Biology subjects

Espinoza-Fonseca, L. M.

Publications and source records attributed to Espinoza-Fonseca, L. M..

16 recordsLinked to original sources

bioRxiv2026

Supercharging the calcium pump: Identification of an activation hotspot on SERCA by cryo-EM.

The sarco-endoplasmic reticulum Ca{superscript 2}-ATPase (SERCA) is a ubiquitous P-type ATPase that restores cytosolic Ca2+ to the sarco-endoplasmic reticulum. SERCA is essential for cardiac Ca2+ cycling and cellular energy metabolism. Several small molecules enhance SERCA function and show promise in models of metabolic and cardiovascular diseases. However, the structural basis for SERCA activation has remained unknown, hindering mechanism-driven lead optimization. Here we present cryo-EM structures of SERCA bound to two chemically distinct activators: the quinoline derivative CDN1163 (2.6 [A] resolution) and a benzofuran derivative UM-52 (3.1 [A] resolution). Biochemical assays show that both compounds stimulate Ca2+-dependent ATPase activity of SERCA without altering the apparent Ca{superscript 2} affinity. The structures reveal a previously unrecognized "activation hotspot" in the transmembrane domain, a shallow groove formed by helices M3 and M4 and capped by M1. Despite low chemical similarity, both activators occupy the same pocket and share conserved interactions with Ser265, Trp272, and Phe296. These residues are unique to SERCA and help explain selectivity relative to other P-type ATPases. Activator binding stabilizes a catalytically competent conformation, shifting SERCA toward an E1-like state poised for ATP binding and coordinated movements of the M1-M4 bundle and the cytosolic domains. Notably, density consistent with a detergent acyl chain bridges an otherwise open cavity adjacent to the compound, suggesting that altered protein-lipid interactions may contribute to activation. Together, these findings define a structural framework for SERCA activation and provide a blueprint for rational design of next-generation SERCA activators. SIGNIFICANCE STATEMENTSERCA pumps Ca{superscript 2} into the sarco-endoplasmic reticulum, enabling muscle relaxation and shaping calcium signals across tissues. Small-molecule SERCA activators improve cardiac and metabolic phenotypes in animal models, but drug development has been limited by the absence of a defined binding site and activation mechanism. We determined cryo-EM structures of SERCA bound to two distinct activators, CDN1163 and UM-52. Both compounds occupy a groove formed by transmembrane segments M3-M4, anchored by a hydrogen bond to the SERCA-specific Ser265 and aromatic contacts near Trp272 and Phe296. An acyl chain bridges a gap in the binding pocket toward M1, suggesting that protein-lipid coupling may play a role in SERCA activation. These results directly enable structure-mechanism guided design of next-generation selective SERCA activators.

biophysics↗

bioRxiv2024

Paradoxical SERCA dysregulation contributes to atrial fibrillation in a model of diet-induced obesity

Obesity is a major risk factor for atrial fibrillation (AF) the most common serious cardiac arrhythmia, but the molecular mechanisms underlying diet-induced AF remain unclear. In this study, we subjected mice to a chronic high-fat diet and acute sympathetic activation ( two-hit model) to study the mechanisms by which diet-induced obesity promotes AF. Surface electrocardiography revealed that diet-induced obesity and sympathetic activation synergize during intracardiac tachypacing to induce AF. At the cellular level, diet-induced obesity and acute adrenergic stimulation facilitate the formation of delayed afterdepolarizations in atrial myocytes, implicating altered Ca2+ dynamics as the underlying cause of AF. We found that diet-induced obesity does not alter the expression of major Ca2+-handling proteins in atria, including the sarcoplasmic reticulum Ca2+-ATPase (SERCA), a major component of beat-to-beat Ca2+ cycling in the heart. Paradoxically, obesity reduces phospholamban phosphorylation, suggesting decreased SERCA activity, yet atrial myocytes from obese mice showed a significantly increased Ca2+ transient amplitude and SERCA-mediated Ca2+ uptake. Adrenergic stimulation further increases the Ca2+ transient amplitude but does not affect Ca2+ reuptake in atrial myocytes from obese mice. Transcriptomics analysis showed that a high-fat diet prompts upregulation of neuronatin, a protein that has been implicated in obesity and is known to stimulate SERCA activity. We propose a mechanism in which obesity primes SERCA for paradoxical activation, and adrenergic stimulation facilitates AF conversion through a Ca2+-induced Ca2+ release gain in atrial myocytes. Overall, this study links obesity, altered Ca2+ signaling, and AF, and targeting this mechanism may prove effective for treating obesity-induced AF.

