Synthetic null-cysteine phospholamban analogue and the corresponding transmembrane domain inhibit the Ca-ATPase

Interaction of DWORF with SERCA and PLB as determined by EPR spectroscopy

Insufficient sarco/endoplasmic reticulum calcium ATPase (SERCA) activity significantly contributes to heart failure, which is a leading cause of death worldwide. A characteristic pathology of cardiac disease is the slow and incomplete Ca2+ removal from the myocyte cytoplasm in diastole, which is primarily driven by SERCA, the integral transmembrane Ca2+ pump. Phospholamban (PLB) allosterically inhibits SERCA by reducing its apparent Ca2+ affinity. Recently, the 34-codon novel dwarf open reading frame (DWORF) micropeptide has been identified as a muscle-specific SERCA effector, capable of reversing the inhibitory effects of PLB and independently activating SERCA in the absence of PLB. However, the structural basis for these functions has not yet been determined in a system of defined molecular components. We have used electron paramagnetic resonance (EPR) spectroscopy to investigate the protein-protein interactions of DWORF, co-reconstituted in proteoliposomes with SERCA and spin-labeled PLB. We analyzed the change of PLB rotational mobility in response to varying DWORF concentration, to quantify competitive binding of DWORF and PLB. We determined that DWORF competes with PLB for binding to SERCA at low [Ca2+], although the measured affinity of DWORF for SERCA is an order of magnitude weaker than that of PLB for SERCA, indicating cooperativity. The sensitivity of EPR to structural dynamics, using stereospecifically attached spin labels, allows us to obtain new information needed to refine the molecular model for regulation of SERCA activity, as needed for development of novel therapeutic remedies against cardiac pathologies.

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.

A hallmark of phospholamban functional divergence is located in the N-terminal phosphorylation domain

Sarcoplasmic reticulum Ca²⁺ pump (SERCA) is a critical component of the Ca²⁺ transport machinery in myocytes. There is clear evidence for regulation of SERCA activity by PLB, whose activity is modulated by phosphorylation of its N-terminal domain (residues 1–25), but there is less clear evidence for the role of this domain in PLB’s functional divergence. It is widely accepted that only sarcolipin (SLN), a protein that shares substantial homology with PLB, uncouples SERCA Ca²⁺ transport from ATP hydrolysis by inducing a structural change of its energy-transduction domain; yet, experimental evidence shows that the transmembrane domain of PLB (residues 26–52, PLB_26–52) partially uncouples SERCA in vitro. These apparently conflicting mechanisms suggest that PLB’s uncoupling activity is encoded in its transmembrane domain, and that it is controlled by the N-terminal phosphorylation domain. To test this hypothesis, we performed molecular dynamics simulations (MDS) of the binary complex between PLB_26–52 and SERCA. Comparison between PLB_26–52 and wild-type PLB (PLB_WT) showed no significant changes in the stability and orientation of the transmembrane helix, indicating that PLB_26–52 forms a native-like complex with SERCA. MDS showed that PLB_26–52 produces key intermolecular contacts and structural changes required for inhibition, in agreement with studies showing that PLB_26–52 inhibits SERCA. However, deletion of the N-terminal phosphorylation domain facilitates an order-to-disorder shift in the energy-transduction domain associated with uncoupling of SERCA, albeit weaker than that induced by SLN. This mechanistic evidence reveals that the N-terminal phosphorylation domain of PLB is a primary contributor to the functional divergence among homologous SERCA regulators.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Effect of Phosphorylation on Interactions between Transmembrane Domains of SERCA and Phospholamban

We have used site-directed spin labeling and electron paramagnetic resonance (EPR) to map interactions between the transmembrane (TM) domains of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) and phospholamban (PLB) as affected by PLB phosphorylation. In the cardiac sarcoplasmic reticulum, PLB binding to SERCA results in Ca-dependent enzyme inhibition, which is reversed by PLB phosphorylation at Ser16. Previous spectroscopic studies on SERCA-PLB have largely focused on the cytoplasmic domain of PLB, showing that phosphorylation induces a structural shift in this domain relative to SERCA. However, SERCA inhibition is due entirely to TM domain interactions. Therefore, we focus here on PLB's TM domain, attaching Cys-reactive spin labels at five different positions. In each case, continuous-wave EPR indicated moderate spin-label mobility, with the addition of SERCA revealing two populations, one indistinguishable from PLB alone and another with more restricted rotational mobility, presumably due to SERCA-binding. Phosphorylation had no effect on the rotational mobility of either component but significantly decreased the mole fraction of the restricted component. Solvent-accessibility experiments using power-saturation EPR and saturation-recovery EPR confirmed that these two spectral components were SERCA-bound and unbound PLB and showed that phosphorylation increased the overall lipid accessibility of the TM domain by increasing the fraction of unbound PLB. However-based on these results-at physiological levels of SERCA and PLB, most SERCA would have bound PLB even after phosphorylation. Additionally, no structural shift in the TM domain of SERCA-bound PLB was detected, as there were no significant changes in membrane insertion depth or its accessibility. Therefore, we conclude that under physiological conditions, the phosphorylation of PLB induces little or no change in the interaction of the TM domain with SERCA, so relief of inhibition is predominantly due to the previously observed structural shift in the cytoplasmic domain.

Nitroxyl (HNO) targets phospholamban cysteines 41 and 46 to enhance cardiac function

Nitroxyl (HNO) positively modulates myocardial function by accelerating Ca²⁺ reuptake into the sarcoplasmic reticulum (SR). HNO-induced enhancement of myocardial Ca²⁺ cycling and function is due to the modification of cysteines in the transmembrane domain of phospholamban (PLN), which results in activation of SR Ca²⁺-ATPase (SERCA2a) by functionally uncoupling PLN from SERCA2a. However, which cysteines are modified by HNO, and whether HNO induces reversible disulfides or single cysteine sulfinamides (RS(O)NH₂) that are less easily reversed by reductants, remain to be determined. Using an ¹⁵N-edited NMR method for sulfinamide detection, we first demonstrate that Cys46 and Cys41 are the main targets of HNO reactivity with PLN. Supporting this conclusion, mutation of PLN cysteines 46 and 41 to alanine reduces the HNO-induced enhancement of SERCA2a activity. Treatment of WT-PLN with HNO leads to sulfinamide formation when the HNO donor is in excess, whereas disulfide formation is expected to dominate when the HNO/thiol stoichiometry approaches a 1:1 ratio that is more similar to that anticipated in vivo under normal, physiological conditions. Thus, ¹⁵N-edited NMR spectroscopy detects redox changes on thiols that are unique to HNO, greatly advancing the ability to detect HNO footprints in biological systems, while further differentiating HNO-induced post-translational modifications from those imparted by other reactive nitrogen or oxygen species. The present study confirms the potential of HNO as a signaling molecule in the cardiovascular system.

