but produced distinctive effects on channel inactivation kinetics. The degree of enhancement of whole-cell current density was non-uniform between beta subunits, with a rank order of potency beta(2a) approximately equal to beta(4) > beta(1b) > beta(3). For each beta subunit, the increase in L-type current density was accompanied by a correlative increase in the maximal gating charge (Q(max)) moved with depolarization. However, beta subunits produced characteristic effects on single L-type channel gating, resulting in divergent effects on channel open probability (P(o)). Quantitative analysis and modelling of single-channel data provided a kinetic signature for each channel type. Spurred on by ambiguities regarding the molecular identity of the actual endogenous cardiac L-type channel beta subunit, we cloned a new rat beta(2) splice variant, beta(2b), from heart using 5' rapid amplification of cDNA ends (RACE) PCR. By contrast with beta(2a), expression of beta(2b) in heart cells yielded channels with a microscopic gating signature virtually identical to that of native unmodified channels. Our results provide novel insights into beta subunit functions that are unattainable in traditional heterologous expression studies, and also provide new perspectives on the molecular identity of the beta subunit component of cardiac L-type Ca(2+) channels. Overall, the work establishes a powerful experimental paradigm to explore novel functions of ion channel subunits in their native environments.

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.

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.

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.

Identification of inactivation determinants in the domain IIS6 region of high voltage-activated calcium channels

We have recently reported that transfer of the domain IIS6 region from rapidly inactivating R-type (alpha(1E)) calcium channels to slowly inactivating L-type (alpha(1C)) calcium channel confers rapid inactivation (Stotz, S. C., Hamid, J., Spaetgens, R. L., Jarvis, S. E., and Zamponi, G. W. (2000) J. Biol. Chem. 275, 24575-24582). Here we have identified individual amino acid residues in the IIS6 regions that are responsible for these effects. In this region, alpha(1C) and alpha(1E) channels differ in seven residues, and exchanging five of those residues individually or in combination did not significantly affect inactivation kinetics. By contrast, replacement of residues Phe-823 or Ile-829 of alpha(1C) with the corresponding alpha(1E) residues significantly accelerated inactivation rates and, when substituted concomitantly, approached the rapid inactivation kinetics of R-type channels. A systematic substitution of these residues with a series of other amino acids revealed that decreasing side chain size at position 823 accelerates inactivation, whereas a dependence of the inactivation kinetics on the degree of hydrophobicity could be observed at position 829. Although these point mutations facilitated rapid entry into the inactivated state of the channel, they had little to no effect on the rate of recovery from inactivation. This suggests that the development of and recovery from inactivation are governed by separate structural determinants. Finally, the effects of mutations that accelerated alpha(1C) inactivation could still be antagonized following coexpression of the rat beta(2a) subunit or by domain I-II linker substitutions that produce ultra slow inactivation of wild type channels, indicating that the inactivation kinetics seen with the mutants remain subject to regulation by the domain I-II linker. Overall, our results provide novel insights into a complex process underlying calcium channel inactivation.

Modulation of L-type Ca2+ current but not activation of Ca2+ release by the gamma1 subunit of the dihydropyridine receptor of skeletal muscle

BACKGROUND

The multisubunit (alpha1S,alpha2-delta, beta1a and gamma1) skeletal muscle dihydropyridine receptor (DHPR) transduces membrane depolarization into release of Ca2+ from the sarcoplasmic reticulum (SR) and also acts as an L-type Ca2+ channel. To more fully investigate the function of the gamma1 subunit in these two processes, we produced mice lacking this subunit by gene targeting.

RESULTS

Mice lacking the DHPR gamma1 subunit (gamma1 null) survive to adulthood, are fertile and have no obvious gross phenotypic abnormalities. The gamma1 subunit is expressed at approximately half the normal level in heterozygous mice (gamma1 het). The density of the L-type Ca2+ current in gamma1 null and gamma1 het myotubes was higher than in controls. Inactivation of the Ca2+ current produced by a long depolarization was slower and incomplete in gamma1 null and gamma1 het myotubes, and was shifted to a more positive potential than in controls. However, the half-activation potential of intramembrane charge movements was not shifted, and the maximum density of the total charge was unchanged. Also, no shift was observed in the voltage-dependence of Ca2+ transients. gamma1 null and gamma1 het myotubes had the same peak Ca2+ amplitude vs. voltage relationship as control myotubes.