physiology↗

bioRxiv2023

Activation mechanism of the cardiac calcium pump by a small-molecule allosteric modulator

The discovery of small-molecule 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 (TCSPC) imaging approach to investigate the activation mechanism of a druggable protein by a small-molecule allosteric modulator. We tested this approach using the cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), an important pharmacological target that transports Ca2+ at the expense of ATP hydrolysis in the heart. We found that CDN1163, a validated SERCA2a activator, does not dissociate the endogenous complex between SERCA2a and its regulator phospholamban (PLN) in the presence of either Ca2+ or AMP-PCP, a non-hydrolyzable ATP analog. CDN1163 does not influence SERCA2as affinity for Ca2+ ions at functionally relevant conditions. Global analysis of the fluorescence lifetimes showed that ATP is both a substrate and a modulator that populates competent SERCA2a conformations. Interestingly, CDN1163 alone does not significantly induce changes in the structural populations of SERCA2a. Instead, CDN1163 potentiates the effects of ATP to further shift the equilibrium toward a competent SERCA2a conformation. Importantly, this population shift occurs at sub-physiological conditions, and within physiological Ca2+ concentrations at which SERCA2a operates. We propose an activation mechanism whereby a small-molecule modulator synergizes with ATP to stabilize a conformation of SERCA2a primed for activation. This study demonstrates the power of TCSPC to reveal novel insights into how structural and biochemical states are coupled to allosterically activate a pharmacological target in the heart.

biophysics↗

bioRxiv2023

Phospholamban Inhibits the Cardiac Calcium Pump Through Reversing the Allosteric Enhancement of Calcium Affinity by ATP

Phospholamban (PLB) is a transmembrane micropeptide that regulates the Ca2+ pump SERCA in cardiac muscle, but the physical mechanism of this regulation remains poorly understood. PLB reduces the Ca2+ sensitivity of active SERCA, increasing the Ca2+ concentration required for pump cycling. However, PLB does not decrease Ca2+ binding to SERCA when ATP is absent, suggesting PLB does not inhibit SERCA Ca2+ affinity. The prevailing explanation for these seemingly conflicting results is that PLB slows the Ca2+-binding step in the SERCA enzymatic cycle, altering the Ca2+ dependence of cycling without affecting the true affinity of the Ca2+ binding sites. Here, we consider another hypothesis, that measurements of Ca2+ binding in the absence of ATP overlook important allosteric effects of nucleotide binding that increase SERCA Ca2+ binding affinity. We speculated that PLB inhibits SERCA by reversing this allostery. To test this, we used a fluorescent SERCA biosensor to quantify the Ca2+ affinity of non-cycling SERCA in the presence and absence of a non-hydrolyzable ATP-analog, AMPPCP. Nucleotide activation increased SERCA Ca2+ affinity, and this effect was reversed by co-expression of PLB. Interestingly, PLB had no effect on Ca2+ affinity in the absence of nucleotide. These results reconcile the previous conflicting observations from ATPase assays versus Ca2+ binding assays. Moreover, structural analysis of SERCA revealed a novel allosteric pathway connecting the ATP- and Ca2+-binding sites. We propose this pathway is interrupted by PLB binding. Thus, PLB reduces the true Ca2+ affinity of SERCA by reversing allosteric activation of the pump by ATP. O_TEXTBOXSignificance StatementThe micropeptide phospholamban (PLB) serves a vital role as the "adrenaline trigger" for the heart. PLB inhibits the calcium pump SERCA at rest, and relief of this inhibition increases cardiac performance during exercise. Still, the physical mechanism for PLB inhibition of SERCA remains poorly understood. Here, our results reveal that PLB regulates SERCA by reversing an allosteric effect of ATP that enhances SERCAs calcium affinity. We mapped a novel allosteric pathway that structurally couples SERCAs ATP- and calcium-binding sites. PLB binds residues along this pathway to disrupt communication between the ligand-binding sites. The data reveal an allosteric mechanism that may be conserved in other ATP-dependent ion pumps. This allosteric pathway may provide new opportunities for pharmacological targeting of SERCA. C_TEXTBOX