Effects of the Arg9Cys and Arg25Cys mutations on phospholamban's conformational equilibrium in membrane bilayers

Approximately, 70% of the Ca²⁺ ion transport into the sarcoplasmic reticulum is catalyzed by the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), whose activity is endogenously regulated by phospholamban (PLN). PLN comprises a TM inhibitory region and a cytoplasmic regulatory region that harbors a consensus sequence for cAMP-dependent protein kinase (PKA). The inhibitory region binds the ATPase, reducing its apparent Ca²⁺ binding affinity. β-adrenergic stimulation activates PKA, which phosphorylates PLN at Ser 16, reversing its inhibitory function. Mutations and post-translational modifications of PLN may lead to dilated cardiomyopathy (DCM) and heart failure. PLN's cytoplasmic region interconverts between a membrane-associated T state and a membrane-detached R state. The importance of these structural transitions on SERCA regulation is emerging, but the effects of natural occurring mutations and their relevance to the progression of heart disease are unclear. Here we use solid-state NMR spectroscopy to investigate the structural dynamics of two lethal PLN mutations, R9C and R25C, which lead to DCM. We found that the R25C mutant enhances the dynamics of PLN and shifts the conformational equilibrium toward the R state confirmation, whereas the R9C mutant drives the amphipathic cytoplasmic domain toward the membrane-associate state, enriching the T state population. The changes in membrane interactions caused by these mutations may explain the aberrant regulation of SERCA.

Synthetic phosphopeptides enable quantitation of the content and function of the four phosphorylation states of phospholamban in cardiac muscle

We have studied the differential effects of phospholamban (PLB) phosphorylation states on the activity of the sarcoplasmic reticulum Ca-ATPase (SERCA). It has been shown that unphosphorylated PLB (U-PLB) inhibits SERCA and that phosphorylation of PLB at Ser-16 or Thr-17 relieves this inhibition in cardiac sarcoplasmic reticulum. However, the levels of the four phosphorylation states of PLB (U-PLB, P16-PLB, P17-PLB, and doubly phosphorylated 2P-PLB) have not been measured quantitatively in cardiac tissue, and their functional effects on SERCA have not been determined directly. We have solved both problems through the chemical synthesis of all four PLB species. We first used the synthetic PLB as standards for a quantitative immunoblot assay, to determine the concentrations of all four PLB phosphorylation states in pig cardiac tissue, with and without left ventricular hypertrophy (LVH) induced by aortic banding. In both LVH and sham hearts, all phosphorylation states were significantly populated, but LVH hearts showed a significant decrease in U-PLB, with a corresponding increase in the ratio of total phosphorylated PLB to U-PLB. To determine directly the functional effects of each PLB species, we co-reconstituted each of the synthetic peptides in phospholipid membranes with SERCA and measured calcium-dependent ATPase activity. SERCA inhibition was maximally relieved by P16-PLB (the most highly populated PLB state in cardiac tissue homogenates), followed by 2P-PLB, then P17-PLB. These results show that each PLB phosphorylation state uniquely alters Ca(2+) homeostasis, with important implications for cardiac health, disease, and therapy.

Time-resolved FRET reveals the structural mechanism of SERCA-PLB regulation

We have used time-resolved fluorescence resonance energy transfer (TR-FRET) to characterize the interaction between phospholamban (PLB) and the sarcoplasmic reticulum (SR) Ca-ATPase (SERCA) under conditions that relieve SERCA inhibition. Unphosphorylated PLB inhibits SERCA in cardiac SR, but inhibition is relieved by either micromolar Ca(2+) or PLB phosphorylation. In both cases, it has been proposed that inhibition is relieved by dissociation of the complex. To test this hypothesis, we attached fluorophores to the cytoplasmic domains of SERCA and PLB, and reconstituted them functionally in lipid bilayers. TR-FRET, which permitted simultaneous measurement of SERCA-PLB binding and structure, was measured as a function of PLB phosphorylation and [Ca(2+)]. In all cases, two structural states of the SERCA-PLB complex were resolved, probably corresponding to the previously described T and R structural states of the PLB cytoplasmic domain. Phosphorylation of PLB at S16 completely relieved inhibition, partially dissociated the SERCA-PLB complex, and shifted the T/R equilibrium within the bound complex toward the R state. Since the PLB concentration in cardiac SR is at least 10 times that in our FRET measurements, we calculate that most of SERCA contains bound phosphorylated PLB in cardiac SR, even after complete phosphorylation. 4 μM Ca(2+) completely relieved inhibition but did not induce a detectable change in SERCA-PLB binding or cytoplasmic domain structure, suggesting a mechanism involving structural changes in SERCA's transmembrane domain. We conclude that Ca(2+) and PLB phosphorylation relieve SERCA-PLB inhibition by distinct mechanisms, but both are achieved primarily by structural changes within the SERCA-PLB complex, not by dissociation of that complex.

Allosteric regulation of SERCA by phosphorylation-mediated conformational shift of phospholamban

The membrane protein complex between the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) and phospholamban (PLN) controls Ca(2+) transport in cardiomyocytes, thereby modulating cardiac contractility. β-Adrenergic-stimulated phosphorylation of PLN at Ser-16 enhances SERCA activity via an unknown mechanism. Using solid-state nuclear magnetic resonance spectroscopy, we mapped the physical interactions between SERCA and both unphosphorylated and phosphorylated PLN in membrane bilayers. We found that the allosteric regulation of SERCA depends on the conformational equilibrium of PLN, whose cytoplasmic regulatory domain interconverts between three different states: a ground T state (helical and membrane associated), an excited R state (unfolded and membrane detached), and a B state (extended and enzyme-bound), which is noninhibitory. Phosphorylation at Ser-16 of PLN shifts the populations toward the B state, increasing SERCA activity. We conclude that PLN's conformational equilibrium is central to maintain SERCA's apparent Ca(2+) affinity within a physiological window. This model represents a paradigm shift in our understanding of SERCA regulation by posttranslational phosphorylation and suggests strategies for designing innovative therapeutic approaches to enhance cardiac muscle contractility.

Structures of the excited states of phospholamban and shifts in their populations upon phosphorylation

Phospholamban is an integral membrane protein that controls the calcium balance in cardiac muscle cells. As the function and regulation of this protein require the active involvement of low populated states in equilibrium with the native state, it is of great interest to acquire structural information about them. In this work, we calculate the conformations and populations of the ground state and the three main excited states of phospholamban by incorporating nuclear magnetic resonance residual dipolar couplings as replica-averaged structural restraints in molecular dynamics simulations. We then provide a description of the manner in which phosphorylation at Ser16 modulates the activity of the protein by increasing the sizes of the populations of its excited states. These results demonstrate that the approach that we describe provides a detailed characterization of the different states of phospholamban that determine the function and regulation of this membrane protein. We anticipate that the knowledge of conformational ensembles enable the design of new dominant negative mutants of phospholamban by modulating the relative populations of its conformational substates.

