The guanylate kinase domain of the beta-subunit of voltage-gated calcium channels suffices to modulate gating

Nanoenviroments of the β-Subunit of L-Type Voltage-Gated Calcium Channels in Adult Cardiomyocytes

In cardiomyocytes, Ca²⁺ influx through L-type voltage-gated calcium channels (LTCCs) following membrane depolarization regulates crucial Ca²⁺-dependent processes including duration and amplitude of the action potentials and excitation-contraction coupling. LTCCs are heteromultimeric proteins composed of the Ca_vα₁, Ca_vβ, Ca_vα₂δ and Ca_vγ subunits. Here, using ascorbate peroxidase (APEX2)-mediated proximity labeling and quantitative proteomics, we identified 61 proteins in the nanoenvironments of Ca_vβ₂ in cardiomyocytes. These proteins are involved in diverse cellular functions such as cellular trafficking, cardiac contraction, sarcomere organization and excitation-contraction coupling. Moreover, pull-down assays and co-immunoprecipitation analyses revealed that Ca_vβ₂ interacts with the ryanodine receptor 2 (RyR2) in adult cardiomyocytes, probably coupling LTCCs and the RyR2 into a supramolecular complex at the dyads. This interaction is mediated by the Src-homology 3 domain of Ca_vβ₂ and is necessary for an effective pacing frequency-dependent increase of the Ca²⁺-induced Ca²⁺ release mechanism in cardiomyocytes.

Direct inhibition of CaV2.3 by Gem is dynamin dependent and does not require a direct alfa/beta interaction

The Rad, Rem, Rem2, and Gem/Kir (RGK) sub-family of small GTP-binding proteins are crucial in regulating high voltage-activated (HVA) calcium channels. RGK proteins inhibit calcium current by either promoting endocytosis or reducing channel activity. They all can associate directly with Ca²⁺ channel β subunit (Ca_Vβ), and the binding between Ca_Vα1/Ca_Vβ appears essential for the endocytic promotion of Ca_V1.X, Ca_V2.1, and Ca_V2.2 channels. In this study, we investigated the inhibition of Ca_V2.3 channels by RGK proteins in the absence of Ca_Vβ. To this end, Xenopus laevis oocytes expressing Ca_V2.3 channels devoid of auxiliary subunit were injected with purified Gem and Rem and found that only Gem had an effect. Ca currents and charge movements were reduced by injection of Gem, pointing to a reduction in the number of channels in the plasma membrane. Since this reduction was ablated by co-expression of the dominant-negative mutant of dynamin K44A, enhanced endocytosis appears to mediate this reduction in the number of channels. Thus, Gem inhibition of Ca_V2.3 channels would be the only example of a Ca_Vβ independent promotion of dynamin-dependent endocytosis.

CaV β controls the endocytic turnover of CaV 1.2 L-type calcium channel

Membrane depolarization activates the multisubunit Ca_V 1.2 L-type calcium channel initiating various excitation coupling responses. Intracellular trafficking into and out of the plasma membrane regulates the channel's surface expression and stability, and thus, the strength of Ca_V 1.2-mediated Ca²⁺ signals. The mechanisms regulating the residency time of the channel at the cell membrane are unclear. Here, we coexpressed the channel core complex Ca_V 1.2α₁ pore-forming and auxiliary Ca_V β subunits and analyzed their trafficking dynamics from single-particle-tracking trajectories. Speed histograms obtained for each subunit were best fitted to a sum of diffusive and directed motion terms. The same mean speed for the highest-mobility state underlying directed motion was found for all subunits. The frequency of this component increased by covalent linkage of Ca_V β to Ca_V 1.2α₁ suggesting that high-speed transport occurs in association with Ca_V β. Selective tracking of Ca_V 1.2α₁ along the postendocytic pathway failed to show the highly mobile state, implying Ca_V β-independent retrograde transport. Retrograde speeds of Ca_V 1.2α₁ are compatible with myosin VI-mediated backward transport. Moreover, residency time at the cell surface was significantly prolonged when Ca_V 1.2α₁ was covalently linked to Ca_V β. Thus, Ca_V β promotes fast transport speed along anterograde trafficking and acts as a molecular switch controlling the endocytic turnover of L-type calcium channels.

