Cross-linking of C-terminal residues of phospholamban to the Ca2+ pump of cardiac sarcoplasmic reticulum to probe spatial and functional interactions within the transmembrane domain

Mechanisms for cardiac calcium pump activation by its substrate and a synthetic allosteric modulator using fluorescence lifetime imaging

The discovery of allosteric modulators is an emerging paradigm in drug discovery, and signal transduction is a subtle and dynamic process that is challenging to characterize. We developed a time-correlated single photon-counting imaging approach to investigate the structural mechanisms for small-molecule activation of the cardiac sarcoplasmic reticulum Ca2+-ATPase, a pharmacologically important pump that transports Ca2+ at the expense of adenosine triphosphate (ATP) hydrolysis. We first tested whether the dissociation of sarcoplasmic reticulum Ca2+-ATPase from its regulatory protein phospholamban is required for small-molecule activation. We found that CDN1163, a validated sarcoplasmic reticulum Ca2+-ATPase activator, does not have significant effects on the stability of the sarcoplasmic reticulum Ca2+-ATPase-phospholamban complex. Time-correlated single photon-counting imaging experiments using the nonhydrolyzable ATP analog β,γ-Methyleneadenosine 5'-triphosphate (AMP-PCP) showed ATP is an allosteric modulator of sarcoplasmic reticulum Ca2+-ATPase, increasing the fraction of catalytically competent structures at physiologically relevant Ca2+ concentrations. Unlike ATP, CDN1163 alone has no significant effects on the Ca2+-dependent shifts in the structural populations of sarcoplasmic reticulum Ca2+-ATPase, and it does not increase the pump's affinity for Ca2+ ions. However, we found that CDN1163 enhances the ATP-mediated modulatory effects to increase the population of catalytically competent sarcoplasmic reticulum Ca2+-ATPase structures. Importantly, this structural shift occurs within the physiological window of Ca2+ concentrations at which sarcoplasmic reticulum Ca2+-ATPase operates. We demonstrated that ATP is both a substrate and modulator of sarcoplasmic reticulum Ca2+-ATPase and showed that CDN1163 and ATP act synergistically to populate sarcoplasmic reticulum Ca2+-ATPase structures that are primed for phosphorylation. This study provides novel insights into the structural mechanisms for sarcoplasmic reticulum Ca2+-ATPase activation by its substrate and a synthetic allosteric modulator.

Structural basis for allosteric control of the SERCA-Phospholamban membrane complex by Ca2+ and phosphorylation

Phospholamban (PLN) is a mini-membrane protein that directly controls the cardiac Ca²⁺-transport response to β-adrenergic stimulation, thus modulating cardiac output during the fight-or-flight response. In the sarcoplasmic reticulum membrane, PLN binds to the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), keeping this enzyme's function within a narrow physiological window. PLN phosphorylation by cAMP-dependent protein kinase A or increase in Ca²⁺ concentration reverses the inhibitory effects through an unknown mechanism. Using oriented-sample solid-state NMR spectroscopy and replica-averaged NMR-restrained structural refinement, we reveal that phosphorylation of PLN's cytoplasmic regulatory domain signals the disruption of several inhibitory contacts at the transmembrane binding interface of the SERCA-PLN complex that are propagated to the enzyme's active site, augmenting Ca²⁺ transport. Our findings address long-standing questions about SERCA regulation, epitomizing a signal transduction mechanism operated by posttranslationally modified bitopic membrane proteins.

Selective cysteines oxidation in soluble guanylyl cyclase catalytic domain is involved in NO activation

Nitric oxide (NO) binds to soluble guanylyl cyclase (GC1) and stimulates its catalytic activity to produce cGMP. Despite the key role of the NO-cGMP signaling in cardiovascular physiology, the mechanisms of GC1 activation remain ill-defined. It is believed that conserved cysteines (Cys) in GC1 modulate the enzyme's activity through thiol-redox modifications. We previously showed that GC1 activity is modulated via mixed-disulfide bond by protein disulfide isomerase and thioredoxin 1. Herein we investigated the novel concept that NO-stimulated GC1 activity is mediated by thiol/disulfide switches and aimed to map the specific Cys that are involved. First, we showed that the dithiol reducing agent Tris (2-carboxyethyl)-phosphine reduces GC1 response to NO, indicating the significance of Cys oxidation in NO activation. Second, using dibromobimane, which fluoresces when crosslinking two vicinal Cys thiols, we demonstrated decreased fluorescence in NO-stimulated GC1 compared to unstimulated conditions. This suggested that NO-stimulated GC1 contained more bound Cys, potentially disulfide bonds. Third, to identify NO-regulated Cys oxidation using mass spectrometry, we compared the redox status of all Cys identified in tryptic peptides, among which, ten were oxidized and two were reduced in NO-stimulated GC1. Fourth, we resorted to computational modeling to narrow down the Cys candidates potentially involved in disulfide bond and identified Cys489 and Cys571. Fifth, our mutational studies showed that Cys489 and Cys571 were involved in GC1'response to NO, potentially as a thiol/disulfide switch. These findings imply that specific GC1 Cys sensitivity to redox environment is critical for NO signaling in cardiovascular physiology.

Protein docking and steered molecular dynamics suggest alternative phospholamban-binding sites on the SERCA calcium transporter

The transport activity of the sarco(endo)plasmic reticulum calcium ATPase (SERCA) in cardiac myocytes is modulated by an inhibitory interaction with a transmembrane peptide, phospholamban (PLB). Previous biochemical studies have revealed that PLB interacts with a specific inhibitory site on SERCA, and low-resolution structural evidence suggests that PLB interacts with distinct alternative sites on SERCA. High-resolution details of the structural determinants of SERCA regulation have been elusive because of the dynamic nature of the regulatory complex. In this study, we used computational approaches to develop a structural model of SERCA-PLB interactions to gain a mechanistic understanding of PLB-mediated SERCA transport regulation. We combined steered molecular dynamics and membrane protein-protein docking experiments to achieve both a global search and all-atom force calculations to determine the relative affinities of PLB for candidate sites on SERCA. We modeled the binding of PLB to several SERCA conformations, representing different enzymatic states sampled during the calcium transport catalytic cycle. The results of the steered molecular dynamics and docking experiments indicated that the canonical PLB-binding site (comprising transmembrane helices M2, M4, and M9) is the preferred site. This preference was even more stringent for a superinhibitory PLB variant. Interestingly, PLB-binding specificity became more ambivalent for other SERCA conformers. These results provide evidence for polymorphic PLB interactions with novel sites on M3 and with the outside of the SERCA helix M9. Our findings are compatible with previous physical measurements that suggest that PLB interacts with multiple binding sites, conferring dynamic responsiveness to changing physiological conditions.