Solution NMR studies on the orientation of membrane-bound peptides and proteins by paramagnetic probes

Many peptides and proteins are attached to or immersed in a biological membrane. In order to understand their function not only the structure but also their topology in the membrane is important. Solution NMR spectroscopy is one of the most often used approaches to determine the orientation and localization of membrane-bound peptides and proteins. Here we give an application-oriented overview on the use of paramagnetic probes for the investigation of membrane-bound peptides and proteins. The examples discussed range from the large pool of antimicrobial peptides, bacterial toxins, cell penetrating peptides to domains of larger proteins or the calcium regulating protein phospholamban. Topological information is obtained in all these examples by the use of either attached or freely mobile paramagnetic tags. For some examples information obtained from the paramagnetic probes was included in the structure determination.

Protein-protein interactions in calcium transport regulation probed by saturation transfer electron paramagnetic resonance

We have used electron paramagnetic resonance (EPR) to probe the homo- and heterooligomeric interactions of reconstituted sarcoplasmic reticulum Ca-ATPase (SERCA) and its regulator phospholamban (PLB). SERCA is responsible for restoring calcium to the sarcoplasmic reticulum to allow muscle relaxation, whereas PLB inhibits cardiac SERCA unless phosphorylated at Ser(16). To determine whether changes in protein association play essential roles in regulation, we detected the microsecond rotational diffusion of both proteins using saturation transfer EPR. Peptide synthesis was used to create a fully functional and monomeric PLB mutant with a spin label rigidly coupled to the backbone of the transmembrane helix, while SERCA was reacted with a Cys-specific spin label. Saturation transfer EPR revealed that sufficiently high lipid/protein ratios minimized self-association for both proteins. Under these dilute conditions, labeled PLB was substantially immobilized after co-reconstitution with unlabeled SERCA, reflecting their association to form the regulatory complex. Ser(16) phosphorylation slightly increased this immobilization. Complementary measurements with labeled SERCA showed no change in mobility after co-reconstitution with unlabeled PLB, regardless of its phosphorylation state. We conclude that phosphorylating monomeric PLB can relieve SERCA inhibition without changes in the oligomeric states of these proteins, indicating a structural rearrangement within the heterodimeric regulatory complex.

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.

Tuning the structural coupling between the transmembrane and cytoplasmic domains of phospholamban to control sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) function

Phospholamban (PLN) is the endogenous inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), the integral membrane enzyme responsible for 70 % of the removal of Ca(2+) from the cytosol, inducing cardiac muscle relaxation in humans. Dysfunctions in SERCA:PLN interactions have been implicated as having a critical role in cardiac disease, and targeting Ca(2+) transport has been demonstrated to be a promising avenue in treating conditions of heart failure. Here, we designed a series of new mutants able to tune SERCA function, targeting the loop sequence that connects the transmembrane and cytoplasmic helices of PLN. We found that a variable degree of loss of inhibition mutants is attainable by engineering glycine mutations along PLN's loop domain. Remarkably, a double glycine mutation results in a complete loss-of-function mutant, fully mimicking the phosphorylated state of PLN. Using nuclear magnetic resonance spectroscopy, we rationalized the effects of these mutations in terms of entropic control on PLN function, whose inhibitory function can be modulated by increasing its conformational dynamics. However, if PLN mutations go past a threshold set by the phosphorylated state, they break the structural coupling between the transmembrane and cytoplasmic domains, resulting in a species that behaves as the inhibitory transmembrane domain alone. These studies provide new potential candidates for gene therapy to reverse the effects of heart failure.

High-throughput FRET assay yields allosteric SERCA activators

Using fluorescence resonance energy transfer (FRET), we performed a high-throughput screen (HTS) in a reconstituted membrane system, seeking compounds that reverse inhibition of sarcoplasmic reticulum Ca-ATPase (SERCA) by its cardiac regulator, phospholamban (PLB). Such compounds have long been sought to correct aberrant Ca(2+) regulation in heart failure. Donor-SERCA was reconstituted in phospholipid membranes with or without acceptor-PLB, and FRET was measured in a steady-state fluorescence microplate reader. A 20 000-compound library was tested in duplicate. Compounds that decreased FRET by more than three standard deviations were considered hits. From 43 hits (0.2%), 31 (72%) were found to be false-positives upon more thorough FRET testing. The remaining 12 hits were tested in assays of Ca-ATPase activity, and six of these activated SERCA significantly, by as much as 60%, and several also enhanced cardiomyocyte contractility. These compounds directly activated SERCA from heart and other tissues. These results validate our FRET approach and set the stage for medicinal chemistry and preclinical testing. We were concerned about the high rate of false-positives, resulting from the low precision of steady-state fluorescence. Preliminary studies with a novel fluorescence lifetime plate reader show 20-fold higher precision. This instrument can dramatically increase the quality of future HTS.

Structural and functional dynamics of an integral membrane protein complex modulated by lipid headgroup charge

We have used membrane surface charge to modulate the structural dynamics of an integral membrane protein, phospholamban (PLB), and thereby its functional inhibition of the sarcoplasmic reticulum Ca-ATPase (SERCA). It was previously shown by electron paramagnetic resonance, in vesicles of neutral lipids, that the PLB cytoplasmic domain is in equilibrium between an ordered T state and a dynamically disordered R state and that phosphorylation of PLB increases the R state and relieves SERCA inhibition, suggesting that R is less inhibitory. Here, we sought to control the T/R equilibrium by an alternative means-varying the lipid headgroup charge, thus perturbing the electrostatic interaction of PLB's cationic cytoplasmic domain with the membrane surface. We resolved the T and R states not only by electron paramagnetic resonance in the absence of SERCA but also by time-resolved fluorescence resonance energy transfer from SERCA to PLB, thus probing directly the SERCA-PLB complex. Compared to neutral lipids, anionic lipids increased both the T population and SERCA inhibition, while cationic lipids had the opposite effects. In contrast to conventional models, decreased inhibition was not accompanied by decreased binding. We conclude that PLB binds to SERCA in two distinct structural states of the cytoplasmic domain: an inhibitory T state that interacts strongly with the membrane surface and a less inhibitory R state that interacts more strongly with the anionic SERCA cytoplasmic domain. Modulating membrane surface charge provides an effective way of investigating the correlation between structural dynamics and function of integral membrane proteins.