AHNAK: The quiet giant in calcium homeostasis

The phosphoprotein AHNAK is a large, ubiquitously expressed scaffolding protein involved in mediating a host of protein-protein interactions. This enables AHNAK to participate in various multi-protein complexes thereby orchestrating a range of diverse biological processes, including tumour suppression, immune regulation and cell architecture maintenance. A less studied but nonetheless equally important function occurs in calcium homeostasis. It does so by largely interacting with the L-type voltage-gated calcium channel (LVGCC) present in the plasma membrane of excitable cells such as muscles and neurons. Several studies have characterized the underlying basis of AHNAK's functional role in calcium channel modulation, which has led to a greater understanding of this cellular process and its associated pathologies. In this article we review and examine recent advances relating to the physiological aspects of AHNAK in calcium regulation. Specifically, we will provide a broad overview of AHNAK including its structural makeup and its interaction with several isoforms of LVGCC, and how these molecular interactions regulate calcium modulation across various tissues and their implication in muscle and neuronal function.

An ancestral MAGUK protein supports the modulation of mammalian voltage-gated Ca2+ channels through a conserved CaVβ-like interface

Eukaryote voltage-gated Ca²⁺ channels of the Ca_V2 channel family are hetero-oligomers formed by the pore-forming Ca_Vα1 protein assembled with auxiliary Ca_Vα2δ and Ca_Vβ subunits. Ca_Vβ subunits are formed by a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain connected through a HOOK domain. The GK domain binds a conserved cytoplasmic region of the pore-forming Ca_Vα1 subunit referred as the "AID". Herein we explored the phylogenetic and functional relationship between Ca_V channel subunits in distant eukaryotic organisms by investigating the function of a MAGUK protein (XM_004990081) cloned from the choanoflagellate Salpingoeca rosetta (Sro). This MAGUK protein (Sroβ) features SH3 and GK structural domains with a 25% primary sequence identity to mammalian Ca_Vβ. Recombinant expression of its cDNA with mammalian high-voltage activated Ca²⁺ channel Ca_V2.3 in mammalian HEK cells produced robust voltage-gated inward Ca²⁺ currents with typical activation and inactivation properties. Like Ca_Vβ, Sroβ prevents fast degradation of total Ca_V2.3 proteins in cycloheximide assays. The three-dimensional homology model predicts an interaction between the GK domain of Sroβ and the AID motif of the pore-forming Ca_Vα1 protein. Substitution of AID residues Trp (W386A) and Tyr (Y383A) significantly impaired co-immunoprecipitation of Ca_V2.3 with Sroβ and functional upregulation of Ca_V2.3 currents. Likewise, a 6-residue deletion within the GK domain of Sroβ, similar to the locus found in mammalian Ca_Vβ, significantly reduced peak current density. Altogether our data demonstrate that an ancestor MAGUK protein reconstitutes the biophysical and molecular features responsible for channel upregulation by mammalian Ca_Vβ through a minimally conserved molecular interface.

Direct interaction of CaVβ with actin up-regulates L-type calcium currents in HL-1 cardiomyocytes