biochemistry↗

bioRxiv2021

STOICHIOMETRY OF THE SODIUM PUMP-PHOSPHOLEMMAN REGULATORY COMPLEX

The sodium-potassium ATPase (NKA) establishes ion gradients that facilitate many physiological processes. In the heart, NKA activity is regulated by its interaction with phospholemman (PLM, FXYD1). Here we used a novel fluorescence lifetime-based assay to investigate the structure, stoichiometry, and affinity of the NKA-PLM regulatory complex. We observed concentration dependent association of the subunits of NKA-PLM regulatory complex, with avid association of the alpha subunit with the essential beta subunit followed by lower affinity alpha-alpha and alpha-PLM interactions. The data provide the first evidence that the regulatory complex is composed of two alpha subunits associated with two beta subunits, decorated with two PLM regulatory subunits in intact cells. Docking and molecular dynamics simulations generated a structural model of the complex that is consistent with our experimental observations. We propose that alpha-alpha subunit interactions support conformational coupling of the catalytic subunits, which may enhance NKA turnover rate. These observations provide insight into the pathophysiology of heart failure, wherein low NKA expression may be insufficient to support formation of the complete regulatory complex with stoichiometry (alpha-beta-PLM)2.

biophysics↗

bioRxiv2021

From atoms to cells: bridging the gap between potency, efficacy, and safety of small molecules directed at a membrane protein

Membrane proteins constitute a substantial fraction of the human proteome, thus representing a vast source of therapeutic drug targets. Indeed, newly devised technologies now allow targeting "undruggable" regions of membrane proteins to modulate protein function in the cell. Despite the advances in technology, the rapid translation of basic science discoveries into potential drug candidates targeting transmembrane protein domains remains challenging. We address this issue by harmonizing single molecule-based and ensemble-based atomistic simulations of ligand-membrane interactions with patient-derived induced pluripotent stem cell (iPSC)-based experiments to gain insights into drug delivery, cellular efficacy, and safety of molecules directed at membrane proteins. In this study, we interrogated the pharmacological activation of the cardiac Ca2+ pump (Sarcoplasmic reticulum Ca2+-ATPase, SERCA2a) in human iPSC-derived cardiac cells as a proof-of-concept model. The combined computational-experimental approach serves as a platform to explain the differences in the cell-based activity of candidates with similar functional profiles, thus streamlining the identification of drug-like candidates that directly target SERCA2a activation in human cardiac cells. Systematic cell-based studies further showed that a direct SERCA2a activator does not induce cardiotoxic pro-arrhythmogenic events in human cardiac cells, demonstrating that pharmacological stimulation of SERCA2a activity is a safe therapeutic approach targeting the heart. Overall, this novel platform encompasses organ-specific drug potency, efficacy, and safety, and opens new avenues to accelerate the bench-to-patient research aimed at designing effective therapies directed at membrane protein domains.

pharmacology and toxicology↗

bioRxiv2020

Dwarf open reading frame (DWORF) peptide is a direct activator of the sarcoplasmic reticulum calcium pump SERCA.