Structural basis for relief of phospholamban-mediated inhibition of the sarcoplasmic reticulum Ca2+-ATPase at saturating Ca2+ conditions

Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) is critical for cardiac Ca²⁺ transport. Reversal of phospholamban (PLB)-mediated SERCA inhibition by saturating Ca²⁺ conditions operates as a physiological rheostat to reactivate SERCA function in the absence of PLB phosphorylation. Here, we performed extensive atomistic molecular dynamics simulations to probe the structural mechanism of this process. Simulation of the inhibitory complex at superphysiological Ca²⁺ concentrations ([Ca²⁺] = 10 mm) revealed that Ca²⁺ ions interact primarily with SERCA and the lipid headgroups, but not with PLB's cytosolic domain or the cytosolic side of the SERCA-PLB interface. At this [Ca²⁺], a single Ca²⁺ ion was translocated from the cytosol to the transmembrane transport sites. We used this Ca²⁺-bound complex as an initial structure to simulate the effects of saturating Ca²⁺ at physiological conditions ([Ca²⁺]_total ≈ 400 μm). At these conditions, ∼30% of the Ca²⁺-bound complexes exhibited structural features consistent with an inhibited state. However, in ∼70% of the Ca²⁺-bound complexes, Ca²⁺ moved to transport site I, recruited Glu⁷⁷¹ and Asp⁸⁰⁰, and disrupted key inhibitory contacts involving the conserved PLB residue Asn³⁴ Structural analysis showed that Ca²⁺ induces only local changes in interresidue inhibitory interactions, but does not induce repositioning or changes in PLB structural dynamics. Upon relief of SERCA inhibition, Ca²⁺ binding produced a site I configuration sufficient for subsequent SERCA activation. We propose that at saturating [Ca²⁺] and in the absence of PLB phosphorylation, binding of a single Ca²⁺ ion in the transport sites rapidly shifts the equilibrium toward a noninhibited SERCA-PLB complex.

Atomic-level mechanisms for phospholamban regulation of the calcium pump

We performed protein pKa calculations and molecular dynamics (MD) simulations of the calcium pump (sarcoplasmic reticulum Ca(2+)-ATPase (SERCA)) in complex with phospholamban (PLB). X-ray crystallography studies have suggested that PLB locks SERCA in a low-Ca(2+)-affinity E2 state that is incompatible with metal-ion binding, thereby blocking the conversion toward a high-Ca(2+)-affinity E1 state. Estimation of pKa values of the acidic residues in the transport sites indicates that at normal intracellular pH (7.1-7.2), PLB-bound SERCA populates an E1 state that is deprotonated at residues E309 and D800 yet protonated at residue E771. We performed three independent microsecond-long MD simulations to evaluate the structural dynamics of SERCA-PLB in a solution containing 100 mM K(+) and 3 mM Mg(2+). Principal component analysis showed that PLB-bound SERCA lies exclusively along the structural ensemble of the E1 state. We found that the transport sites of PLB-bound SERCA are completely exposed to the cytosol and that K(+) ions bind transiently (≤5 ns) and nonspecifically (nine different positions) to the two transport sites, with a total occupancy time of K(+) in the transport sites of 80%. We propose that PLB binding to SERCA populates a novel (to our knowledge) E1 intermediate, E1⋅H(+)771. This intermediate serves as a kinetic trap that controls headpiece dynamics and depresses the structural transitions necessary for Ca(2+)-dependent activation of SERCA. We conclude that PLB-mediated regulation of SERCA activity in the heart results from biochemical and structural transitions that occur primarily in the E1 state of the pump.

Phospholamban and sarcolipin: Are they functionally redundant or distinct regulators of the Sarco(Endo)Plasmic Reticulum Calcium ATPase?

In muscle, the Sarco(Endo)plasmic Reticulum Calcium ATPase (SERCA) activity is regulated by two distinct proteins, PLB and SLN, which are highly conserved throughout vertebrate evolution. PLB is predominantly expressed in the cardiac muscle, while SLN is abundant in skeletal muscle. SLN is also found in the cardiac atria and to a lesser extent in the ventricle. PLB regulation of SERCA is central to cardiac function, both at rest and during extreme physiological demand. Compared to PLB, the physiological relevance of SLN remained a mystery until recently and some even thought it was redundant in function. Studies on SLN suggest that it is an uncoupler of the SERCA pump activity and can increase ATP hydrolysis resulting in heat production. Using genetically engineered mouse models for SLN and PLB, we showed that SLN, not PLB, is required for muscle-based thermogenesis. However, the mechanism of how SLN binding to SERCA results in uncoupling SERCA Ca(2+) transport from its ATPase activity remains unclear. In this review, we discuss recent advances in understanding how PLB and SLN differ in their interaction with SERCA. We will also explore whether structural differences in the cytosolic domain of PLB and SLN are the basis for their unique function and physiological roles in cardiac and skeletal muscle.