CONCLUSIONS

The L-type Ca2+ channel function, but not the SR Ca2+ release triggering function of the skeletal muscle dihydropyridine receptor, is modulated by the gamma1 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 [beta]2a subunit is a molecular groom for the Ca2+ channel inactivation gate

Ca(2+) channel inactivation is a key element in controlling the level of Ca(2+) entry through voltage-gated Ca(2+) channels. Interaction between the pore-forming alpha(1) subunit and the auxiliary beta subunit is known to be a strong modulator of voltage-dependent inactivation. Here, we demonstrate that an N-terminal membrane anchoring site (MAS) of the beta(2a) subunit strongly reduces alpha(1A) (Ca(V)2.1) Ca(2+) channel inactivation. This effect can be mimicked by the addition of a transmembrane segment to the N terminus of the beta(2a) subunit. Inhibition of inactivation by beta(2a) also requires a link between MAS and another important molecular determinant, the beta interaction domain (BID). Our data suggest that mobility of the Ca(2+) channel I-II loop is necessary for channel inactivation. Interaction of this loop with other identified intracellular channel domains may constitute the basis of voltage-dependent inactivation. We thus propose a conceptually novel mechanism for slowing of inactivation by the beta(2a) subunit, in which the immobilization of the channel inactivation gate occurs by means of MAS and BID.

Functional coupling between 'R-type' Ca2+ channels and insulin secretion in the insulinoma cell line INS-1

Among voltage-gated Ca2+ channels the non-dihydropyridine-sensitive alpha1E subunit is functionally less well characterized than the structurally related alpha1A (omega-agatoxin-IVA sensitive, P- /Q-type) and alpha1B (omega-conotoxin-GVIA sensitive, N-type) subunits. In the rat insulinoma cell line, INS-1, a tissue-specific splice variant of alpha1E (alpha1Ee) has been characterized at the mRNA and protein levels, suggesting that INS-1 cells are a suitable model for investigating the function of alpha1Ee. In alpha1E-transfected human embryonic kidney (HEK-293) cells the alpha1E-selective peptide antagonist SNX-482 (100 nM) reduces alpha1Ed- and alpha1Ee-induced Ba2+ inward currents in the absence and presence of the auxiliary subunits beta3 and alpha2delta-2 by more than 80%. The inhibition is fast and only partially reversible. No effect of SNX-482 was detected on the recombinant T-type Ca2+ channel subunits alpha1G, alpha1H, and alpha1I showing that the toxin from the venom of Hysterocrates gigas is useful as an alpha1E-selective antagonist. After blocking known components of Ca2+ channel inward current in INS-1 cells by 2 microM (+/-)-isradipine plus 0.5 microM omega-conotoxin-MVIIC, the remaining current is reduced by 100 nM SNX-482 from -12.4 +/- 1.2 pA/pF to -7.6 +/- 0.5 pA/pF (n = 9). Furthermore, in INS-1 cells, glucose- and KCl-induced insulin release are reduced by SNX-482 in a dose-dependent manner leading to the conclusion that alpha1E, in addition to L-type and non-L-type (alpha1A-mediated) Ca2+ currents, is involved in Ca2+ dependent insulin secretion of INS-1 cells.

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.

Greater diversity than previously thought of chromaffin cell Ca2+ channels, derived from mRNA identification studies

Using reverse transcription followed by PCR amplification (RT-PCR), we have identified multiple messenger RNAs encoding for the neuronal pore-forming Ca(2+) channel subunits alpha(1A) (P/Q channel), alpha(1B) (N channel), alpha(1D) (neuronal/endocrine L channel), alpha(1E) (R channel), alpha(1G-H) (T channel) and alpha(1S) (skeletal muscle L channel) in bovine chromaffin cells. mRNAs for the auxiliary beta(2), beta(3), beta(4), alpha(2)/delta and gamma(2) subunits were also identified. In agreement with these molecular data, perforated patch-clamp recordings of whole-cell Ca(2+) currents reveal the existence of functional R-type Ca(2+) channels in these cells that were previously undetected with other techniques. Our results provide a molecular frame for a much wider functional diversity of Ca(2+) channels in chromaffin cells than that previously established using pharmacological and electrophysiological approaches.

