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

Diffusion behavior of peptide amphiphiles containing different numbers of alkyl tails at a hydrophobic solid-liquid interface: single molecule tracking investigation

Using the single molecule tracking technique, the diffusion behavior of peptide amphiphiles (PAs) with different numbers of alkyl tails at a hydrophobic solid-liquid interface has been investigated. The effect of the number of alkyl tails of PAs on molecular trajectories at the hydrophobic solid-liquid interface has been systematically studied. PA molecules display an intermittent motion consisting of immobilization and hopping processes, which has been well simulated by the continuous time random walk (CTRW) model. The results reveal that the hydrophobic interaction between the PAs and hydrophobic surface plays an important role in the diffusion behavior of PAs. Increasing the number of alkyl tails in PAs systematically reduces the mobility of PAs on the hydrophobic surface. Moreover, the diffusion behavior of PAs at the hydrophobic interface also shows pH dependence. A decrease in pH is beneficial to the motion of all PAs on the hydrophobic surface, which can be ascribed to the protonation of PAs in acidic solutions. Therefore, the hydrophobic interaction is crucial to the transport of peptide amphiphiles at hydrophobic interfaces which would be important for the design of peptides in biological applications.

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.

Targeting protein-protein interactions for therapeutic discovery via FRET-based high-throughput screening in living cells

We have developed a structure-based high-throughput screening (HTS) method, using time-resolved fluorescence resonance energy transfer (TR-FRET) that is sensitive to protein-protein interactions in living cells. The membrane protein complex between the cardiac sarcoplasmic reticulum Ca-ATPase (SERCA2a) and phospholamban (PLB), its Ca-dependent regulator, is a validated therapeutic target for reversing cardiac contractile dysfunction caused by aberrant calcium handling. However, efforts to develop compounds with SERCA2a-PLB specificity have yet to yield an effective drug. We co-expressed GFP-SERCA2a (donor) in the endoplasmic reticulum membrane of HEK293 cells with RFP-PLB (acceptor), and measured FRET using a fluorescence lifetime microplate reader. We screened a small-molecule library and identified 21 compounds (Hits) that changed FRET by >3SD. 10 of these Hits reproducibly alter SERCA2a-PLB structure and function. One compound increases SERCA2a calcium affinity in cardiac membranes but not in skeletal, suggesting that the compound is acting specifically on the SERCA2a-PLB complex, as needed for a drug to mitigate deficient calcium transport in heart failure. The excellent assay quality and correlation between structural and functional assays validate this method for large-scale HTS campaigns. This approach offers a powerful pathway to drug discovery for a wide range of protein-protein interaction targets that were previously considered “undruggable”.

Structural dynamics of calmodulin-ryanodine receptor interactions: electron paramagnetic resonance using stereospecific spin labels

We have used electron paramagnetic resonance, with rigid and stereospecific spin labels, to resolve structural states in calmodulin (CaM), as affected by binding of Ca and a CaM-binding peptide (RyRp) derived from the ryanodine receptor (RyR), the Ca channel that triggers muscle contraction. CaM mutants containing a pair of cysteines in the N-lobe and/or C-lobe were engineered and labeled with a stereospecifically bound bifunctional spin label (BSL). RyRp was synthesized with and without TOAC (a stereospecifically attached spin-labeled amino acid) substituted for a single amino acid near the N-terminus. Intramolecular DEER distance measurements of doubly-labeled BSL-CaM revealed that CaM exists in dynamic equilibrium among multiple states, consistent with open, closed, and compact structural models. Addition of RyRp shifted the equilibrium partially toward the compact state in the absence of Ca, and completely toward the compact state in the presence of Ca, supporting a conformational selection model. Inter-protein distance measurements show that Ca stabilizes the compact state primarily by inducing ordered binding of the CaM N-lobe to RyRp, while only slightly affecting the C-lobe. The results provide insight into the structural mechanism of CaM-mediated RyR regulation, while demonstrating the power of using two types of rigidly and stereospecifically bound spin labels.