Role of nucleotides in stabilization of the phospholamban/cardiac Ca²⁺ pump inhibitory complex examined with use of metal fluorides

Phospholamban (PLB) inhibits the activity of the cardiac calcium pump SERCA2a. We previously showed that PLB with engineered Cys residues only cross-linked with the Ca(2+) -free E2 intermediate of SERCA2a. Formation of E2•PLB prevents Ca(2+) binding at the high-affinity Ca(2+) binding sites, blocking the enzyme kinetic cycle. Here we further studied the synergistic action of PLB and ATP on E2 in terms of prevention of formation of the phosphorylated E2P-like states stabilized by metal fluorides. SERCA2a was co-expressed in insect cell microsomes with PLB mutants of normal or super-inhibitory strength, with cross-linkable mutations at either the cytosolic side (N30C) or the luminal side (V49C) of PLB. For normal-strength PLB mutants, in the absence of nucleotide, metal fluorides totally inhibited both SERCA2a enzyme activity and cross-linking of PLB to SERCA2a at both sites, suggesting that PLB dissociates from SERCA2a in the E2P-like states. However, under the same conditions, super-inhibitory PLB mutants prevented total enzyme inhibition by metal fluorides. Further, the cross-linking of super-inhibitory PLB to SERCA2a was only partially inhibited by metal fluorides, but was drastically restored upon sequential addition of ATP. These results revealed the equilibrium between E2•PLB, E2•ATP, or E2•ATP•PLB states and E2P-like states, suggesting that the synergistic binding of ATP and PLB to SERCA is very strong, sufficient to prevent formation of E2 phosphoenzymes, even when stabilized by metal fluorides.

Phospholamban interactome in cardiac contractility and survival: A new vision of an old friend

Depressed sarcoplasmic reticulum (SR) calcium cycling, reflecting impaired SR Ca-transport and Ca-release, is a key and universal characteristic of human and experimental heart failure. These SR processes are regulated by multimeric protein complexes, including protein kinases and phosphatases as well as their anchoring and regulatory subunits that fine-tune Ca-handling in specific SR sub-compartments. SR Ca-transport is mediated by the SR Ca-ATPase (SERCA2a) and its regulatory phosphoprotein, phospholamban (PLN). Dephosphorylated PLN is an inhibitor of SERCA2a and phosphorylation by protein kinase A (PKA) or calcium-calmodulin-dependent protein kinases (CAMKII) relieves these inhibitory effects. Recent studies identified additional regulatory proteins, associated with PLN, that control SR Ca-transport. These include the inhibitor-1 (I-1) of protein phosphatase 1 (PP1), the small heat shock protein 20 (Hsp20) and the HS-1 associated protein X-1 (HAX1). In addition, the intra-luminal histidine-rich calcium binding protein (HRC) has been shown to interact with both SERCA2a and triadin. Notably, there is physical and direct interaction between these protein players, mediating a fine-cross talk between SR Ca-uptake, storage and release. Importantly, regulation of SR Ca-cycling by the PLN/SERCA interactome does not only impact cardiomyocyte contractility, but also survival and remodeling. Indeed, naturally occurring variants in these Ca-cycling genes modulate their activity and interactions with other protein partners, resulting in depressed contractility and accelerated remodeling. These genetic variants may serve as potential prognostic or diagnostic markers in cardiac pathophysiology.

Phospholamban C-terminal residues are critical determinants of the structure and function of the calcium ATPase regulatory complex

To determine the structural and regulatory role of the C-terminal residues of phospholamban (PLB) in the membranes of living cells, we fused fluorescent protein tags to PLB and sarco/endoplasmic reticulum calcium ATPase (SERCA). Alanine substitution of PLB C-terminal residues significantly altered fluorescence resonance energy transfer (FRET) from PLB to PLB and SERCA to PLB, suggesting a change in quaternary conformation of PLB pentamer and SERCA-PLB regulatory complex. Val to Ala substitution at position 49 (V49A) had particularly large effects on PLB pentamer structure and PLB-SERCA regulatory complex conformation, increasing and decreasing probe separation distance, respectively. We also quantified a decrease in oligomerization affinity, an increase in binding affinity of V49A-PLB for SERCA, and a gain of inhibitory function as quantified by calcium-dependent ATPase activity. Notably, deletion of only a few C-terminal residues resulted in significant loss of PLB membrane anchoring and mislocalization to the cytoplasm and nucleus. C-terminal truncations also resulted in progressive loss of PLB-PLB FRET due to a decrease in the apparent affinity of PLB oligomerization. We quantified a similar decrease in the binding affinity of truncated PLB for SERCA and loss of inhibitory potency. However, despite decreased SERCA-PLB binding, intermolecular FRET for Val(49)-stop (V49X) truncation mutant was paradoxically increased as a result of an 11.3-Å decrease in the distance between donor and acceptor fluorophores. We conclude that PLB C-terminal residues are critical for localization, oligomerization, and regulatory function. In particular, the PLB C terminus is an important determinant of the quaternary structure of the SERCA regulatory complex.

Istaroxime stimulates SERCA2a and accelerates calcium cycling in heart failure by relieving phospholamban inhibition

BACKGROUND AND PURPOSE

Calcium handling is known to be deranged in heart failure. Interventions aimed at improving cell Ca(2) (+) cycling may represent a promising approach to heart failure therapy. Istaroxime is a new luso-inotropic compound that stimulates cardiac contractility and relaxation in healthy and failing animal models and in patients with acute heart failure (AHF) syndrome. Istaroxime is a Na-K ATPase inhibitor with the unique property of increasing sarcoplasmic reticulum (SR) SERCA2a activity as shown in heart microsomes from humans and guinea pigs. The present study addressed the molecular mechanism by which istaroxime increases SERCA2a activity.

EXPERIMENTAL APPROACH

To study the effect of istaroxime on SERCA2a-phospholamban (PLB) complex, we applied different methodologies in native dog healthy and failing heart preparations and heterologous canine SERCA2a/PLB co-expressed in Spodoptera frugiperda (Sf21) insect cells.

KEY RESULTS

We showed that istaroxime enhances SERCA2a activity, Ca(2) (+) uptake and the Ca(2) (+) -dependent charge movements into dog healthy and failing cardiac SR vesicles. Although not directly demonstrated, the most probable explanation of these activities is the displacement of PLB from SERCA2a.E2 conformation, independently from cAMP/PKA. We propose that this displacement may favour the SERCA2a conformational transition from E2 to E1, thus resulting in the acceleration of Ca(2) (+) cycling.

CONCLUSIONS AND IMPLICATIONS

Istaroxime represents the first example of a small molecule that exerts a luso-inotropic effect in the failing human heart through the stimulation of SERCA2a ATPase activity and the enhancement of Ca(2) (+) uptake into the SR by relieving the PLB inhibitory effect on SERCA2a in a cAMP/PKA independent way.