Accurate quantitation of phospholamban phosphorylation by immunoblot

We have developed a quantitative immunoblot method to measure the mole fraction of phospholamban (PLB) phosphorylated at Ser16 (X(p)) in biological samples. In cardiomyocytes, PLB phosphorylation activates the sarcoplasmic reticulum calcium ATPase (SERCA), which reduces cytoplasmic Ca(2+) to relax the heart during diastole. Unphosphorylated PLB (uPLB) inhibits SERCA at low [Ca(2+)] but phosphorylated PLB (pPLB) is less inhibitory, so myocardial physiology and pathology depend critically on X(p). Current methods of X(p) determination by immunoblot provide moderate precision but poor accuracy. We have solved this problem using purified uPLB and pPLB standards produced by solid-phase peptide synthesis. In each assay, a pair of blots is performed with identical standards and unknowns using antibodies partially selective for uPLB and pPLB, respectively. When performed on mixtures of uPLB and pPLB, the assay measures both total PLB (tPLB) and X(p) with accuracy of 96% or better. We assayed pig cardiac sarcoplasmic reticulum (SR) and found that X(p) varied widely among four animals, from 0.08 to 0.38, but there was remarkably little variation in the ratios of X(p)/tPLB and uPLB/SERCA, suggesting that PLB phosphorylation is tuned to maintain homeostasis in SERCA regulation.

Topology and immersion depth of an integral membrane protein by paramagnetic rates from dissolved oxygen

In studies of membrane proteins, knowledge of protein topology can provide useful insight into both structure and function. In this work, we present a solution NMR method for the measurement the tilt angle and average immersion depth of alpha helices in membrane proteins, from analysis of the paramagnetic relaxation rate enhancements arising from dissolved oxygen. No modification to the micelle or protein is necessary, and the topology of both transmembrane and amphipathic helices are readily determined. We apply this method to the measure the topology of a monomeric mutant of phospholamban (AFA-PLN), a 52-residue membrane protein containing both an amphipathic and a transmembrane alpha helix. In dodecylphosphocholine micelles, the amphipathic helix of AFA-PLN was found to have a tilt angle of 87° ± 1° and an average immersion depth of 13.2 Å. The transmembrane helix was found to have an average immersion depth of 5.4 Å, indicating residues 41 and 42 are closest to the micelle centre. The resolution of paramagnetic relaxation rate enhancements from dissolved oxygen compares favourably to those from Ni (II), a hydrophilic paramagnetic species.

Probing ground and excited states of phospholamban in model and native lipid membranes by magic angle spinning NMR spectroscopy

In this paper, we analyzed the ground and excited states of phospholamban (PLN), a membrane protein that regulates sarcoplasmic reticulum calcium ATPase (SERCA), in different membrane mimetic environments. Previously, we proposed that the conformational equilibria of PLN are central to SERCA regulation. Here, we show that these equilibria detected in micelles and bicelles are also present in native sarcoplasmic reticulum lipid membranes as probed by MAS solid-state NMR. Importantly, we found that the kinetics of conformational exchange and the extent of ground and excited states in detergent micelles and lipid bilayers are different, revealing a possible role of the membrane composition on the allosteric regulation of SERCA. Since the extent of excited states is directly correlated to SERCA inhibition, these findings open up the exciting possibility that calcium transport in the heart can be controlled by the lipid bilayer composition. This article is part of a Special Issue entitled: Membrane protein structure and function.

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

Lipid-mediated folding/unfolding of phospholamban as a regulatory mechanism for the sarcoplasmic reticulum Ca2+-ATPase

The integral membrane protein complex between phospholamban (PLN) and sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) regulates cardiac contractility. In the unphosphorylated form, PLN binds SERCA and inhibits Ca(2+) flux. Upon phosphorylation of PLN at Ser16, the inhibitory effect is reversed. Although structural details on both proteins are emerging from X-ray crystallography, cryo-electron microscopy, and NMR studies, the molecular mechanisms of their interactions and regulatory process are still lacking. It has been speculated that SERCA regulation depends on PLN structural transitions (order to disorder, i.e., folding/unfolding). Here, we investigated PLN conformational changes upon chemical unfolding by a combination of electron paramagnetic resonance and NMR spectroscopies, revealing that the conformational transitions involve mostly the cytoplasmic regions, with two concomitant phenomena: (1) membrane binding and folding of the amphipathic domain Ia and (2) folding/unfolding of the juxtamembrane domain Ib of PLN. Analysis of phosphorylated and unphosphorylated PLN with two phosphomimetic mutants of PLN (S16E and S16D) shows that the population of an unfolded state in domains Ia and Ib (T' state) is linearly correlated to the extent of SERCA inhibition measured by activity assays. Inhibition of SERCA is carried out by the folded ground state (T state) of the protein (PLN), while the relief of inhibition involves promotion of PLN to excited conformational states (Ser16 phosphorylated PLN). We propose that PLN population shifts (folding/unfolding) are a key regulatory mechanism for SERCA.

Paramagnetic-based NMR restraints lift residual dipolar coupling degeneracy in multidomain detergent-solubilized membrane proteins

Residual dipolar couplings (RDCs) are widely used as orientation-dependent NMR restraints to improve the resolution of the NMR conformational ensemble of biomacromolecules and define the relative orientation of multidomain proteins and protein complexes. However, the interpretation of RDCs is complicated by the intrinsic degeneracy of analytical solutions and protein dynamics that lead to ill-defined orientations of the structural domains (ghost orientations). Here, we illustrate how restraints from paramagnetic relaxation enhancement (PRE) experiments lift the orientational ambiguity of multidomain membrane proteins solubilized in detergent micelles. We tested this approach on monomeric phospholamban (PLN), a 52-residue membrane protein, which is composed of two helical domains connected by a flexible loop. We show that the combination of classical solution NMR restraints (NOEs and dihedral angles) with RDC and PRE constraints resolves topological ambiguities, improving the convergence of the PLN structural ensemble and giving the depth of insertion of the protein within the micelle. The combination of RDCs with PREs will be necessary for improving the accuracy and precision of membrane protein conformational ensembles, where three-dimensional structures are dictated by interactions with the membrane-mimicking environment rather than compact tertiary folds common in globular proteins.

Lethal Arg9Cys phospholamban mutation hinders Ca2+-ATPase regulation and phosphorylation by protein kinase A

The regulatory interaction of phospholamban (PLN) with Ca(2+)-ATPase controls the uptake of calcium into the sarcoplasmic reticulum, modulating heart muscle contractility. A missense mutation in PLN cytoplasmic domain (R9C) triggers dilated cardiomyopathy in humans, leading to premature death. Using a combination of biochemical and biophysical techniques both in vitro and in live cells, we show that the R9C mutation increases the stability of the PLN pentameric assembly via disulfide bridge formation, preventing its binding to Ca(2+)-ATPase as well as phosphorylation by protein kinase A. These effects are enhanced under oxidizing conditions, suggesting that oxidative stress may exacerbate the cardiotoxic effects of the PLN(R9C) mutant. These results reveal a regulatory role of the PLN pentamer in calcium homeostasis, going beyond the previously hypothesized role of passive storage for active monomers.