The cardiac sarcoplasmic reticulum calcium pump, SERCA, sequesters calcium in the sarco-endoplasmic reticulum (SR/ER) and plays a critical role in the contraction-relaxation cycle of the heart. A well-known regulator of SERCA in cardiac muscle is phospholamban (PLN), which interacts with the pump and reduces its apparent calcium affinity. A newly discovered SERCA regulatory subunit in cardiac muscle, dwarf open reading frame (DWORF), has added a new level of SERCA regulation. In this report, we modeled the structure of DWORF and evaluated it using molecular dynamics simulations. DWORF structure was modeled as a discontinuous helix with an unwound region at Pro15. This model orients an N-terminal amphipathic helix along the membrane surface and leaves a relatively short C-terminal transmembrane helix. We determined the functional regulation of SERCA by DWORF using a membrane reconstitution system. Surprisingly, we observed that DWORF directly activated SERCA by increasing its turnover rate. Furthermore, in-cell imaging of calcium dynamics demonstrated that DWORF increased SERCA-dependent ER calcium load, calcium reuptake rate, and spontaneous calcium release. Together, these functional assays suggest opposing effects of DWORF and PLN on SERCA function. The results agree with fluorescence resonance energy transfer experiments, which revealed changes in the affinity of DWORF for SERCA at low versus high cytosolic calcium concentrations. We found that DWORF has a higher affinity for SERCA in the presence of calcium, while PLN had the opposite behavior, a higher affinity for SERCA in low calcium. We propose a new mechanism for DWORF regulation of cardiac calcium handling in which DWORF directly enhances SERCA turnover by stabilizing the conformations of SERCA that predominate during elevated cytosolic calcium.

biophysics↗

bioRxiv2020

Sarcolambans are phospholamban- and sarcolipin-like regulators of the sarcoplasmic reticulum calcium pump SERCA

From insects to humans, calcium signaling is essential for life. An important part of this process is the sarco-endoplasmic reticulum calcium pump SERCA, which maintains low cytosolic calcium levels required for intracellular calcium homeostasis. In higher organisms, this is a tightly controlled system where SERCA interacts with tissuespecific regulatory subunits such as phospholamban in cardiac muscle and sarcolipin in skeletal muscle. With the recent discovery of the sarcolambans, the family of calcium pump regulatory subunits also appears to be ancient, spanning more than 550 million years of evolutionary divergence from insects to humans. This evolutionary divergence is reflected in the peptide sequences, which vary enormously from one another and range from vaguely phospholamban-like to vaguely sarcolipin-like. Here, our goal was to investigate select sarcolamban peptides for their ability to regulate calcium pump activity. For a side-by-side comparison of diverse sarcolamban peptides, we tested them against mammalian skeletal muscle SERCA1a. This allowed us to determine if the sarcolamban peptides mimic phospholamban and sarcolipin in their regulatory activities. Four sarcolamban peptides were chosen from different invertebrate species. Of these, we were able to express and purify sarcolamban peptides from bumble bee, water flea, and tadpole shrimp. Sarcolamban peptides were co-reconstituted into proteoliposomes with mammalian SERCA1a and the effect of each peptide on the apparent calcium affinity and maximal activity of SERCA was measured. While all peptides were super-inhibitors of SERCA, they exhibited either phospholamban-like or sarcolipin-like characteristics. Molecular modeling, protein-protein docking, and molecular dynamics simulations were used to reveal novel features of insect versus mammalian calcium pumps and the sarcolamban regulatory subunits.

biochemistry↗

bioRxiv2020

Multiscale Simulation Reveals Passive Proton Transport Through SERCA on the Microsecond Timescale

The sarcoplasmic reticulum Ca2+-ATPase (SERCA) transports two Ca2+ ions from the cytoplasm to the reticulum lumen at the expense of ATP hydrolysis. In addition to transporting Ca2+, SERCA facilitates bidirectional proton transport across the sarcoplasmic reticulum to maintain the charge balance of the transport sites and to balance the charge deficit generated by the exchange of Ca2+. Previous studies have shown the existence of a transient water-filled pore in SERCA that connects the Ca2+-binding sites with the lumen, but the capacity of this pathway to sustain passive proton transport has remained unknown. In this study, we used the multiscale reactive molecular dynamics (MS-RMD) method and free energy sampling to quantify the free energy profile and timescale of the proton transport across this pathway while also explicitly accounting for the dynamically coupled hydration changes of the pore. We find that proton transport from the central binding site to the lumen has a microsecond timescale, revealing a novel passive cytoplasm-to-lumen proton flow beside the well-known inverse proton countertransport occurring in active Ca2+ transport. We propose that this proton transport mechanism is operational and serves as a functional conduit for passive proton transport across the sarcoplasmic reticulum. SIGNIFICANCEMultiscale reactive molecular dynamics combined with free energy sampling was applied to study proton transport through a transient water pore connecting the Ca2+-binding site to the lumen in SERCA. This is the first computational study of this large biomolecular system that treats the hydrated excess proton and its transport through water structures and amino acids explicitly. When also correctly accounting for the hydration fluctuations of the pore, it is found that a transiently hydrated channel can transport protons on a microsecond timescale. These results quantitatively support the hypothesis of the proton intake into the sarcoplasm via SERCA, in addition to the well-known proton pumping by SERCA to the cytoplasm along with Ca2+ transport.