The structural basis for phospholamban inhibition of the calcium pump in sarcoplasmic reticulum

P-type ATPases are a large family of enzymes that actively transport ions across biological membranes by interconverting between high (E1) and low (E2) ion-affinity states; these transmembrane transporters carry out critical processes in nearly all forms of life. In striated muscle, the archetype P-type ATPase, SERCA (sarco(endo)plasmic reticulum Ca(2+)-ATPase), pumps contractile-dependent Ca(2+) ions into the lumen of sarcoplasmic reticulum, which initiates myocyte relaxation and refills the sarcoplasmic reticulum in preparation for the next contraction. In cardiac muscle, SERCA is regulated by phospholamban (PLB), a small inhibitory phosphoprotein that decreases the Ca(2+) affinity of SERCA and attenuates contractile strength. cAMP-dependent phosphorylation of PLB reverses Ca(2+)-ATPase inhibition with powerful contractile effects. Here we present the long sought crystal structure of the PLB-SERCA complex at 2.8-Å resolution. The structure was solved in the absence of Ca(2+) in a novel detergent system employing alkyl mannosides. The structure shows PLB bound to a previously undescribed conformation of SERCA in which the Ca(2+) binding sites are collapsed and devoid of divalent cations (E2-PLB). This new structure represents one of the key unsolved conformational states of SERCA and provides a structural explanation for how dephosphorylated PLB decreases Ca(2+) affinity and depresses cardiac contractility.

A separate pool of cardiac phospholemman that does not regulate or associate with the sodium pump: multimers of phospholemman in ventricular muscle

BACKGROUND

Phospholemman regulates the plasmalemmal sodium pump in excitable tissues.

RESULTS

In cardiac muscle, a subpopulation of phospholemman with a unique phosphorylation signature associates with other phospholemman molecules but not with the pump.

CONCLUSION

Phospholemman oligomers exist in cardiac muscle.

SIGNIFICANCE

Much like phospholamban regulation of SERCA, phospholemman exists as both a sodium pump inhibiting monomer and an unassociated oligomer. Phospholemman (PLM), the principal quantitative sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. Much like phospholamban, which regulates the related ATPase SERCA, PLM is reported to oligomerize. We investigated subpopulations of PLM in adult rat ventricular myocytes based on phosphorylation status. Co-immunoprecipitation identified two pools of PLM: one not associated with the sodium pump phosphorylated at Ser(63) and one associated with the pump, both phosphorylated at Ser(68) and unphosphorylated. Phosphorylation of PLM at Ser(63) following activation of PKC did not abrogate association of PLM with the pump, so its failure to associate with the pump was not due to phosphorylation at this site. All pools of PLM co-localized to cell surface caveolin-enriched microdomains with sodium pump α subunits, despite the lack of caveolin-binding motif in PLM. Mass spectrometry analysis of phosphospecific immunoprecipitation reactions revealed no unique protein interactions for Ser(63)-phosphorylated PLM, and cross-linking reagents also failed to identify any partner proteins for this pool. In lysates from hearts of heterozygous transgenic animals expressing wild type and unphosphorylatable PLM, Ser(63)-phosphorylated PLM co-immunoprecipitated unphosphorylatable PLM, confirming the existence of PLM multimers. Dephosphorylation of the PLM multimer does not change sodium pump activity. Hence like phospholamban, PLM exists as a pump-inhibiting monomer and an unassociated oligomer. The distribution of different PLM phosphorylation states to different pools may be explained by their differential proximity to protein phosphatases rather than a direct effect of phosphorylation on PLM association with the pump.

Sarco(endo)plasmic reticulum calcium ATPase (SERCA) inhibition by sarcolipin is encoded in its luminal tail

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) is regulated in a tissue-dependent manner via interaction with the short integral membrane proteins phospholamban (PLN) and sarcolipin (SLN). Although defects in SERCA activity are known to cause heart failure, the regulatory mechanisms imposed by PLN and SLN could have clinical implications for both heart and skeletal muscle diseases. PLN and SLN have significant sequence homology in their transmembrane regions, suggesting a similar mode of binding to SERCA. However, unlike PLN, SLN has a conserved C-terminal luminal tail composed of five amino acids ((27)RSYQY), which may contribute to a distinct SERCA regulatory mechanism. We have functionally characterized alanine mutants of the C-terminal tail of SLN using co-reconstituted proteoliposomes of SERCA and SLN. We found that Arg(27) and Tyr(31) are essential for SLN function. We also tested the effect of a truncated variant of SLN (Arg(27)stop) and extended chimeras of PLN with the five luminal residues of SLN added to its C terminus. The Arg(27)stop form of SLN resulted in loss of function, whereas the PLN chimeras resulted in superinhibition with characteristics of both PLN and SLN. Based on our results, we propose that the C-terminal tail of SLN is a distinct, essential domain in the regulation of SERCA and that the functional properties of the SLN tail can be transferred to PLN.

Sarcolipin protein interaction with sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is distinct from phospholamban protein, and only sarcolipin can promote uncoupling of the SERCA pump

Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent data suggest that SLN could play a role in muscle thermogenesis by promoting uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 1575-1579), but the mechanistic details are unknown. To better define how binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 interaction with that of PLB in detail. The homo-bifunctional cross-linker (1,6-bismaleimidohexane) was employed to detect dynamic protein interaction during the SERCA cycle. Our studies reveal that SLN differs significantly from PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally show that PLB and SLN can bind to the same groove but interact with a different set of residues on SERCA. These data collectively suggest that SLN is functionally distinct from PLB; its ability to interact with SERCA in the presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat production.

Two-dimensional crystallization of membrane proteins by reconstitution through dialysis

Studies of membrane proteins by two-dimensional (2D) crystallization and electron crystallography have provided crucial information on the structure and function of a rapidly growing number of these intricate proteins within a close-to-native lipid bilayer. Here we provide protocols for planning and executing 2D crystallization trials by detergent removal through dialysis, including the preparation of phospholipids and the dialysis setup. General factors to be considered, such as the protein preparation, solubilizing detergent, lipid for reconstitution, and buffer conditions are discussed. Several 2D crystallization conditions are highlighted that have shown great promise to grow 2D crystals within a surprisingly short amount of time. Finally, conditions for optimizing order and size of 2D crystals are outlined.