Solid-state NMR measurements of the kinetics of the interaction between phospholamban and Ca2 + -ATPase in lipid bilayers

Phospholamban (PLB) is a small transmembrane protein that regulates calcium transport across the sarcoplasmic reticulum (SR) of cardiac cells via a reversible inhibitory interaction with Ca2+-ATPase. In this work solid-state NMR methods have been used to investigate the dynamics of the inhibitory association between PLB and Ca2+-ATPase. Skeletal muscle Ca2+-ATPase was incorporated into phosphatidylcholine membranes together with a ten-fold excess of a null-cysteine mutant of PLB labelled with 13C at Leu-44 in the transmembrane domain ([alpha-13C-L44]AAA-PLB). In these membranes the PLB variant was found to partially inhibit Ca2+-ATPase by reducing the affinity of the enzyme for calcium. Cross-polarization magic angle spinning (CP-MAS) 13C NMR spectra of the membranes exhibited a signature peak from [alpha-13C-L44]AAA-PLB at 56 ppm. Changes in the intensity of the peak were observed at different temperatures, which was diagnostic of direct interaction between [alpha-13C-L44]AAA-PLB and Ca2+-ATPase. Measurements of dipolar couplings between the 13C label and neighbouring protons were analysed to show that the mean residency time for the association of AAA-PLB with Ca2+-ATPase was on the order of 2.5 ms at temperatures between 0 degrees C and 30 degrees C. This new NMR approach will be useful for examining how the association of the two proteins is affected by physiological stimuli such as kinases and the elevation of calcium concentration.

Structure and topology of monomeric phospholamban in lipid membranes determined by a hybrid solution and solid-state NMR approach

Phospholamban (PLN) is an essential regulator of cardiac muscle contractility. The homopentameric assembly of PLN is the reservoir for active monomers that, upon deoligomerization form 1:1 complexes with the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), thus modulating the rate of calcium uptake. In lipid bilayers and micelles, monomeric PLN exists in equilibrium between a bent (or resting) T state and a more dynamic (or active) R state. Here, we report the high-resolution structure and topology of the T state of a monomeric PLN mutant in lipid bilayers, using a hybrid of solution and solid-state NMR restraints together with molecular dynamics simulations in explicit lipid environments. Unlike the previous structural ensemble determined in micelles, this approach gives a complete picture of the PLN monomer structure in a lipid bilayer. This hybrid ensemble exemplifies the tilt, rotation, and depth of membrane insertion, revealing the interaction with the lipids for all protein domains. The N-terminal amphipathic helical domain Ia (residues 1-16) rests on the surface of the lipid membrane with the hydrophobic face of domain Ia embedded in the membrane bilayer interior. The helix comprised of domain Ib (residues 23-30) and transmembrane domain II (residues 31-52) traverses the bilayer with a tilt angle of approximately 24 degrees . The specific interactions between PLN and lipid membranes may represent an additional regulatory element of its inhibitory function. We propose this hybrid method for the simultaneous determination of structure and topology for membrane proteins with compact folds or proteins whose spatial arrangement is dictated by their specific interactions with lipid bilayers.

Rotational dynamics of HIV-1 nucleocapsid protein NCp7 as probed by a spin label attached by peptide synthesis

2,2,6,6-Tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) spin label was attached at the N-terminal position to interrogate the dynamics of the HIV-1 nucleocapsid Zn-finger protein, NCp7. NCp7 is a 6.4-kDa 55-mer critical to the recognition, packaging, and efficient reverse transcription of viral RNA that has stem-loop structures, such as the RNA stem-loop 3 used in this work. The NCp7, made by solid-phase peptide synthesis with TOAC incorporated into the alpha-carbon backbone at the N-terminal "0" position, showed analytical purity and biological activity. Electron Paramagnetic Resonance (EPR) spectra of the N-terminal TOAC indicated rapid temperature-sensitive motion of the probe (< or =0.33 ns correlation time) on the flexible N-terminal segment. This N-terminal TOAC-NCp7 reported a RNA-NCp7 interaction at a 1:1 ratio of NCp7 to RNA, which caused the tumbling time to be slowed from about 0.3 ns to about 0.5 ns. NCp7 is a largely disordered protein that adapts to its RNA targets. However, as shown by circular dichroism, > or =90% trifluoroethanol [(TFE), an alpha-helix enhancer] caused the TOAC-NCp7 without zinc in its fingers to change to a fully helical conformation, while the TOAC spin label was concurrently reporting a tumbling time of well over a nanosecond, as the N-terminal TOAC became inflexibly enfolded. Even with TFE present, the existence of intact Zn-finger regions in NCp7 prevented complete formation of helical structure, as shown by circular dichroism, and decreased the N-terminal TOAC tumbling time, as shown by EPR. This study demonstrated TOAC at the N-terminal of NCp7 to be a reporter for the considerable conformational lability of NCp7. (

Cytoplasmic residues of phospholamban interact with membrane surfaces in the presence of SERCA: a new role for phospholipids in the regulation of cardiac calcium cycling?

The 52-amino acid transmembrane protein phospholamban (PLB) regulates calcium cycling in cardiac cells by forming a complex with the sarco(endo)plasmic reticulum calcium ATPase (SERCA) and reversibly diminishing the rate of calcium uptake by the sarcoplasmic reticulum. The N-terminal cytoplasmic domain of PLB interacts with the cytoplasmic domain of SERCA, but, in the absence of the enzyme, can also associate with the surface of anionic phospholipid membranes. This work investigates whether the cytoplasmic domain of PLB can also associate with membrane surfaces in the presence of SERCA, and whether such interactions could influence the regulation of the enzyme. It is shown using solid-state NMR and isothermal titration calorimetry (ITC) that an N-terminally acetylated peptide representing the first 23 N-terminal amino acids of PLB (PLB1-23) interacts with membranes composed of zwitterionic phosphatidylcholine (PC) and anionic phosphatidylglycerol (PG) lipids in the absence and presence of SERCA. Functional measurements of SERCA in sarcoplasmic reticulum (SR) vesicles, planar SR membranes and reconstituted into PC/PG membranes indicate that PLB1-23 lowers the maximal rate of ATP hydrolysis by acting at the cytoplasmic face of the enzyme. A small, but statistically significant, reduction in the inhibitory effect of the peptide is observed for SERCA reconstituted into PC/PG membranes compared to SERCA in membranes of PC alone. It is suggested that interactions between the cytoplasmic domain of PLB and negatively charged phospholipids might play a role in moderating the regulation of SERCA, with implications for cardiac muscle contractility.

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.