Modulation of slow inactivation in class A Ca2+ channels by beta-subunits

beta-subunit modulation of slow inactivation of class A calcium (Ca2+) channels was studied with two-microlectrode voltage clamp after expression of the alpha1A- (BI-2) together with beta1a-, beta2a-, beta3- or beta4-subunits in Xenopus oocytes. On- and off-rates of slow inactivation were estimated from the kinetics of recovery from slow inactivation. Ca2+ channels with an alpha1A/beta-subunit composition inducing the slower rate of fast inactivation displayed the faster rate of slow inactivation. The corresponding order of slow inactivation time constants (tau[onset]) was: alpha1A/beta2a, 33 +/- 3 s; alpha1A/beta4, 42 +/- 4 s; alpha1A/beta1a, 59 +/- 4 s; alpha1A/beta3, 67 +/- 5 s (n >= 7). Recovery of class A Ca2+ channels from slow inactivation was voltage dependent and accelerated at hyperpolarized voltages. At a given holding potential recovery kinetics were not significantly modulated by different beta-subunits. Two mutations in segment IIIS6 (IF1612/1613AA) slowed fast inactivation and accelerated the onset of slow inactivation in the resulting mutant (alpha1A/IF-AA/beta3) in a similar manner as coexpression of the beta2a-subunit. Recovery from slow inactivation was slightly slowed in the double mutant. Our data suggest that class A Ca2+ channels enter the 'slow inactivated' state more willingly from the open than from the 'fast inactivated' state. The rate of slow inactivation is, therefore, indirectly modulated by different beta-subunits. Fast and slow inactivation in class A Ca2+ channels appears to represent structurally independent conformational changes. Fast inactivation is not a prerequisite for slow inactivation.

The role of dynamic palmitoylation in Ca2+ channel inactivation

N- and P/Q-type Ca(2+) channels regulate a number of critical physiological processes including synaptic transmission and hormone secretion. These Ca(2+) channels are multisubunit proteins, consisting of a pore-forming alpha(1), and accessory beta and alpha(2)delta subunits each encoded by multiple genes and splice variants. beta subunits alter current amplitude and kinetics. The beta(2a) subunit is associated with slowed inactivation, an effect that requires the palmitoylation of two N-terminal cysteine residues in beta(2a). In the current manuscript, we studied steady state inactivation properties of native N- and P/Q-type Ca(2+) channels and recombinant N-type Ca(2+) channels. When bovine alpha(1B) and beta(2a) and human alpha(2)delta were coexpressed in tsA 201 cells, we observed significant variations in inactivation; some cells exhibited virtually no inactivation as the holding potential was altered whereas others exhibited significant inactivation. A similar variability in inactivation was observed in native channels from bovine chromaffin cells. In individual chromaffin cells, the amount of inactivation exhibited by N-type channels was correlated with the inactivation of P/Q-type channels, suggesting a shared mechanism. Our results with recombinant channels with known beta subunit composition indicated that inactivation could be dynamically regulated, possibly by alterations in beta subunit palmitoylation. Tunicamycin, which inhibits palmitoylation, increased steady-state inactivation of Ca(2+) channels in chromaffin cells. Cerulenin, another drug that inhibits palmitoylation, also increased inactivation. Tunicamycin produced a similar effect on recombinant N-type Ca(2+) channels containing beta(2a) but not beta(2b) or beta(2a) subunits mutated to be palmitoylation deficient. Our results suggest that Ca(2+) channels containing beta(2a) subunits may be regulated by dynamic palmitoylation.

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.

Expression of the alpha(2)delta subunit interferes with prepulse facilitation in cardiac L-type calcium channels

We investigated the role of the accessory alpha(2)delta subunit on the voltage-dependent facilitation of cardiac L-type Ca(2+) channels (alpha(1C)). alpha(1C) Channels were coexpressed in Xenopus oocytes with beta(3) and alpha(2)delta calcium channel subunits. In alpha(1C) + beta(3), the amplitude of the ionic current (measured during pulses to 10 mV) was in average approximately 1.9-fold larger after the application of a 200-ms prepulse to +80 mV. This phenomenon, commonly referred to as voltage-dependent facilitation, was not observed when alpha(2)delta was coexpressed with alpha(1C) + beta(3). In alpha(1C) + beta(3), the prepulse produced a left shift ( approximately 40 mV) of the activation curve. Instead, the activation curve for alpha(1C) + beta(3) + alpha(2)delta was minimally affected by the prepulse and had a voltage dependence very similar to the G-V curve of the alpha(1C) + beta(3) channel facilitated by the prepulse. Coexpression of alpha(2)delta with alpha(1C) + beta(3) seems to mimic the prepulse effect by shifting the activation curve toward more negative potentials, leaving little room for facilitation. The facilitation of alpha(1C) + beta(3) was associated with an increase of the charge movement. In the presence of alpha(2)delta, the charge remained unaffected after the prepulse. Coexpression of alpha(2)delta seems to set all the channels in a conformational state from where the open state can be easily reached, even without prepulse.

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.