Role of conformational sampling of Ser16 and Thr17-phosphorylated phospholamban in interactions with SERCA

Phosphorylation of phospholamban (PLB) at Ser16 and/ or Thr17 is believed to release its inhibitory effect on sarcoplasmic reticulum calcium ATPase. Ser16 phosphorylation of PLB has been suggested to cause a conformational change that alters the interaction between the enzyme and protein. Using computer simulations, the conformational sampling of Ser16 phosphorylated PLB in implicit membrane environment is compared here with the unphosphorylated PLB system to investigate these conformational changes. The results suggest that conformational changes in the cytoplasmic domain of PLB upon phosphorylation at Ser16 increase the likelihood of unfavorable interactions with SERCA in the E2 state prompting a conformational switch of SERCA from E2 to E1. Phosphorylation of PLB at Thr17 on the other hand does not appear to affect interactions with SERCA significantly suggesting that the mechanism of releasing the inhibitory effect is different between Thr17 phosphorylated and Ser16 phosphorylated PLB.

Characterizing phospholamban to sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a) protein binding interactions in human cardiac sarcoplasmic reticulum vesicles using chemical cross-linking

Chemical cross-linking was used to study protein binding interactions between native phospholamban (PLB) and SERCA2a in sarcoplasmic reticulum (SR) vesicles prepared from normal and failed human hearts. Lys(27) of PLB was cross-linked to the Ca(2+) pump at the cytoplasmic extension of M4 (at or near Lys(328)) with the homobifunctional cross-linker, disuccinimidyl glutarate (7.7 Å). Cross-linking was augmented by ATP but abolished by Ca(2+) or thapsigargin, confirming in native SR vesicles that PLB binds preferentially to E2 (low Ca(2+) affinity conformation of the Ca(2+)-ATPase) stabilized by ATP. To assess the functional effects of PLB binding on SERCA2a activity, the anti-PLB antibody, 2D12, was used to disrupt the physical interactions between PLB and SERCA2a in SR vesicles. We observed a tight correlation between 2D12-induced inhibition of PLB cross-linking to SERCA2a and 2D12 stimulation of Ca(2+)-ATPase activity and Ca(2+) transport. The results suggest that the inhibitory effect of PLB on Ca(2+)-ATPase activity in SR vesicles results from mutually exclusive binding of PLB and Ca(2+) to the Ca(2+) pump, requiring PLB dissociation for catalytic activation. Importantly, the same result was obtained with SR vesicles prepared from normal and failed human hearts; therefore, we conclude that PLB binding interactions with the Ca(2+) pump are largely unchanged in failing myocardium.

Phospholamban binds with differential affinity to calcium pump conformers

To investigate the mechanism of regulation of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) by phospholamban (PLB), we expressed Cerulean-SERCA and yellow fluorescent protein (YFP)-PLB in adult rabbit ventricular myocytes using adenovirus vectors. SERCA and PLB were localized in the sarcoplasmic reticulum and were mobile over multiple sarcomeres on a timescale of tens of seconds. We also observed robust fluorescence resonance energy transfer (FRET) from Cerulean-SERCA to YFP-PLB. Electrical pacing of cardiac myocytes elicited cytoplasmic Ca(2+) elevations, but these increases in Ca(2+) produced only modest changes in SERCA-PLB FRET. The data suggest that the regulatory complex is not disrupted by elevations of cytosolic calcium during cardiac contraction (systole). This conclusion was also supported by parallel experiments in heterologous cells, which showed that FRET was reduced but not abolished by calcium. Thapsigargin also elicited a small decrease in PLB-SERCA binding affinity. We propose that PLB is not displaced from SERCA by high calcium during systole, and relief of functional inhibition does not require dissociation of the regulatory complex. The observed modest reduction in the affinity of the PLB-SERCA complex with Ca(2+) or thapsigargin suggests that the binding interface is altered by SERCA conformational changes. The results are consistent with multiple modes of PLB binding or alternative binding sites.

A phospholamban-tethered cardiac Ca2+ pump reveals stoichiometry and dynamic interactions between the two proteins

To study PLB (phospholamban) inhibition of the cardiac Ca2+ pump [SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a)], a fusion protein (SER-20G-PLB) was engineered by tethering SERCA2a with PLB through a 20-glycine residue chain, allowing the PLB tether to either bind to or dissociate from the inhibition site on SERCA2a. When expressed in insect cells, SER-20G-PLB produced active Ca2+ uptake, which was stimulated by the anti-PLB antibody, both similar to that which occurred with the control sample co-expressing WT (wild-type)-SERCA2a and WT-PLB. The KCa values of Ca2+-dependent ATPase were similar for SER-20G-PLB (0.29±0.02 μM) and for the control sample (0.30±0.02 μM), both greater than 0.17±0.01 μM for WT-SERCA2a expressed alone. Thus SER-20G-PLB retains a fully active Ca2+ pump, but its apparent Ca2+ affinity was decreased intrinsically by tethered PLB at a 1:1 molar stoichiometry. Like WT-PLB, SER-20G-PLB ran as both monomers and homo-pentamers on SDS/PAGE. As Ca2+ concentrations increase from 0 to the micromolar range, the proportion of non-inhibiting pentamers increased from 32% to 52%, suggesting that Ca2+ activation of the pump completely dissociates the PLB tether from the inhibition site on SERCA2a, with concurrent association of PLB pentamers. Collectively, the regulation of SERCA2a is achieved through the Ca2+-dependent equilibria involving PLB association and dissociation from SERCA2a, and assembling and disassembling of SER-20G-PLB pentamers.