Spectroscopic validation of the pentameric structure of phospholamban

Phospholamban (PLN) regulates calcium translocation within cardiac myocytes by shifting sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) affinity for calcium. Although the monomeric form of PLN (6 kDa) is the principal inhibitory species, recent evidence suggests that the PLN pentamer (30 kDa) also is able to bind SERCA. To date, several membrane architectures of the pentamer have been proposed, with different topological orientations for the cytoplasmic domain: (i) extended from the bilayer normal by 50-60 degrees; (ii) continuous alpha-helix tilted 28 degrees relative to the bilayer normal; (iii) pinwheel geometry, with the cytoplasmic helix perpendicular to the bilayer normal and in contact with the surface of the bilayer; and (iv) bellflower structure, in which the cytoplasmic domain helix makes approximately 20 degrees angle with respect to the membrane bilayer normal. Using a variety of cell membrane mimicking systems (i.e., lipid vesicles, oriented lipid bilayers, and detergent micelles) and a combination of multidimensional solution/solid-state NMR and EPR spectroscopies, we tested the different structural models. We conclude that the pinwheel topology is the predominant conformation of pentameric PLN, with the cytoplasmic domain interacting with the membrane surface. We propose that the interaction with the bilayer precedes SERCA binding and may mediate the interactions with other proteins such as protein kinase A and protein phosphatase 1.

Synthesis of TOAC spin-labeled proteins and reconstitution in lipid membranes

A procedure is described for the synthetic incorporation into membrane proteins of the non-natural amino acid TOAC (2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxylic acid), which is coupled rigidly to the alpha-carbon, providing direct detection of peptide backbone dynamics by electron paramagnetic resonance (EPR). Also included is a protocol for the functional reconstitution of the spin-labeled protein in lipid vesicles. This protocol can be completed in 17 d.

Backbone dynamics determined by electron paramagnetic resonance to optimize solid-phase peptide synthesis of TOAC-labeled phospholamban

Electron paramagnetic resonance (EPR) was used to optimize the solid-phase peptide synthesis of a membrane-bound peptide labeled with TOAC (2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxylic acid). The incorporation of this paramagnetic amino acid results in a nitroxide spin label coupled rigidly to the alpha-carbon, providing direct detection of peptide backbone dynamics by EPR. We applied this approach to phospholamban, which regulates cardiac calcium transport. The synthesis of this amphipathic 52-amino-acid membrane peptide including TOAC is a challenge, especially in the addition of TOAC and the next several amino acids. Therefore, EPR of synthetic intermediates, reconstituted into lipid bilayers, was used to ensure complete coupling and 9-fluorenylmethoxycarbonyl (Fmoc) deprotection. The attachment of Fmoc-TOAC-OH leads to strong immobilization of the spin label, whereas Fmoc deprotection dramatically mobilizes it, producing an EPR spectral peak that is completely resolved from that observed before deprotection. Similarly, coupling of the next amino acid (Ser) restores the spin label to strong immobilization, giving a peak that is completely resolved from that of the preceding step. For several subsequent steps, the effect of coupling and deprotection is similar but less dramatic. Thus, the sensitivity and resolution of EPR provides a quantitative monitor of completion at each of these critical steps in peptide synthesis. Mass spectrometry, circular dichroism, and Edman degradation were used in concert with EPR to verify the chemistry and characterize the secondary structure. In conclusion, the application of conventional analytical methods in combination with EPR offers an improved approach to optimize the accurate synthesis of TOAC spin-labeled membrane peptides.

Phosphorylation-dependent Conformational Switch in Spin-labeled Phospholamban Bound to SERCA

We have used chemical synthesis, functional reconstitution, and electron paramagnetic resonance (EPR) to probe the functional dynamics of phospholamban (PLB), which regulates the Ca-ATPase (SERCA) in cardiac sarcoplasmic reticulum. The transmembrane domain of PLB inhibits SERCA at low [Ca(2+)], but the cytoplasmic domain relieves this inhibition upon Ser16 phosphorylation. Monomeric PLB was synthesized with Ala11 replaced by the 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) spin label, which reports peptide backbone dynamics directly. PLB was reconstituted into membranes in the presence or absence of SERCA. TOAC-PLB showed normal inhibitory function, which was reversed by phosphorylation at Ser16 or by micromolar [Ca(2+)]. EPR showed that the PLB cytoplasmic domain exhibits two resolved conformations, a tense T state that is ordered and a relaxed R state that is dynamically disordered and extended. PLB phosphorylation shifts this equilibrium toward the R state and makes it more dynamic (hyperextended). Phosphorylation strongly perturbs the dynamics of SERCA-bound PLB without dissociating the complex, while micromolar [Ca(2+)] has no effect on PLB dynamics. A lipid anchor synthetically attached to the N terminus of PLB permits Ca-dependent SERCA inhibition but prevents the phosphorylation-induced disordering and reversal of inhibition. We conclude that the relief of SERCA inhibition by PLB phosphorylation is due to an order-to-disorder transition in the cytoplasmic domain of PLB, which allows this domain to extend above the membrane surface and induce a structural change in the cytoplasmic domain of SERCA. This mechanism is distinct from the one that relieves PLB-dependent SERCA inhibition upon the addition of micromolar [Ca(2+)].

Effects of Ser16 phosphorylation on the allosteric transitions of phospholamban/Ca(2+)-ATPase complex

Phosphorylation by protein kinase A and dephosphorylation by protein phosphatase 1 modulate the inhibitory activity of phospholamban (PLN), the endogenous regulator of the sarco(endo)plasmic reticulum calcium Ca(2+) ATPase (SERCA). This cyclic mechanism constitutes the driving force for calcium reuptake from the cytoplasm into the myocite lumen, regulating cardiac contractility. PLN undergoes a conformational transition between a relaxed (R) and tense (T) state, an equilibrium perturbed by the addition of SERCA. Here, we show that the single phosphoryl transfer at Ser16 induces a more pronounced conformational switch to the R state in phosphorylated PLN (pPLN). The binding affinity of PLN to SERCA is not affected (K(d) values for the transmembrane domains of pPLN and PLN are approximately 60 microM), supporting the hypothesis that phosphorylation at Ser16 does not dissociate PLN from SERCA. However, the binding surface and dynamics in domain Ib (residues 22-31) change substantially upon phosphorylation. Since PLN can be singly or doubly phosphorylated at Ser16 and Thr17, we propose that these sites remotely control the conformation of domain Ib. These findings constitute a paradigm for how post-translational modifications such as phosphorylation in the cytoplasmic portion of membrane proteins control intramembrane protein-protein interactions.

Rapid, high-yield expression and purification of Ca2+-ATPase regulatory proteins for high-resolution structural studies

Phospholamban (PLB) and sarcolipin (SLN) are small integral membrane proteins that regulate the Ca(2+)-ATPases of cardiac and skeletal muscle, respectively, and directly alter their calcium transport properties. PLB interacts with and regulates the cardiac Ca(2+)-ATPase at submaximal calcium concentrations, thereby slowing relaxation rates and reducing contractility in the heart. SLN interacts with and regulates the skeletal muscle Ca(2+)-ATPase in a mechanism analogous to that used by PLB. While these regulatory interactions are biochemically and physiologically well characterized, structural details are lacking. To pursue structural studies, such as electron cryo-microscopy and X-ray crystallography, large quantities of over-expressed and purified protein are required. Herein, we report a modified method for producing large quantities of PLB and SLN in a rapid and efficient manner. Briefly, recombinant wild-type PLB and SLN were over-produced in Escherichia coli as maltose binding protein fusion proteins. A tobacco etch virus protease site allowed specific cleavage of the fusion protein and release of recombinant PLB or SLN. Selective solubilization with guanidine-hydrochloride followed by reverse-phase HPLC permitted the rapid, large-scale production of highly pure protein. Reconstitution and measurement of ATPase activity confirmed the functional interaction between our recombinant regulatory proteins and Ca(2+)-ATPase. The inhibitory properties of the over-produced proteins were consistent with previous studies, where the inhibition was relieved by elevated calcium concentrations. In addition, we show that our recombinant PLB and SLN are suitable for high-resolution structural studies.