Calcium-Dependent Structural Dynamics of a Spin-Labeled RyR Peptide Bound to Calmodulin

We have used chemical synthesis, electron paramagnetic resonance (EPR), and circular dichroism to detect and analyze the structural dynamics of a ryanodine receptor (RyR) peptide bound to calmodulin (CaM). The skeletal muscle calcium release channel RyR1 is activated by Ca²⁺-free CaM and inhibited by Ca²⁺-bound CaM. To probe the structural mechanism for this regulation, wild-type RyRp and four spin-labeled derivatives were synthesized, each containing the nitroxide probe 2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxylic acid substituted for a single amino acid. In 2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxylic acid, the probe is rigidly and stereospecifically coupled to the α-carbon, enabling direct detection by EPR of peptide backbone structural dynamics. In the absence of CaM, circular dichroism indicates a complete lack of secondary structure, while 40% trifluoroethanol (TFE) induces >90% helicity and is unperturbed by the spin label. The EPR spectrum of each spin-labeled peptide indicates nanosecond dynamic disorder that is substantially reduced by TFE, but a significant gradient in dynamics is observed, decreasing from N- to C-terminus, both in the presence and absence of TFE. When bound to CaM, the probe nearest RyRp's N-terminus shows rapid rotational motion consistent with peptide backbone dynamics of a locally unfolded peptide, while the other three sites show substantial restriction of dynamics, consistent with helical folding. The two N-terminal sites, which bind to the C-lobe of CaM, do not show a significant Ca²⁺-dependence in mobility, while both C-terminal sites, which bind to the N-lobe of CaM, are significantly less mobile in the presence of bound Ca²⁺. These results support a model in which the interaction of RyR with CaM is nonuniform along the peptide, and the primary effect of Ca²⁺ is to increase the interaction of the C-terminal portion of the peptide with the N-terminal lobe of CaM. These results provide, to our knowledge, new insight into the Ca²⁺-dependent regulation of RyR by CaM.

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.

Self-assembled peptide amphiphiles function as multivalent binder with increased hemagglutinin affinity

BACKGROUND

A promising way in diagnostic and therapeutic applications is the development of peptide amphiphiles (PAs). Peptides with a palmitic acid alkylchain were designed and characterized to study the effect of the structure modifications on self-assembling capabilities and the multiple binding capacity to hemagglutinin (HA), the surface protein of influenza virus type A. The peptide amphiphiles consists of a hydrophilic headgroup with a biological functionality of the peptide sequence and a chemically conjugated hydrophobic tail. In solution they self-assemble easily to micelles with a hydrophobic core surrounded by a closely packed peptide-shell.

RESULTS

In this study the effect of a multiple peptide binding partner to the receptor binding site of HA could be determined with surface plasmon resonance measurements. The applied modification of the peptides causes signal amplification in relationship to the unmodified peptide wherein the high constant specificity persists. The molecular assembly of the peptides was characterized by the determination of critical micelle concentration (CMC) with concentration of 10⁻⁵ M and the colloidal size distribution.

CONCLUSION

The modification of the physico-chemical parameters by producing peptide amphiphiles form monomeric structures which enhances the binding affinity and allows a better examination of the interaction with the virus surface protein hemagglutinin.

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.

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.

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.

cAMP-dependent protein kinase A selects the excited state of the membrane substrate phospholamban

Phosphorylation of membrane proteins is a central regulatory and signaling mechanism across cell compartments. However, the recognition process and phosphorylation mechanism of membrane-bound substrates by kinases are virtually unknown. cAMP-dependent protein kinase A (PKA) is a ubiquitous enzyme that phosphorylates several soluble and membrane-bound substrates. In cardiomyocytes, PKA targets phospholamban (PLN), a membrane protein that inhibits the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA). In the unphosphorylated state, PLN binds SERCA, reducing the calcium uptake and generating muscle contraction. PKA phosphorylation of PLN at S16 in the cytoplasmic helix relieves SERCA inhibition, initiating muscle relaxation. Using steady-state kinetic assays, NMR spectroscopy, and molecular modeling, we show that PKA recognizes and phosphorylates the excited, membrane-detached R-state of PLN. By promoting PLN from a ground state to an excited state, we obtained a linear relationship between rate of phosphorylation and population of the excited state of PLN. The conformational equilibrium of PLN is crucial to regulate the extent of PLN phosphorylation and SERCA inhibition.

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.