Oligomeric interactions of sarcolipin and the Ca-ATPase

We have detected directly the interactions of sarcolipin (SLN) and the sarcoplasmic reticulum Ca-ATPase (SERCA) by measuring fluorescence resonance energy transfer (FRET) between fusion proteins labeled with cyan fluorescent protein (donor) and yellow fluorescent protein (acceptor). SLN is a membrane protein that helps control contractility by regulating SERCA activity in fast-twitch and atrial muscle. Here we used FRET microscopy and spectroscopy with baculovirus expression in insect cells to provide direct evidence for: 1) oligomerization of SLN and 2) regulatory complex formation between SLN and the fast-twitch muscle Ca-ATPase (SERCA1a isoform). FRET experiments demonstrated that SLN monomers self-associate into dimers and higher order oligomers in the absence of SERCA, and that SLN monomers also bind to SERCA monomers in a 1:1 binary complex when the two proteins are coexpressed. FRET experiments further demonstrated that the binding affinity of SLN for itself is similar to that for SERCA. Mutating SLN residue isoleucine-17 to alanine (I17A) decreased the binding affinity of SLN self-association and converted higher order oligomers into monomers and dimers. The I17A mutation also decreased SLN binding affinity for SERCA but maintained 1:1 stoichiometry in the regulatory complex. Thus, isoleucine-17 plays dual roles in determining the distribution of SLN homo-oligomers and stabilizing the formation of SERCA-SLN heterodimers. FRET results for SLN self-association were supported by the effects of SLN expression in bacterial cells. We propose that SLN exists as multiple molecular species in muscle, including SERCA-free (monomer, dimer, oligomer) and SERCA-bound (heterodimer), with transmembrane zipper residues of SLN serving to stabilize oligomeric interactions.

Phosphorylation and mutation of phospholamban alter physical interactions with the sarcoplasmic reticulum calcium pump

Phospholamban physically interacts with the sarcoplasmic reticulum calcium pump (SERCA) and regulates contractility of the heart in response to adrenergic stimuli. We studied this interaction using electron microscopy of 2D crystals of SERCA in complex with phospholamban. In earlier studies, phospholamban oligomers were found interspersed between SERCA dimer ribbons and a 3D model was constructed to show interactions with SERCA. In this study, we examined the oligomeric state of phospholamban and the effects of phosphorylation and mutation of phospholamban on the interaction with SERCA in the 2D crystals. On the basis of projection maps from negatively stained and frozen-hydrated crystals, phosphorylation of Ser16 selectively disordered the cytoplasmic domain of wild type phospholamban. This was not the case for a pentameric gain-of-function mutant (Lys27Ala), which retained inhibitory activity and remained ordered in the phosphorylated state. A partial loss-of-function mutation that altered the charge state of phospholamban (Arg14Ala) retained an ordered state, while a complete loss-of-function mutation (Asn34Ala) was also disordered. The functional state of phospholamban was correlated with an order-to-disorder transition of the phospholamban cytoplasmic domain in the 2D co-crystals. Furthermore, co-crystals of the gain-of-function mutant (Lys27Ala) facilitated data collection from frozen-hydrated crystals. An improved projection map was calculated to a resolution of 8 Å, which supports the pentamer as the oligomeric state of phospholamban in the crystals. The 2D co-crystals with SERCA require a functional pentameric form of phospholamban, which physically interacts with SERCA at an accessory site distinct from that used by the phospholamban monomer for the inhibitory association.

Superinhibitory phospholamban mutants compete with Ca2+ for binding to SERCA2a by stabilizing a unique nucleotide-dependent conformational state

Three cross-linkable phospholamban (PLB) mutants of increasing inhibitory strength (N30C-PLB < N27A,N30C,L37A-PLB (PLB3) < N27A,N30C,L37A,V49G-PLB (PLB4)) were used to determine whether PLB decreases the Ca(2+) affinity of SERCA2a by competing for Ca(2+) binding. The functional effects of N30C-PLB, PLB3, and PLB4 on Ca(2+)-ATPase activity and E1 approximately P formation were correlated with their binding interactions with SERCA2a measured by chemical cross-linking. Successively higher Ca(2+) concentrations were required to both activate the enzyme co-expressed with N30C-PLB, PLB3, and PLB4 and to dissociate N30C-PLB, PLB3, and PLB4 from SERCA2a, suggesting competition between PLB and Ca(2+) for binding to SERCA2a. This was confirmed with the Ca(2+) pump mutant, D351A, which is catalytically inactive but retains strong Ca(2+) binding. Increasingly higher Ca(2+) concentrations were also required to dissociate N30C-PLB, PLB3, and PLB4 from D351A, demonstrating directly that PLB antagonizes Ca(2+) binding. Finally, the specific conformation of E2 (Ca(2+)-free state of SERCA2a) that binds PLB was investigated using the Ca(2+)-pump inhibitors thapsigargin and vanadate. Cross-linking assays conducted in the absence of Ca(2+) showed that PLB bound preferentially to E2 with bound nucleotide, forming a remarkably stable complex that is highly resistant to both thapsigargin and vanadate. In the presence of ATP, N30C-PLB had an affinity for SERCA2a approaching that of vanadate (micromolar), whereas PLB3 and PLB4 had much higher affinities, severalfold greater than even thapsigargin (nanomolar or higher). We conclude that PLB decreases Ca(2+) binding to SERCA2a by stabilizing a unique E2.ATP state that is unable to bind thapsigargin or vanadate.

Phospholamban modulates the functional coupling between nucleotide domains in Ca-ATPase oligomeric complexes in cardiac sarcoplasmic reticulum

Oligomeric interactions between Ca-ATPase polypeptide chains and their modulation by phospholamban (PLB) were measured in native cardiac sarcoplasmic reticulum (SR) microsomes. Progressive modification of Lys(514) with fluorescein 5-isothiocyanate (FITC), which physically blocks access to the nucleotide binding site by ATP, demonstrates that Ca-ATPase active sites function independently of one another prior to the phosphorylation of PLB. However, upon cAMP-dependent protein kinase (PKA) phosphorylation of PLB, a second-order dependence between residual enzyme activity and the fraction of active sites is observed, consistent with a dimeric functional complex. Complementary distance measurements were made using FITC or 5-iodoacetamidofluorescein (IAF) bound to Cys(674) within the N- or P-domains, respectively, to detect structural coupling within oligomeric complexes. Accompanying the phosphorylation of PLB, neighboring Ca-ATPase polypeptide chains exhibit a 4 +/- 2 A decrease in the proximity between FITC sites within the N-domain and a 9 +/- 3 A increase in the proximity between IAF sites within P-domains. Thus, the phosphorylation of PLB induces spatial rearrangements between the N- and P-domain elements of proximal Ca-ATPase polypeptide chains which restore functional interactions between neighboring polypeptide chains and, in turn, result in increased rates of catalytic turnover. These results are interpreted in terms of a structural model, calculated through optimization of shape complementarity, desolvation, and electrostatic energies, which suggests a dimeric arrangement of Ca-ATPase polypeptide chains through the proximal association of N-domains that accommodates interaction with PLB. We suggest that the phosphorylation of PLB acts to release constraints involving interdomain subunit interactions that enhance catalytically important N-domain motions.