biophysics↗

bioRxiv2020

Dynamics-driven allostery underlies pre-activation of the regulatory Ca2+-ATPase/phospholamban complex

Sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and phospholamban (PLB) are essential for intracellular Ca2+ transport in myocytes. Ca2+-dependent activation of SERCA-PLB provides a rheostat function that regulates cytosolic and SR Ca2+ levels. While experimental and computational studies alone have led to a greater insight into the mechanisms for SERCA-PLB regulation, the structural changes induced by Ca2+ binding and how those are communicated to couple enzymatic activity with active transport remain poorly understood. Therefore, we have performed atomistic simulations totaling 32.7 s and cell-based intramolecular fluorescence resonance energy transfer (FRET) experiments to determine structural changes of PLB-bound SERCA in response to Ca2+ binding. Complementary simulations and experiments showed structural disorder underlies PLB inhibition of SERCA, and Ca2+ binding is sufficient to shift the protein population toward a structurally ordered state of the complex. This structural transition results in a redistribution of structural states toward a partially closed conformation of SERCAs cytosolic headpiece. Closure is accompanied by functional interactions between the N-domain {beta}5-{beta}6 loop and the A-domain. Regulation of these key structural elements indicate that Ca2+ is a critical mediator of allosteric signaling that dictates structural changes and motions that pre-activate SERCA-PLB. These findings provide direct support that dynamically driven protein allostery underlies PLB regulation of SERCA. These functional insights at unprecedented spatiotemporal resolution suggest a general modular architecture mechanism for dynamic regulation of the SERCA-PLB complex. Understanding these mechanisms is of paramount importance to guide therapeutic modulation of SERCA and other evolutionarily related ion-motive ATPases.

biophysics↗

bioRxiv2020

Conserved luminal C-terminal domain dynamically controls interdomain communication in sarcolipin

ABSTRACTSarcolipin (SLN) mediates Ca2+ transport and metabolism in muscle by regulating the activity of the Ca2+ pump SERCA. SLN has a conserved luminal C-terminal domain that contributes to the its functional divergence among homologous SERCA regulators, but the precise mechanistic role of this domain remains poorly understood. We used all-atom molecular dynamics (MD) simulations of SLN totaling 77.5 μs to show that the N- (NT) and C-terminal (CT) domains function in concert. Analysis of the MD simulations showed that serial deletions of SLN C-terminus does not affect the stability of the peptide nor induce dissociation of SLN from the membrane but promotes a gradual decrease in both tilt angle of the transmembrane helix and the local thickness of the lipid bilayer. Mutual information analysis showed that the NT and CT domains communicate with each other in SLN, and that interdomain communication is partially or completely abolished upon deletion of the conserved segment Tyr29-Tyr31 as well as by serial deletions beyond this domain. Phosphorylation of SLN at residue Thr5 also induces changes in the communication between the CT and NT domains, thus providing additional evidence for interdomain communication within SLN. We found that interdomain communication is independent of the force field used and lipid composition, thus demonstrating that communication between the NT and CT domains is an intrinsic functional feature of SLN. We propose the novel hypothesis that the conserved C-terminus is an essential element required for dynamic control of SLN regulatory function.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biophysics↗

bioRxiv2019

Horse gluteus is a null-sarcolipin muscle with enhanced sarcoplasmic reticulum calcium transport