The protein–lipid interface: perspectives from magnetic resonance and crystal structures

Lipid-protein interactions in membranes are dynamic, and consequently are well studied by magnetic resonance spectroscopy. More recently, lipids associated with integral membrane proteins have been resolved in crystals by X-ray diffraction, mostly at cryogenic temperatures. The conformation and chain ordering of lipids in crystals of integral proteins are reviewed here and are compared and contrasted with results from magnetic resonance and with the crystal structures of phospholipid bilayers. Various aspects of spin-label magnetic resonance studies on lipid interactions with single integral proteins are also reviewed: specificity for phosphatidylcholine, competition with local anaesthetics, oligomer formation of single transmembrane helices, and protein-linked lipid chains. Finally, the interactions between integral proteins and peripheral or lipid-linked proteins, as reflected by the lipid-protein interactions in double reconstitutions, are considered.

Phospholamban structural dynamics in lipid bilayers probed by a spin label rigidly coupled to the peptide backbone

We have used chemical synthesis and electron paramagnetic resonance to probe the structural dynamics of phospholamban (PLB) in lipid bilayers. Derivatives of monomeric PLB were synthesized, each of which contained a single spin-labeled 2,2,6,6,-Tetramethyl-piperidine-N-oxyl-4-amino-4-carboxylic acid amino acid, with the nitroxide-containing ring covalently and rigidly attached to the alpha-carbon, providing direct insight into the conformational dynamics of the peptide backbone. 2,2,6,6,-tetramethyl-piperidine-N-oxyl-4-amino-4-carboxylic acid was attached at positions 0, 11, and 24 in the cytoplasmic domain or at position 46 in the transmembrane domain. The electron paramagnetic resonance spectrum of the transmembrane domain site (position 46) indicates a single spectral component corresponding to strong immobilization of the probe, consistent with the presence of a stable and highly ordered transmembrane helix. In contrast, each of the three cytoplasmic domain probes has two clearly resolved spectral components (conformational states), one of which indicates nearly isotropic nanosecond dynamic disorder. For the probe at position 11, an N-terminal lipid anchor shifts the equilibrium toward the restricted component, whereas Mg(2+) shifts it in the opposite direction. Relaxation enhancement, due to Ni(2+) ions chelated to lipid head-groups, provides further information about the membrane topology of PLB, allowing us to confirm and refine a structural model based on previous NMR data. We conclude that the cytoplasmic domain of PLB is in a dynamic equilibrium between an ordered conformation, which is in direct contact with the membrane surface, and a dynamically disordered form, which is detached from the membrane and poised to interact with its regulatory target.

(1)H/(15)N heteronuclear NMR spectroscopy shows four dynamic domains for phospholamban reconstituted in dodecylphosphocholine micelles

We report the backbone dynamics of monomeric phospholamban in dodecylphosphocholine micelles using (1)H/(15)N heteronuclear NMR spectroscopy. Phospholamban is a 52-amino acid membrane protein that regulates Ca-ATPase in cardiac muscle. Phospholamban comprises three structural domains: a transmembrane domain from residues 22 to 52, a connecting loop from 17 to 21, and a cytoplasmic domain from 1 to 16 that is organized in an "L"-shaped structure where the transmembrane and the cytoplasmic domain form an angle of approximately 80 degrees (Zamoon et al., 2003; Mascioni et al., 2002). T(1), T(2), and (1)H/(15)N nuclear Overhauser effect values measured for the amide backbone resonances were interpreted using the model-free approach of Lipari and Szabo. The results point to the existence of four dynamic domains, revealing the overall plasticity of the cytoplasmic helix, the flexible loop, and part of the transmembrane domain (residues 22-30). In addition, using Carr-Purcell-Meiboom-Gill-based experiments, we have characterized phospholamban dynamics in the micros-ms timescale. We found that the majority of the residues in the cytoplasmic domain, the flexible loop, and the first ten residues of the transmembrane domain undergo dynamics in the micros-ms range, whereas minimal dynamics were detected for the transmembrane domain. Hydrogen/deuterium exchange factors measured at different temperatures support the existence of slow motion in both the loop and the cytoplasmic helix. We propose that these dynamic properties are critical factors in the biomolecular recognition of phospholamban by Ca-ATPase and other interacting proteins such as protein kinase A and protein phosphatase 1.

NMR solution structure and topological orientation of monomeric phospholamban in dodecylphosphocholine micelles

Phospholamban is an integral membrane protein that regulates the contractility of cardiac muscle by maintaining cardiomyocyte calcium homeostasis. Abnormalities in association of protein kinase A with PLB have recently been linked to human heart failure, where a single mutation is responsible for dilated cardiomyopathy. To date, a high-resolution structure of phospholamban in a lipid environment has been elusive. Here, we describe the first structure of recombinant, monomeric, biologically active phospholamban in lipid-mimicking dodecylphosphocholine micelles as determined by multidimensional NMR experiments. The overall structure of phospholamban is "L-shaped" with the hydrophobic domain approximately perpendicular to the cytoplasmic portion. This is in agreement with our previously published solid-state NMR data. In addition, there are two striking discrepancies between our structure and those reported previously for synthetic phospholamban in organic solvents: a), in our structure, the orientation of the cytoplasmic helix is consistent with the amphipathic nature of these residues; and b), within the hydrophobic helix, residues are positioned on two discrete faces of the helix as consistent with their functional roles ascribed by mutagenesis. This topology renders the two phosphorylation sites, Ser-16 and Thr-17, more accessible to kinases.

The affinity of GXXXG motifs in transmembrane helix-helix interactions is modulated by long-range communication

Sequence motifs are responsible for ensuring the proper assembly of transmembrane (TM) helices in the lipid bilayer. To understand the mechanism by which the affinity of a common TM-TM interactive motif is controlled at the sequence level, we compared two well characterized GXXXG motif-containing homodimers, those formed by human erythrocyte protein glycophorin A (GpA, high-affinity dimer) and those formed by bacteriophage M13 major coat protein (MCP, low affinity dimer). In both constructs, the GXXXG motif is necessary for TM-TM association. Although the remaining interfacial residues (underlined) in GpA (LIXXGVXXGVXXT) differ from those in MCP (VVXXGAXXGIXXF), molecular modeling performed here indicated that GpA and MCP dimers possess the same overall fold. Thus, we could introduce GpA interfacial residues, alone and in combination, into the MCP sequence to help decrypt the determinants of dimer affinity. Using both in vivo TOXCAT assays and SDS-PAGE gel migration rates of synthetic peptides derived from TM regions of the proteins, we found that the most distal interfacial sites, 12 residues apart (and approximately 18 A in structural space), work in concert to control TM-TM affinity synergistically.