Ca2+ binding to site I of the cardiac Ca2+ pump is sufficient to dissociate phospholamban

Phospholamban (PLB) inhibits the activity of SERCA2a, the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum, by decreasing the apparent affinity of the enzyme for Ca(2+). Recent cross-linking studies have suggested that PLB binding and Ca(2+) binding to SERCA2a are mutually exclusive. PLB binds to the E2 conformation of the Ca(2+)-ATPase, preventing formation of E1, the conformation that binds two Ca(2+) (at sites I and II) with high affinity and is required for ATP hydrolysis. Here we determined whether Ca(2+) binding to site I, site II, or both sites is sufficient to dissociate PLB from the Ca(2+) pump. Seven SERCA2a mutants with amino acid substitutions at Ca(2+)-binding site I (E770Q, T798A, and E907Q), site II (E309Q and N795A), or both sites (D799N and E309Q/E770Q) were made, and the effects of Ca(2+) on N30C-PLB cross-linking to Lys(328) of SERCA2a were measured. In agreement with earlier reports with the skeletal muscle Ca(2+)-ATPase, none of the SERCA2a mutants (except E907Q) hydrolyzed ATP in the presence of Ca(2+); however, all were phosphorylatable by P(i) to form E2P. Ca(2+) inhibition of E2P formation was observed only in SERCA2a mutants retaining site I. In cross-linking assays, strong cross-linking between N30C-PLB and each Ca(2+)-ATPase mutant was observed in the absence of Ca(2+). Importantly, however, micromolar Ca(2+) inhibited PLB cross-linking only to mutants retaining a functional Ca(2+)-binding site I. The dynamic equilibrium between Ca(2+) pumps and N30C-PLB was retained by all mutants, demonstrating normal regulation of cross-linking by ATP, thapsigargin, and anti-PLB antibody. From these results we conclude that site I is the key Ca(2+)-binding site regulating the physical association between PLB and SERCA2a.

Effects of phospholamban transmembrane mutants on the calcium affinity, maximal activity, and cooperativity of the sarcoplasmic reticulum calcium pump

Regulation of the SERCA calcium pump by phospholamban (PLB) is largely due to interactions between their respective transmembrane domains. In spite of numerous mutagenesis and kinetic studies, we still do not have a clear mechanistic picture of how PLB influences the calcium transport cycle of SERCA. Herein, we have created alanine mutants for each residue in the transmembrane domain of PLB, we have co-reconstituted these mutants with SERCA into proteoliposomes, and we have performed kinetic simulations of the calcium-dependent ATPase activity isotherms. The PLB transmembrane mutants had a variable effect on the calcium affinity, maximal activity, and cooperativity of SERCA, such that a range of values was observed. Kinetic simulations using a well-established reaction scheme for SERCA then allowed us to correlate the effects on SERCA activity with changes in the reaction scheme rate constants. Only three steps in the reaction scheme were affected by the presence of PLB, namely, binding of the first calcium ion, a subsequent conformational change in SERCA, and binding of the second calcium ion. The ability of wild-type and mutant forms of PLB to alter the apparent calcium affinity of SERCA correlated with a decreased rate of binding of the second calcium ion. In addition, the ability of wild-type and mutant forms of PLB to alter the maximal activity of SERCA correlated with a change in the forward rate constant for the slow conformational change in SERCA following binding of the first calcium ion.

Peptide inhibitors use two related mechanisms to alter the apparent calcium affinity of the sarcoplasmic reticulum calcium pump

The primary sequence of phospholamban (PLB) has provided a template for the rational design of peptide inhibitors of the sarcoplasmic reticulum calcium ATPase (SERCA). In the transmembrane domain of PLB, there are few polar residues and only one is essential (Asn (34)). Using synthetic peptides, we have previously investigated the role of Asn (34) in the context of simple hydrophobic transmembrane peptides. Herein we propose that the role of Asn in SERCA inhibition is position-sensitive and dependent upon the distribution of hydrophobic residues. To test this hypothesis, we synthesized a series of transmembrane peptides based on a 24 amino acid polyalanine sequence having either an alternating Leu-Ala sequence (Leu 12) or Leu residues at the native positions found in PLB (Leu 9). Asn-containing Leu 9 and Leu 12 peptides were synthesized with a single Asn residue located either one amino acid (N+/-1) or one turn of the helix (N+/-4) in either direction from its native position. Co-reconstitution of these peptides with SERCA into proteoliposomes revealed effects on the apparent calcium affinity and cooperativity of SERCA that correlated with the positions of the Asn and Leu residues. The most inhibitory peptides increased the cooperativity of SERCA as indicated by the Hill coefficients, suggesting that calcium-dependent reversibility is an inherent part of the inhibitory mechanism. Kinetic simulations combined with molecular modeling of the interaction between the peptides and SERCA reveal two related mechanisms of inhibition. Peptides that resemble PLB use the same inhibitory mechanism, whereas peptides that are more divergent from PLB alter an additional step in the calcium transport cycle.

Structural characterization of Ca(2+)-ATPase-bound phospholamban in lipid bilayers by solid-state nuclear magnetic resonance (NMR) spectroscopy

Phospholamban (PLN) regulates cardiac contractility by modulation of sarco(endo)plasmic reticulum calcium ATPase (SERCA) activity. While PLN and SERCA1a, an isoform from skeletal muscle, have been structurally characterized in great detail, direct information about the conformation of PLN in complex with SERCA has been limited. We used solid-state NMR (ssNMR) spectroscopy to deduce structural properties of both the A 36F 41A 46 mutant (AFA-PLN) and wild-type PLN (WT-PLN) when bound to SERCA1a after reconstitution in a functional lipid bilayer environment. Chemical-shift assignments in all domains of AFA-PLN provide direct evidence for the presence of two terminal alpha helices connected by a linker region of reduced structural order that differs from previous findings on free PLN. ssNMR experiments on WT-PLN show no significant difference in binding compared to AFA-PLN and do not support the coexistence of a significantly populated dynamic state of PLN after formation of the PLN/SERCA complex. A combination of our spectroscopic data with biophysical and biochemical data using flexible protein-protein docking simulations provides a structural basis for understanding the interaction between PLN and SERCA1a.