Expression of the β-subunit (CaVβ) is required for normal function of cardiac L-type calcium channels, and its up-regulation is associated with heart failure. CaVβ binds to the α1 pore-forming subunit of L-type channels and augments calcium current density by facilitating channel opening and increasing the number of channels in the plasma membrane, by a poorly understood mechanism. Actin, a key component of the intracellular trafficking machinery, interacts with Src homology 3 domains in different proteins. Although CaVβ encompasses a highly conserved Src homology 3 domain, association with actin has not yet been explored. Here, using co-sedimentation assays and FRET experiments, we uncover a direct interaction between CaVβ and actin filaments. Consistently, single-molecule localization analysis reveals streaklike structures composed by CaVβ2 that distribute over several micrometers along actin filaments in HL-1 cardiomyocytes. Overexpression of CaVβ2-N3 in HL-1 cells induces an increase in L-type current without altering voltage-dependent activation, thus reflecting an increased number of channels in the plasma membrane. CaVβ mediated L-type up-regulation, and CaVβ-actin association is prevented by disruption of the actin cytoskeleton with cytochalasin D. Our study reveals for the first time an interacting partner of CaVβ that is directly involved in vesicular trafficking. We propose a model in which CaVβ promotes anterograde trafficking of the L-type channels by anchoring them to actin filaments in their itinerary to the plasma membrane.

Structure-function of proteins interacting with the α1 pore-forming subunit of high-voltage-activated calcium channels

Openings of high-voltage-activated (HVA) calcium channels lead to a transient increase in calcium concentration that in turn activate a plethora of cellular functions, including muscle contraction, secretion and gene transcription. To coordinate all these responses calcium channels form supramolecular assemblies containing effectors and regulatory proteins that couple calcium influx to the downstream signal cascades and to feedback elements. According to the original biochemical characterization of skeletal muscle Dihydropyridine receptors, HVA calcium channels are multi-subunit protein complexes consisting of a pore-forming subunit (α1) associated with four additional polypeptide chains β, α2, δ, and γ, often referred to as accessory subunits. Twenty-five years after the first purification of a high-voltage calcium channel, the concept of a flexible stoichiometry to expand the repertoire of mechanisms that regulate calcium channel influx has emerged. Several other proteins have been identified that associate directly with the α1-subunit, including calmodulin and multiple members of the small and large GTPase family. Some of these proteins only interact with a subset of α1-subunits and during specific stages of biogenesis. More strikingly, most of the α1-subunit interacting proteins, such as the β-subunit and small GTPases, regulate both gating and trafficking through a variety of mechanisms. Modulation of channel activity covers almost all biophysical properties of the channel. Likewise, regulation of the number of channels in the plasma membrane is performed by altering the release of the α1-subunit from the endoplasmic reticulum, by reducing its degradation or enhancing its recycling back to the cell surface. In this review, we discuss the structural basis, interplay and functional role of selected proteins that interact with the central pore-forming subunit of HVA calcium channels.

The N-terminal domain tethers the voltage-gated calcium channel β2e-subunit to the plasma membrane via electrostatic and hydrophobic interactions

The β-subunit associates with the α1 pore-forming subunit of high voltage-activated calcium channels and modulates several aspects of ion conduction. Four β-subunits are encoded by four different genes with multiple splice variants. Only two members of this family, β2a and β2e, associate with the plasma membrane in the absence of the α1-subunit. Palmitoylation on a di-cysteine motif located at the N terminus of β2a promotes membrane targeting and correlates with the unique ability of this protein to slow down inactivation. In contrast, the mechanism by which β2e anchors to the plasma membrane remains elusive. Here, we identified an N-terminal segment in β2e encompassing a cluster of positively charged residues, which is strictly required for membrane anchoring, and when transferred to the cytoplasmic β1b isoform it confers membrane localization to the latter. In the presence of negatively charged phospholipid vesicles, this segment binds to acidic liposomes dependently on the ionic strength, and the intrinsic fluorescence emission maxima of its single tryptophan blue shifts considerably. Simultaneous substitution of more than two basic residues impairs membrane targeting. Coexpression of the fast inactivating R-type calcium channels with wild-type β2e, but not with a β2e membrane association-deficient mutant, slows down inactivation. We propose that a predicted α-helix within this domain orienting parallel to the membrane tethers the β2e-subunit to the lipid bilayer via electrostatic interactions. Penetration of the tryptophan side chain into the lipidic core stabilizes the membrane-bound conformation. This constitutes a new mechanism for membrane anchoring among the β-subunit family that also sustains slowed inactivation.