Polar residue tagging of transmembrane peptides

Studies that focus on packing interactions between transmembrane (TM) helices in membrane proteins would greatly benefit from the ability to investigate their association and packing interactions in multi-spanning TM domains. However, the production, purification, and characterization of such units have been impeded by their high intrinsic hydrophobicity. We describe the polar tagging approach to biophysical analysis of TM segment peptides, where incorporation of polar residues of suitable type and number at one or both peptide N- and C-termini can serve to counterbalance the apolar nature of a native TM segment, and render it aqueous-soluble. Using the native TM sequences of the human erythrocyte protein glycophorin A (GpA) and bacteriophage M13 major coat protein (MCP), properties of tags such as Lys, His, Asp, sarcosine, and Pro-Gly are evaluated, and general procedures for tagging a given TM segment are presented. Gel-shift assays on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) establish that various tagged GpA TM segments spontaneously insert into micellar membranes, and exhibit native TM dimeric states. Sedimentation equilibrium analytical centrifugation is used to confirm that Lys-tagged GpA peptides retain the native dimer state. Two-dimensional nuclear magnetic resonance (NMR) spectroscopy studies on Lys-tagged TM MCP peptides selectively enriched with N-15 illustrate the usefulness of this system for evaluating monomer-dimer equilibria in micelle environments. The overall results suggest that polar-tagging of hydrophobic (TM) peptides approach constitutes a valuable tool for the study of protein-protein interactions in membranes.

Overexpression, purification, and characterization of recombinant Ca-ATPase regulators for high-resolution solution and solid-state NMR studies

Phospholamban (PLB) and Sarcolipin (SLN) are integral membrane proteins that regulate muscle contractility via direct interaction with the Ca-ATPase in cardiac and skeletal muscle, respectively. The molecular details of these protein-protein interactions are as yet undetermined. Solution and solid-state NMR spectroscopies have proven to be effective tools for deciphering such regulatory mechanisms to a high degree of resolution; however, large quantities of pure recombinant protein are required for these studies. Thus, recombinant PLB and SLN production in Escherichia coli was optimized for use in NMR experiments. Fusions of PLB and SLN to maltose binding protein (MBP) were constructed and optimal conditions for protein expression and purification were screened. This facilitated the large-scale production of highly pure protein. To confirm their functionality, the biological activities of recombinant PLB and SLN were compared to those of their synthetic counterparts. The regulation of Ca-ATPase activity by recombinant PLB and SLN was indistinguishable from the regulation by synthetic proteins, demonstrating the functional integrity of the recombinant constructs and ensuring the biological relevance of our future structural studies. Finally, NMR spectroscopic conditions were established and optimized for use in investigations of the mechanism of Ca-ATPase regulation by PLB and SLN.

Structure and function of integral membrane protein domains resolved by peptide-amphiphiles: application to phospholamban

We have used synthetic lipidated peptides ("peptide-amphiphiles") to study the structure and function of isolated domains of integral transmembrane proteins. We used 9-fluorenylmethyloxycarbonyl (Fmoc) solid-phase peptide synthesis to prepare full-length phospholamban (PLB(1-52)) and its cytoplasmic (PLB(1-25)K: phospholamban residues 1-25 plus a C-terminal lysine), and transmembrane (PLB(26-52)) domains, and a 38-residue model alpha-helical sequence as a control. We created peptide-amphiphiles by linking the C-terminus of either the isolated cytoplasmic domain or the model peptide to a membrane-anchoring, lipid-like hydrocarbon tail. Circular dichroism measurements showed that the model peptide-amphiphile, either in aqueous suspension or in lipid bilayers, had a higher degree of alpha-helical secondary structure than the unlipidated model peptide. We hypothesized that the peptide-amphiphile system would allow us to study the function and structure of the PLB(1-25)K cytoplasmic domain in a native-like configuration. We compared the function (inhibition of the Ca-ATPase in reconstituted membranes) and structure (via CD) of the PLB(1-25) amphiphile to that of PLB and its isolated transmembrane and cytoplasmic domains. Our results indicate that the cytoplasmic domain PLB(1-25)K has no effect on Ca-ATPase (calcium pump) activity, even when tethered to the membrane in a manner mimicking its native configuration, and that the transmembrane domain of PLB is sufficient for inhibition of the Ca-ATPase.

Solid-state NMR reveals structural changes in phospholamban accompanying the functional regulation of Ca2+-ATPase

Calcium transport across the sarcoplasmic reticulum of cardiac myocytes is regulated by a reversible inhibitory interaction between the Ca2+-ATPase and the small transmembrane protein phospholamban (PLB). A nullcysteine analogue of PLB, containing isotope labels in the transmembrane domain or cytoplasmic domain, was reconstituted into membranes in the absence and presence of the SERCA1 isoform of Ca2+-ATPase for structural investigation by cross-polarization magic-angle spinning (CP-MAS) NMR. PLB lowered the maximal hydrolytic activity of SERCA1 and its affinity for calcium in membrane preparations suitable for structural analysis by NMR. Novel backbone amide proton-deuterium exchange CP-MAS NMR experiments on the two PLB analogues co-reconstituted with SERCA1 indicated that labeled residues Leu42 and Leu44 were situated well within the membrane interior, whereas Pro21 and Ala24 lie exposed outside the membrane. Internuclear distance measurements on PLB using rotational resonance NMR indicated that the sequences Pro21-Ala24 and Leu42-Leu44 adopt an alpha-helical structure in pure lipid bilayers, which is unchanged in the presence of Ca2+-ATPase. By contrast, rotational echo double resonance (REDOR) NMR experiments revealed that the sequence Ala24-Gln26 switches from an alpha-helix in pure lipid membranes to a more extended structure in the presence of SERCA1, which may reflect local structural distortions which change the orientations of the transmembrane and cytoplasmic domains. These results suggest that Ca2+-ATPase has a long-range effect on the structure of PLB around residue 25, which promotes the functional association of the two proteins.

Solid-state NMR and rigid body molecular dynamics to determine domain orientations of monomeric phospholamban

Solid-state NMR spectroscopy, in conjunction with rigid body molecular dynamics calculations, shows that monomeric phospholamban in lipid bilayers has two distinct helical domains, with an interhelical angle within 60-100 degrees, ruling out the possibility of a continuous alpha-helical structure for this protein.