Determinants of voltage-dependent inactivation affect Mibefradil block of calcium channels

The voltage gated calcium channel family is a major target for a range of therapeutic drugs. Mibefradil (Ro 40-5967) belongs to a new chemical class of these molecules which differs from other Ca2+ antagonists by its ability to potently block T-type Ca2+ channels. However, this molecule has also been shown to inhibit other Ca2+ channel subtypes. To further analyze the mechanism governing the Ca2+ channel-Mibefradil interaction, we examined the effect of Mibefradil on various recombinant Ca2+ channels expressed in mammalian cells from their cloned cDNAs, using Ca2+ as the permeant ion at physiological concentration. Expression of alpha1A, alpha1C, and alpha1E in tsA 201 cells resulted in Ca2+ currents with functional characteristics closely related to those of their native counterparts. Mibefradil blocked alpha1A and alpha1E with a Kd comparable to that reported for T-type channels, but had a lower affinity (approximately 30-fold) for alpha1C. For each channel, inhibition by Mibefradil was consistent with high-affinity binding to the inactivated state. Modulation of the voltage-dependent inactivation properties by the nature of the coexpressed beta subunit or the alpha1 splice variant altered block at the Mibefradil receptor site. Therefore, we conclude that the tissue and sub-cellular localization of calcium channel subunits as well as their specific associations are essential parameters to understand the in vivo effects of Mibefradil.

Inactivation properties of human recombinant class E calcium channels

The electrophysiological and pharmacological properties of alpha(1E)-containing Ca(2+) channels were investigated by using the patch-clamp technique in the whole cell configuration, in HEK 293 cells stably expressing the human alpha(1E) together with alpha(2b) and beta(1b) accessory subunits. These channels had current-voltage (I-V) characteristics resembling those of high-voltage-activated (HVA) Ca(2+) channels (threshold at -30 mV and peak amplitude at +10 mV in 5 mM Ca(2+)). The currents activated and deactivated with a fast rate, in a time- and voltage-dependent manner. No difference was found in their relative permeability to Ca(2+) and Ba(2+). Inorganic Ca(2+) channel blockers (Cd(2+), Ni(2+)) blocked completely and potently the alpha(1E,)/alpha(2b)delta/beta(1b) mediated currents (IC(50) = 4 and 24.6 microM, respectively). alpha(1E)-mediated currents inactivated rapidly and mainly in a non-Ca(2+)-dependent manner, as evidenced by the fact that 1) decreasing extracellular Ca(2+) from 10 to 2 mM and 2) changing the intracellular concentration of the Ca(2+) chelator 1. 2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA), did not affect the inactivation characteristics; 3) there was no clear-cut bell-shaped relationship between test potential and inactivation, as would be expected from a Ca(2+)-dependent event. Although Ba(2+) substitution did not affect the inactivation of alpha(1E) channels, Na(+) substitution revealed a small but significant reduction in the extent and rate of inactivation, suggesting that besides the presence of dominant voltage-dependent inactivation, alpha(1E) channels are also affected by a divalent cation-dependent inactivation process. We have analyzed the Ca(2+) currents produced by a range of imposed action potential-like voltage protocols (APVPs). The amplitude and area of the current were dependent on the duration of the waveform employed and were relatively similar to those described for HVA calcium channels. However, the peak latency resembled that obtained for low-voltage-activated (LVA) calcium channels. Short bursts of APVPs applied at 100 Hz produced a depression of the Ca(2+) current amplitude, suggesting an accumulation of inactivation likely to be calcium dependent. The human alpha(1E) gene seems to participate to a Ca(2+) channel type with biophysical and pharmacological properties partly resembling those of LVA and those of HVA channels, with inactivation characteristics more complex than previously believed.

Cloning of the beta(2a) subunit of the voltage-dependent calcium channel from human heart: cooperative effect of alpha(2)/delta and beta(2a) on the membrane expression of the alpha(1C) subunit

Expression and membrane localization of an epitope-tagged human Ca(2+) channel alpha(1C) subunit were monitored in Xenopus oocytes by confocal microscopy and electrophysiological recording. When alpha(2)/delta and beta(2a) were separately coexpressed with the alpha(1C) subunit, assessment by confocal microscopy showed an 86 and 225% increase of the channel density, respectively. Simultaneous coexpression of alpha(2)/delta and beta(2a) subunits resulted in a cooperative (470%) increase. Electrophysiological measurements performed in parallel revealed that the current augmentation by the alpha(2)/delta subunit is totally attributable to an increase in channel density, whereas the beta(2a) subunit, in addition to increasing channel density, also facilitates channel opening.