(15)N Solid-state NMR spectroscopic studies on phospholamban at its phosphorylated form at ser-16 in aligned phospholipid bilayers

Wild-type phospholamban (WT-PLB) is a pentameric transmembrane protein that regulates the cardiac cycle (contraction and relaxation). From a physiological prospective, unphosphorylated WT-PLB inhibits sarcoplasmic reticulum ATPase activity; whereas, its phosphorylated form relieves the inhibition in a mechanism that is not completely understood. In this study, site-specifically (15)N-Ala-11- and (15)N-Leu-7-labeled WT-PLB and the corresponding phosphorylated forms (P-PLB) were incorporated into 1,2-dioleoyl-sn-glycero-3-phosphocholine/2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPC/DOPE) mechanically oriented lipid bilayers. The aligned (15)N-labeled Ala-11 and Leu-7 WT-PLB samples show (15)N resonance peaks at approximately 71ppm and 75ppm, respectively, while the corresponding phosphorylated forms P-PLB show (15)N peaks at 92ppm and 99ppm, respectively. These (15)N chemical shift changes upon phosphorylation are significant and in agreement with previous reports, which indicate that phosphorylation of WT-PLB at Ser-16 alters the structural properties of the cytoplasmic domain with respect to the 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.

Side chain and backbone dynamics of phospholamban in phospholipid bilayers utilizing 2H and 15N solid-state NMR spectroscopy

2H and 15N solid-state NMR spectroscopic techniques were used to investigate both the side chain and backbone dynamics of wild-type phospholamban (WT-PLB) and its phosphorylated form (P-PLB) incorporated into 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine (POPC) phospholipid bilayers. 2H NMR spectra of site-specific CD3-labeled WT-PLB (at Leu51, Ala24, and Ala15) in POPC bilayers were similar under frozen conditions (-25 degrees C). However, significant differences in the line shapes of the 2H NMR spectra were observed in the liquid crystalline phase at and above 0 degrees C. The 2H NMR spectra indicate that Leu51, located toward the lower end of the transmembrane (TM) helix, shows restricted side chain motion, implying that it is embedded inside the POPC lipid bilayer. Additionally, the line shape of the 2H NMR spectrum of CD3-Ala24 reveals more side chain dynamics, indicating that this residue (located in the upper end of the TM helix) has additional backbone and internal side chain motions. 2H NMR spectra of both WT-PLB and P-PLB with CD3-Ala15 exhibit strong isotropic spectral line shapes. The dynamic isotropic nature of the 2H peak can be attributed to side chain and backbone motions to residues located in an aqueous environment outside the membrane. Also, the spectra of 15N-labeled amide WT-PLB at Leu51 and Leu42 residues showed only a single powder pattern component indicating that these two 15N-labeled residues located in the TM helix are motionally restricted at 25 degrees C. Conversely, 15N-labeled amide WT-PLB at Ala11 located in the cytoplasmic domain showed both powder and isotropic components at 25 degrees C. Upon phosphorylation, the mobile component contribution increases at Ala11. The 2H and 15N NMR data indicate significant backbone motion for the cytoplasmic domain of WT-PLB when compared to the transmembrane section.

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.

Self-assembly and applications of biomimetic and bioactive peptide-amphiphiles

Peptide-amphiphiles are amphiphilic structures with a hydrophilic peptide headgroup that incorporates a bioactive sequence and has the potential to form distinct structures, and a hydrophobic tail that serves to align the headgroup, drive self-assembly, and induce secondary and tertiary conformations. In this paper we review the different self-assembled structures of peptide-amphiphiles that range from micelles and nanofibers, to patterned membranes. We also describe several examples where peptide-amphiphiles have found applications as soft bioactive materials for model studies of bioadhesion and characterization of different cellular phenomena, as well as scaffolds for tissue engineering, regenerative medicine, and targeted drug delivery.

Phospholamban and its phosphorylated form interact differently with lipid bilayers: a 31P, 2H, and 13C solid-state NMR spectroscopic study