Somatic and germline CACNA1D calcium channel mutations in aldosterone-producing adenomas and primary aldosteronism

Adrenal aldosterone-producing adenomas (APAs) constitutively produce the salt-retaining hormone aldosterone and are a common cause of severe hypertension. Recurrent mutations in the potassium channel KCNJ5 that result in cell depolarization and Ca²⁺ influx cause ~40% of these tumors¹. We found five somatic mutations (four altering glycine 403, one altering isoleucine 770) in CACNA1D, encoding a voltage-gated calcium channel, among 43 non-KCNJ5-mutant APAs. These mutations lie in S6 segments that line the channel pore. Both result in channel activation at less depolarized potentials, and glycine 403 mutations also impair channel inactivation. These effects are inferred to cause increased Ca²⁺ influx, the sufficient stimulus for aldosterone production and cell proliferation in adrenal glomerulosa². Remarkably, we identified de novo mutations at the identical positions in two children with a previously undescribed syndrome featuring primary aldosteronism and neuromuscular abnormalities. These findings implicate gain of function Ca²⁺ channel mutations in aldosterone-producing adenomas and primary aldosteronism.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

A short polybasic segment between the two conserved domains of the β2a-subunit modulates the rate of inactivation of R-type calcium channel

Besides opening and closing, high voltage-activated calcium channels transit to a nonconducting inactivated state from which they do not re-open unless the plasma membrane is repolarized. Inactivation is critical for temporal regulation of intracellular calcium signaling and prevention of a deleterious rise in calcium concentration. R-type high voltage-activated channels inactivate fully in a few hundred milliseconds when expressed alone. However, when co-expressed with a particular β-subunit isoform, β(2a), inactivation is partial and develops in several seconds. Palmitoylation of a unique di-cysteine motif at the N terminus anchors β(2a) to the plasma membrane. The current view is that membrane-anchored β(2a) immobilizes the channel inactivation machinery and confers slow inactivation phenotype. β-Subunits contain one Src homology 3 and one guanylate kinase domain, flanked by variable regions with unknown structures. Here, we identified a short polybasic segment at the boundary of the guanylate kinase domain that slows down channel inactivation without relocating a palmitoylation-deficient β(2a) to the plasma membrane. Substitution of the positively charged residues within this segment by alanine abolishes its slow inactivation-conferring phenotype. The linker upstream from the polybasic segment, but not the N- and C-terminal variable regions, masks the effect of this determinant. These results reveal a novel mechanism for inhibiting voltage-dependent inactivation of R-type calcium channels by the β(2a)-subunit that might involve electrostatic interactions with an unknown target on the channel's inactivation machinery or its modulatory components. They also suggest that intralinker interactions occlude the action of the polybasic segment and that its functional availability is regulated by the palmitoylated state of the β(2a)-subunit.

A quartet of leucine residues in the guanylate kinase domain of CaVβ determines the plasma membrane density of the CaV2.3 channel

Ca(V)β subunits are formed by a Src homology 3 domain and a guanylate kinase-like (GK) domain connected through a variable HOOK domain. Complete deletion of the Src homology 3 domain (75 residues) as well as deletion of the HOOK domain (47 residues) did not alter plasma membrane density of Ca(V)2.3 nor its typical activation gating. In contrast, six-residue deletions in the GK domain disrupted cell surface trafficking and functional expression of Ca(V)2.3. Mutations of residues known to carry nanomolar affinity binding in the GK domain of Ca(V)β (P175A, P179A, M195A, M196A, K198A, S295A, R302G, R307A, E339G, N340G, and A345G) did not significantly alter cell surface targeting or gating modulation of Ca(V)2.3. Nonetheless, mutations of a quartet of leucine residues (either single or multiple mutants) in the α3, α6, β10, and α9 regions of the GK domain were found to significantly impair cell surface density of Ca(V)2.3 channels. Furthermore, the normalized protein density of Ca(V)2.3 was nearly abolished with the quadruple Ca(V)β3 Leu mutant L200G/L303G/L337G/L342G. Altogether, our observations suggest that the four leucine residues in Ca(V)β3 form a hydrophobic pocket surrounding key residues in the α-interacting domain of Ca(V)2.3. This interaction appears to play an essential role in conferring Ca(V)β-induced modulation of the protein density of Ca(V)α1 subunits in Ca(V)2 channels.