We have analyzed gene transcription, protein expression, and enzymatic activity of the Ca2+-transporting ATPase (SERCA) in horse gluteal muscle. Horses are bred for peak athletic performance but exhibit a high incidence of exertional rhabdomyolysis, with myosolic Ca2+ suggested as a correlative linkage. To assess Ca2+ regulation in horse gluteus, we developed an improved protocol for isolating horse sarcoplasmic reticulum (SR) vesicles. RNA-seq and immunoblotting determined that the ATP2A1 gene (protein product SERCA1) is the predominant Ca2+-ATPase expressed in horse gluteus, as in rabbit muscle. Gene expression was assessed for four regulatory peptides of SERCA, finding that sarcolipin (SLN) is the predominant regulatory peptide transcript expressed in horse gluteus, as in rabbit muscle. Surprisingly, the RNA transcription ratio of SLN-to-ATP2A1 in horse gluteus is an order of magnitude higher than in rabbit muscle, but conversely, the protein expression ratio of SLN-to-SERCA1 in horse gluteus is an order of magnitude lower than in rabbit. Thus, the SLN gene is not translated to a stable protein in horse gluteus, yet the supra-high level of SLN RNA suggests a non-coding role. Gel-stain analysis revealed that horse SR expresses calsequestrin (CASQ) protein abundantly, with a CASQ-to-SERCA ratio [~]3-fold greater than rabbit SR. The Ca2+ transport rate of horse SR vesicles is [~]2-fold greater than rabbit SR, suggesting horse myocytes have enhanced luminal Ca2+ stores that increase intracellular Ca2+ release and muscular performance. The absence of SLN inhibition of SERCA and the abundant expression of CASQ may potentiate horse muscle contractility and susceptibility to exertional rhabdomyolysis.

biochemistry↗

bioRxiv2019

Interaction of a sarcolipin pentamer and monomer with the sarcoplasmic reticulum calcium pump, SERCA

The sequential rise and fall of cytosolic calcium underlies the contraction-relaxation cycle of muscle cells. While contraction is initiated by the release of calcium from the sarcoplasmic reticulum, muscle relaxation involves the active transport of calcium back into the sarcoplasmic reticulum. This re-uptake of calcium is catalysed by the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA), which plays a lead role in muscle contractility. The activity of SERCA is regulated by small membrane protein subunits, most well-known being phospholamban (PLN) and sarcolipin (SLN). SLN physically interacts with SERCA and differentially regulates contractility in skeletal and atrial muscle. SLN has also been implicated in skeletal muscle thermogenesis. Despite these important roles, the structural mechanisms by which SLN modulates SERCA-dependent contractility and thermogenesis remain unclear. Here, we functionally characterized wild-type SLN and a pair of mutants, Asn4-Ala and Thr5-Ala, which yielded gain-of-function behavior comparable to what has been found for PLN. Next, we analyzed twodimensional crystals of SERCA in the presence of wild-type SLN by electron cryo-microscopy. The fundamental units of the crystals are anti-parallel dimer ribbons of SERCA, known for decades as an assembly of calcium-free SERCA molecules induced by the addition of decavanadate. A projection map of the SERCA-SLN complex was determined to a resolution of 8.5 [A], which allowed the direct visualization of a SLN pentamer. The SLN pentamer was found to interact with transmembrane segment M3 of SERCA, though the interaction appeared to be indirect and mediated by an additional density consistent with a SLN monomer. This SERCA-SLN complex correlated with the ability of SLN to decrease the maximal activity of SERCA, which is distinct from the ability of PLN to increase the maximal activity of SLN. Protein-protein docking and molecular dynamics simulations provided models for the SLN pentamer and the novel interaction between SERCA and a SLN monomer.\n\nSTATEMENT OF SIGNIFICANCEThis research article describes a novel complex of the sarcoplasmic reticulum calcium pump SERCA and its regulatory subunit sarcolipin. Given the potential role of sarcolipin in skeletal muscle non-shivering thermogenesis, the interactions between SERCA and sarcolipin are of critical importance. Using complementary approaches of functional analysis, electron crystallography, and molecular dynamics simulations, we demonstrate an inherent interaction between SERCA, a sarcolipin monomer, and a sarcolipin pentamer. The interaction involves transmembrane segment M3 of SERCA, which allows sarcolipin to decrease the maximal activity or turnover rate of SERCA. Protein-protein docking and molecular dynamics simulations provided models for the SLN pentamer and the novel interaction between SERCA and a SLN monomer.