Phospholamban (PLB) is a 52-amino acid integral membrane protein that helps to regulate the flow of Ca(2+) ions in cardiac muscle cells. Recent structural studies on the PLB pentamer and the functionally active monomer (AFA-PLB) debate whether its cytoplasmic domain, in either the phosphorylated or dephosphorylated states, is alpha-helical in structure as well as whether it associates with the lipid head groups (Oxenoid, K. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10870-10875; Karim, C. B. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 14437-14442; Andronesi, C.A. (2005) J. Am. Chem. Soc. 127, 12965-12974; Li, J. (2003) Biochemistry 42, 10674-10682; Metcalfe, E. E. (2005) Biochemistry 44, 4386-4396: Clayton, J. C. (2005) Biochemistry 44, 17016-17026). Comparing the secondary structure of the PLB pentamer and its phosphorylated form (P-PLB) as well as their interaction with the lipid bilayer is crucial in order to understand its regulatory function. Therefore, in this study, the full-length wild-type (WT) PLB and P-PLB were incorporated into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) phospholipid bilayers and studied utilizing solid-state NMR spectroscopy. The analysis of the (2)H and (31)P solid-state NMR data of PLB and P-PLB in POPC multilamellar vesicles (MLVs) indicates that a direct interaction takes place between both proteins and the phospholipid head groups. However, the interaction of P-PLB with POPC bilayers was less significant compared that with PLB. Moreover, the secondary structure using (13)C=O site-specific isotopically labeled Ala15-PLB and Ala15-P-PLB in POPC bilayers suggests that this residue, located in the cytoplasmic domain, is a part of an alpha-helical structure for both PLB and P-PLB.

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+)].

Mapping the interaction surface of a membrane protein: unveiling the conformational switch of phospholamban in calcium pump regulation

We have used magnetic resonance to map the interaction surface of an integral membrane protein for its regulatory target, an integral membrane enzyme. Phospholamban (PLN) regulates cardiac contractility via its modulation of sarco(endo)plasmic reticulum calcium ATPase (SERCA) activity. Impairment of this regulatory process causes heart failure. To map the molecular details of the PLN/SERCA interaction, we have functionally reconstituted SERCA with labeled PLN in dodecylphosphocholine micelles for high-resolution NMR spectroscopy and in both micelles and lipid bilayers for EPR spectroscopy. Differential perturbations in NMR linewidths and chemical shifts, measured as a function of position in the PLN sequence, provide a vivid picture of extensive SERCA contacts in both cytoplasmic and transmembrane domains of PLN and provide structural insight into previously reported functional mutagenesis data. NMR and EPR data show clear and complementary evidence for a dynamic (micros-to-ms) equilibrium between two conformational states in the cytoplasmic domain of PLN. These results support the hypothesis that SERCA attracts the cytoplasmic domain of PLN away from the lipid surface, shifting the preexisting equilibrium of PLN conformers toward a structure that is poised to interact with the regulatory target. EPR shows that this conformational switch behaves similarly in micelles and lipid membranes. Based on structural and dynamics data, we propose a model in which PLN undergoes allosteric activation upon encountering SERCA.

The alpha-helical propensity of the cytoplasmic domain of phospholamban: a molecular dynamics simulation of the effect of phosphorylation and mutation

We have used molecular dynamics simulations to investigate the effect of phosphorylation and mutation on the cytoplasmic domain of phospholamban (PLB), a 52-residue protein that regulates the calcium pump in cardiac muscle. Simulations were carried out in explicit water systems at 300 K for three peptides spanning the first 25 residues of PLB: wild-type (PLB(1-25)), PLB(1-25) phosphorylated at Ser16 and PLB(1-25) with the R9C mutation, which is known to cause human heart disease. The unphosphorylated peptide maintains a helical conformation from 3 to 15 throughout a 26-ns simulation, in agreement with spectroscopic data. Comparison with simulations of a fourth peptide truncated at Pro21 showed the importance of the region from 17 to 21 in preventing local unfolding of the helix. The results suggest that residues 11-16 are more likely to unfold when specific capping motifs are not present. It is proposed that protein kinase A exploits the intrinsic flexibility of the 11-21 region when binding PLB. In agreement with available CD and NMR data, the simulations show a decrease in the helical content upon phosphorylation. The phosphorylated peptide is characterized by helix spanning residues 3-11, followed by a turn that optimizes the salt-bridge interaction between the side chains of the phosphorylated Ser-16 and Arg-13. Replacing Arg-9 with Cys results in unfolding of the helix from C9 and an overall decrease of the helical conformation. The simulations show that initiation of unfolding is due to increased solvent accessibility of the backbone atoms near the smaller Cys. It is proposed that the loss of inhibitory potency upon Ser-16 phosphorylation or R9C mutation of PLB is due to a similar mechanism, in which the partial unfolding of the cytoplasmic helix of PLB results in a conformation that interacts with the cytoplasmic domain of the calcium pump to relieve its inhibition.

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.

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.