Gαi2- and Gαi3-specific regulation of voltage-dependent L-type calcium channels in cardiomyocytes

Background

Two pertussis toxin sensitive G_i proteins, G_i2 and G_i3, are expressed in cardiomyocytes and upregulated in heart failure. It has been proposed that the highly homologous G_i isoforms are functionally distinct. To test for isoform-specific functions of G_i proteins, we examined their role in the regulation of cardiac L-type voltage-dependent calcium channels (L-VDCC).

Methods

Ventricular tissues and isolated myocytes were obtained from mice with targeted deletion of either Gα_i2 (Gα_i2^−/−) or Gα_i3 (Gα_i3^−/−). mRNA levels of Gα_i/o isoforms and L-VDCC subunits were quantified by real-time PCR. Gα_i and Ca_vα₁ protein levels as well as protein kinase B/Akt and extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation levels were assessed by immunoblot analysis. L-VDCC function was assessed by whole-cell and single-channel current recordings.

Results

In cardiac tissue from Gα_i2^−/− mice, Gα_i3 mRNA and protein expression was upregulated to 187±21% and 567±59%, respectively. In Gα_i3^−/− mouse hearts, Gα_i2 mRNA (127±5%) and protein (131±10%) levels were slightly enhanced. Interestingly, L-VDCC current density in cardiomyocytes from Gα_i2^−/− mice was lowered (−7.9±0.6 pA/pF, n = 11, p<0.05) compared to wild-type cells (−10.7±0.5 pA/pF, n = 22), whereas it was increased in myocytes from Gα_i3^−/− mice (−14.3±0.8 pA/pF, n = 14, p<0.05). Steady-state inactivation was shifted to negative potentials, and recovery kinetics slowed in the absence of Gα_i2 (but not of Gα_i3) and following treatment with pertussis toxin in Gα_i3^−/−. The pore forming Ca_vα₁ protein level was unchanged in all mouse models analyzed, similar to mRNA levels of Ca_vα₁ and Ca_vβ₂ subunits. Interestingly, at the cellular signalling level, phosphorylation assays revealed abolished carbachol-triggered activation of ERK1/2 in mice lacking Gα_i2.

Conclusion

Our data provide novel evidence for an isoform-specific modulation of L-VDCC by Gα_i proteins. In particular, loss of Gα_i2 is reflected by alterations in channel kinetics and likely involves an impairment of the ERK1/2 signalling pathway.

Enzyme closure and nucleotide binding structurally lock guanylate kinase

We investigate the conformational dynamics and mechanical properties of guanylate kinase (GK) using a multiscale approach combining high-resolution atomistic molecular dynamics and low-resolution Brownian dynamics simulations. The GK enzyme is subject to large conformational changes, leading from an open to a closed form, which are further influenced by the presence of nucleotides. As suggested by recent work on simple coarse-grained models of apo-GK, we primarily focus on GK's closure mechanism with the aim to establish a detailed picture of the hierarchy and chronology of structural events essential for the enzymatic reaction. We have investigated open-versus-closed, apo-versus-holo, and substrate-versus-product-loaded forms of the GK enzyme. Bound ligands significantly modulate the mechanical and dynamical properties of GK and rigidity profiles of open and closed states hint at functionally important differences. Our data emphasizes the role of magnesium, highlights a water channel permitting active site hydration, and reveals a structural lock that stabilizes the closed form of the enzyme.