biophysics↗

bioRxiv2018

Probing the effects of nonannular lipid binding on the stability of the calcium pump SERCA

The calcium pump SERCA is a transmembrane protein that is critical for calcium transport in cells. SERCA resides in an environment made up largely by the lipid bilayer, so lipids play a central role on its stability and function. Studies have provided insights into the effects of annular and bulk lipids on SERCA activation, but the role of a nonannular lipid site in the E2 intermediate state remains elusive. Here, we have performed microsecond molecular dynamics (MD) simulations to probe the effects of nonannular lipid binding on the stability and structural dynamics of the E2 state of SERCA. We found that the structural integrity and stability of the E2 state is independent of nonannular lipid binding, and that occupancy of a lipid molecule at this site does not modulate destabilization of the E2 state, a step required to initiate the transition toward the competent E1 state. We also found that binding of the nonannular lipid does not induce direct allosteric control of the intrinsic functional dynamics the E2 state. We conclude that nonannular lipid binding is not necessary for the stability of the E2 state, but we speculate that it becomes functionally significant during the E2-to-E1 transition of the pump.

biophysics↗

bioRxiv2018

The phospholamban pentamer interacts with the sarcoplasmic reticulum calcium pump SERCA.

The interaction of phospholamban with the sarcoplasmic reticulum calcium pump (SERCA) is a major regulatory axis in cardiac muscle contractility. The prevailing model involves reversible inhibition of SERCA by monomeric phospholamban and storage of phospholamban as an inactive pentamer. However, this paradigm has been challenged by studies demonstrating that phospholamban remains associated with SERCA and that the phospholamban pentamer is required for cardiac contractility. We have previously used two-dimensional crystallization and electron microscopy to study the interaction between SERCA and phospholamban. To further understand this interaction, we compared small helical crystals and large two-dimensional crystals of SERCA in the absence and presence of phospholamban. In both crystal forms, SERCA molecules are organized into identical anti-parallel dimer ribbons. The dimer ribbons pack together with distinct crystal contacts in the helical versus large two-dimensional crystals, which allow phospholamban differential access to potential sites of interaction with SERCA. Nonetheless, we show that a phospholamban oligomer interacts with SERCA in a similar manner in both crystal forms. In the two-dimensional crystals, a phospholamban pentamer interacts with transmembrane segments M3 of SERCA and participates in a crystal contact that bridges neighboring SERCA dimer ribbons. In the helical crystals, an oligomeric form of phospholamban also interacts with M3 of SERCA, though the phospholamban oligomer straddles a SERCA-SERCA crystal contact. We conclude that the pentameric form of phospholamban interacts with SERCA, and that it plays distinct structural and functional roles in SERCA regulation.

biophysics↗

bioRxiv2018

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

We have performed extensive atomistic molecular dynamics simulations to probe the structural mechanism for relief of sarcoplasmic reticulum Ca2+-ATPase (SERCA) inhibition by phospholamban (PLB) at saturating Ca2+ conditions. Reversal of SERCA-PLB inhibition by saturating Ca2+ operates as a physiological rheostat to reactivate SERCA function in the absence of PLB phosphorylation. Simulation of the inhibitory complex at super-physiological Ca2+ concentrations ([Ca2+]=10 mM) revealed that calcium ions interact primarily with SERCA and the lipid headgroups, but not with the cytosolic domain of PLB or the cytosolic side of the SERCA-PLB interface. At this [Ca2+], a single Ca2+ ion is 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{approx}400 M). At these conditions, ~30% of the Ca2+-bound complexes exhibit structural features that correspond to an inhibited state. However, in ~70% of the Ca2+-bound complexes, Ca2+ moves to transport site I, recruits Glu771 and Asp800, and disrupts key inhibitory contacts involving conserved PLB residue Asn34. Structural analysis showed that Ca2+ induces only local changes in interresidue inhibitory interactions, but does not induce dissociation, repositioning or changes in the structural dynamics of PLB. Upon relief of SERCA inhibition, Ca2+ binding produces a productive site I configuration that is 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 non-inhibited SERCA-PLB complex.

biophysics↗