Förster transfer recovery reveals that phospholamban exchanges slowly from pentamers but rapidly from the SERCA regulatory complex

Phospholamban (PLB) or the sarcoplasmic reticulum Ca2+-ATPase (SERCA) were fused to cyan fluorescent protein (CFP) and coexpressed with PLB fused to yellow fluorescent protein (YFP). The expressed fluorescently tagged proteins were imaged using epifluorescence and total internal reflection fluorescence microscopy. YFP fluorescence was selectively bleached by a focused laser beam. CFP fluorescence at the targeted site increased after YFP photobleaching, indicating fluorescence resonance energy transfer between CFP-SERCA/CFP-PLB and YFP-PLB. The increased donor fluorescence relaxed back toward baseline as a result of donor diffusion and exchange of bleached YFP-PLB for unbleached YFP-PLB, which restored fluorescence resonance energy transfer. Requenching of CFP donors, termed Förster transfer recovery (FTR), was quantified as an index of the rate of PLB subunit exchange from the PLB:SERCA and PLB:PLB membrane complexes. PLB subunit exchange from the PLB:SERCA regulatory complex was rapid, showing diffusion-limited FTR (tau=1.4 second). Conversely, PLB:PLB oligomeric complexes were found to be stable on a much longer time scale. Despite free lateral diffusion in the membrane, they showed no FTR over 80 seconds. Mutation of PLB position 40 from isoleucine to alanine (I40A-PLB) did not abolish PLB:PLB energy transfer, but destabilization of the PLB:PLB complex was apparent from an increased FTR rate (tau=8.4 seconds). Oligomers of I40A-PLB were stabilized by oxidative crosslinking of transmembrane cysteines with diamide. We conclude that PLB exchanges rapidly from its regulatory complex with the SERCA pump, whereas subunit exchange from the PLB oligomeric complex is slow and does not occur on the time scale of the cardiac cycle.

Mechanism of reversal of phospholamban inhibition of the cardiac Ca2+-ATPase by protein kinase A and by anti-phospholamban monoclonal antibody 2D12

Our model of phospholamban (PLB) regulation of the cardiac Ca(2+)-ATPase in sarcoplasmic reticulum (SERCA2a) states that PLB binds to the Ca(2+)-free, E2 conformation of SERCA2a and blocks it from transitioning from E2 to E1, the Ca(2+)-bound state. PLB and Ca(2+) binding to SERCA2a are mutually exclusive, and PLB inhibition of SERCA2a is manifested as a decreased apparent affinity of SERCA2a for Ca(2+). Here we extend this model to explain the reversal of SERCA2a inhibition that occurs after phosphorylation of PLB at Ser(16) by protein kinase A (PKA) and after binding of the anti-PLB monoclonal antibody 2D12, which recognizes residues 7-13 of PLB. Site-specific cysteine variants of PLB were co-expressed with SERCA2a, and the effects of PKA phosphorylation and 2D12 on Ca(2+)-ATPase activity and cross-linking to SERCA2a were monitored. In Ca(2+)-ATPase assays, PKA phosphorylation and 2D12 partially and completely reversed SERCA2a inhibition by decreasing K(Ca) values for enzyme activation, respectively. In cross-linking assays, cross-linking of PKA-phosphorylated PLB to SERCA2a was inhibited at only two of eight sites when conducted in the absence of Ca(2+) favoring E2. However, at a subsaturating Ca(2+) concentration supporting some E1, cross-linking of phosphorylated PLB to SERCA2a was attenuated at all eight sites. K(Ca) values for cross-linking inhibition were decreased nearly 2-fold at all sites by PLB phosphorylation, demonstrating that phosphorylated PLB binds more weakly to SERCA2a than dephosphorylated PLB. In parallel assays, 2D12 blocked PLB cross-linking to SERCA2a at all eight sites regardless of Ca(2+) concentration. Our results demonstrate that 2D12 restores maximal Ca(2+)-ATPase activity by physically disrupting the binding interaction between PLB and SERCA2a. Phosphorylation of PLB by PKA weakens the binding interaction between PLB and SERCA2a (yielding more PLB-free SERCA2a molecules at intermediate Ca(2+) concentrations), only partially restoring Ca(2+) affinity and Ca(2+)-ATPase activity.

Nitroxyl improves cellular heart function by directly enhancing cardiac sarcoplasmic reticulum Ca2+ cycling

Heart failure remains a leading cause of morbidity and mortality worldwide. Although depressed pump function is common, development of effective therapies to stimulate contraction has proven difficult. This is thought to be attributable to their frequent reliance on cAMP stimulation to increase activator Ca(2+). A potential alternative is nitroxyl (HNO), the 1-electron reduction product of nitric oxide (NO) that improves contraction and relaxation in normal and failing hearts in vivo. The mechanism for myocyte effects remains unknown. Here, we show that this activity results from a direct interaction of HNO with the sarcoplasmic reticulum Ca(2+) pump and the ryanodine receptor 2, leading to increased Ca(2+) uptake and release from the sarcoplasmic reticulum. HNO increases the open probability of isolated ryanodine-sensitive Ca(2+)-release channels and accelerates Ca(2+) reuptake into isolated sarcoplasmic reticulum by stimulating ATP-dependent Ca(2+) transport. Contraction improves with no net rise in diastolic calcium. These changes are not induced by NO, are fully reversible by addition of reducing agents (redox sensitive), and independent of both cAMP/protein kinase A and cGMP/protein kinase G signaling. Rather, the data support HNO/thiolate interactions that enhance the activity of intracellular Ca(2+) cycling proteins. These findings suggest HNO donors are attractive candidates for the pharmacological treatment of heart failure.