Homodimerization of the Src homology 3 domain of the calcium channel β-subunit drives dynamin-dependent endocytosis

Voltage-dependent calcium channels constitute the main entry pathway for calcium into excitable cells. They are heteromultimers formed by an α(1) pore-forming subunit (Ca(V)α(1)) and accessory subunits. To achieve a precise coordination of calcium signals, the expression and activity of these channels is tightly controlled. The accessory β-subunit (Ca(V)β), a membrane associated guanylate kinase containing one guanylate kinase (β-GK) and one Src homology 3 (β-SH3) domain, has antagonistic effects on calcium currents by regulating different aspects of channel function. Although β-GK binds to a conserved site within the α(1)-pore-forming subunit and facilitates channel opening, β-SH3 binds to dynamin and promotes endocytosis. Here, we investigated the molecular switch underlying the functional duality of this modular protein. We show that β-SH3 homodimerizes through a single disulfide bond. Substitution of the only cysteine residue abolishes dimerization and impairs internalization of L-type Ca(V)1.2 channels expressed in Xenopus oocytes while preserving dynamin binding. Covalent linkage of the β-SH3 dimerization-deficient mutant yields a concatamer that binds to dynamin and restores endocytosis. Moreover, using FRET analysis, we show in living cells that Ca(V)β form oligomers and that this interaction is reduced by Ca(V)α(1). Association of Ca(V)β with a polypeptide encoding the binding motif in Ca(V)α(1) inhibited endocytosis. Together, these findings reveal that β-SH3 dimerization is crucial for endocytosis and suggest that channel activation and internalization are two mutually exclusive functions of Ca(V)β. We propose that a change in the oligomeric state of Ca(V)β is the functional switch between channel activator and channel internalizer.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Oligomerization of Cavbeta subunits is an essential correlate of Ca2+ channel activity

Voltage-gated calcium channels conduct Ca(2+) ions in response to membrane depolarization. The resulting transient increase in cytoplasmic free calcium concentration is a critical trigger for the initiation of such vital responses as muscle contraction and transcription. L-type Ca(v)1.2 calcium channels are complexes of the pore-forming α(1C) subunit associated with cytosolic Ca(v)β subunits. All major Ca(v)βs share a highly homologous membrane associated guanylate kinase-like (MAGUK) domain that binds to α(1C) at the α-interaction domain (AID), a short motif in the linker between transmembrane repeats I and II. In this study we show that Ca(v)β subunits form multimolecular homo- and heterooligomeric complexes in human vascular smooth muscle cells expressing native calcium channels and in Cos7 cells expressing recombinant Ca(v)1.2 channel subunits. Ca(v)βs oligomerize at the α(1C) subunits residing in the plasma membrane and bind to the AID. However, Ca(v)β oligomerization occurs independently on the association with α(1C). Molecular structures responsible for Ca(v)β oligomerization reside in 3 regions of the guanylate kinase subdomain of MAGUK. An augmentation of Ca(v)β homooligomerization significantly increases the calcium current density, while heterooligomerization may also change the voltage-dependence and inactivation kinetics of the channel. Thus, oligomerization of Ca(v)β subunits represents a novel and essential aspect of calcium channel regulation.

The voltage-gated calcium-channel beta subunit: more than just an accessory

Voltage-gated Ca(2+) channels (VGCCs) are involved in a number of excitatory processes in the cell that regulate muscle contraction, neurotransmitter release, gene regulation, and neuronal migration. They consist of a central pore-forming alpha(1) subunit together with a number of associated auxiliary subunits including a cytoplasmic beta subunit. With the aid of X-ray crystallography, it has been found that the beta subunits of VGCCs (beta(2a), beta(3), and beta(4)) interact strongly with the I-II loop of the pore-forming alpha(1) subunit. Here we discuss the potential interaction sites of beta(1a) with its alpha(1) subunit as well as the skeletal ryanodine receptor. We suggest that not only can beta(1a) interact with the alpha(1) subunit I-II loop, but more subtle interactions may be possible through the II-III loop via the beta(1a) SH3 domain. Such findings could have important implications with respect to EC coupling.