Involvement of the carboxy-terminus region of the dihydropyridine receptor beta1a subunit in excitation-contraction coupling of skeletal muscle

Skeletal muscle knockout cells lacking the beta subunit of the dihydropyridine receptor (DHPR) are devoid of slow L-type Ca(2+) current, charge movements, and excitation-contraction coupling, despite having a normal Ca(2+) storage capacity and Ca(2+) spark activity. In this study we identified a specific region of the missing beta1a subunit critical for the recovery of excitation-contraction. Experiments were performed in beta1-null myotubes expressing deletion mutants of the skeletal muscle-specific beta1a, the cardiac/brain-specific beta2a, or beta2a/beta1a chimeras. Immunostaining was used to determine that all beta constructs were expressed in these cells. We examined the Ca(2+) conductance, charge movements, and Ca(2+) transients measured by confocal fluo-3 fluorescence of transfected myotubes under whole-cell voltage-clamp. All constructs recovered an L-type Ca(2+) current with a density, voltage-dependence, and kinetics of activation similar to that recovered by full-length beta1a. In addition, all constructs except beta2a mutants recovered charge movements with a density similar to full-length beta1a. Thus, all beta constructs became integrated into a skeletal-type DHPR and, except for beta2a mutants, all restored functional DHPRs to the cell surface at a high density. The maximum amplitude of the Ca(2+) transient was not affected by separate deletions of the N-terminus of beta1a or the central linker region of beta1a connecting two highly conserved domains. Also, replacement of the N-terminus half of beta1a with that of beta2a had no effect. However, deletion of 35 residues of beta1a at the C-terminus produced a fivefold reduction in the maximum amplitude of the Ca(2+) transients. A similar observation was made by deletion of the C-terminus of a chimera in which the C-terminus half was from beta1a. The identified domain at the C-terminus of beta1a may be responsible for colocalization of DHPRs and ryanodine receptors (RyRs), or may be required for the signal that opens the RyRs during excitation-contraction coupling. This new role of DHPR beta in excitation-contraction coupling represents a cell-specific function that could not be predicted on the basis of functional expression studies in heterologous cells.

Current modulation and membrane targeting of the calcium channel alpha1C subunit are independent functions of the beta subunit

1. The beta subunits of voltage-sensitive calcium channels facilitate the incorporation of channels into the plasma membrane and modulate calcium currents. In order to determine whether these two effects of the beta subunit are interdependent or independent of each other we studied plasma membrane incorporation of the channel subunits with green fluorescent protein and immunofluorescence labelling, and current modulation with whole-cell and single-channel patch-clamp recordings in transiently transfected human embryonic kidney tsA201 cells. 2. Coexpression of rabbit cardiac muscle alpha1C with rabbit skeletal muscle beta1a, rabbit heart/brain beta2a or rat brain beta3 subunits resulted in the colocalization of alpha1C with beta and in a marked translocation of the channel complexes into the plasma membrane. In parallel, the whole-cell current density and single-channel open probability were increased. Furthermore, the beta2a isoform specifically altered the voltage dependence of current activation and the inactivation kinetics. 3. A single amino acid substitution in the beta subunit interaction domain of alpha1C (alpha1CY467S) disrupted the colocalization and plasma membrane targeting of both subunits without affecting the beta subunit-induced modulation of whole-cell currents and single-channel properties. 4. These results show that the modulation of calcium currents by beta subunits can be explained by beta subunit-induced changes of single-channel properties, but the formation of stable alpha1C-beta complexes and their increased incorporation into the plasma membrane appear not to be necessary for functional modulation.

Measurement of calcium channel inactivation is dependent upon the test pulse potential

We have developed two methods to measure Ca2+ channel inactivation in Lymnaea neurons-one method, based upon the conventional double-pulse protocol, uses currents during a moderately large depolarizing pulse, and the other uses tail currents after a very strong activating pulse. Both methods avoid contamination by proton currents and are unaffected by rundown of Ca2+ current. The magnitude of inactivation measured differs for the two methods; this difference arises because the measurement of inactivation is inherently dependent upon the test pulse voltage used to monitor the Ca2+ channel conductance. We discuss two models that can generate such test pulse dependence of inactivation measurements-a two-channel model and a two-open-state model. The first model accounts for this by assuming the existence of two types of Ca2+ channels, different proportions of which are activated by the different test pulses. The second model assumes only one Ca2+ channel type, with two closed and open states; in this model, the test pulse dependence is due to the differential activation of channels in the two closed states by the test pulses. Test pulse dependence of inactivation measurements of Ca2+ channels may be a general phenomenon that has been overlooked in previous studies.