Structure of the 1-36 N-terminal fragment of human phospholamban phosphorylated at Ser-16 and Thr-17

NMR structures of membrane proteins in phospholipid bilayers

Membrane proteins have always presented technical challenges for structural studies because of their requirement for a lipid environment. Multiple approaches exist including X-ray crystallography and electron microscopy that can give significant insights into their structure and function. However, nuclear magnetic resonance (NMR) is unique in that it offers the possibility of determining the structures of unmodified membrane proteins in their native environment of phospholipid bilayers under physiological conditions. Furthermore, NMR enables the characterization of the structure and dynamics of backbone and side chain sites of the proteins alone and in complexes with both small molecules and other biopolymers. The learning curve has been steep for the field as most initial studies were performed under non-native environments using modified proteins until ultimately progress in both techniques and instrumentation led to the possibility of examining unmodified membrane proteins in phospholipid bilayers under physiological conditions. This review aims to provide an overview of the development and application of NMR to membrane proteins. It highlights some of the most significant structural milestones that have been reached by NMR spectroscopy of membrane proteins, especially those accomplished with the proteins in phospholipid bilayer environments where they function.

Structural dynamics and topology of phosphorylated phospholamban homopentamer reveal its role in the regulation of calcium transport

Phospholamban (PLN) inhibits the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), thereby regulating cardiac diastole. In membranes, PLN assembles into homopentamers that in both the phosphorylated and nonphosphorylated states have been proposed to form ion-selective channels. Here, we determined the structure of the phosphorylated pentamer using a combination of solution and solid-state nuclear magnetic resonance methods. We found that the pinwheel architecture of the homopentamer is preserved upon phosphorylation, with each monomer having an L-shaped conformation. The TM domains form a hydrophobic pore approximately 24 Å long and 2 Å in diameter, which is inconsistent with canonical Ca²⁺-selective channels. Phosphorylation, however, enhances the conformational dynamics of the cytoplasmic region of PLN, causing partial unwinding of the amphipathic helix. We propose that PLN oligomers act as storage for active monomers, keeping SERCA function within a physiological window.

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

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

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

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

Effects of PKA phosphorylation on the conformation of the Na,K-ATPase regulatory protein FXYD1

FXYD1 (phospholemman) is a member of an evolutionarily conserved family of membrane proteins that regulate the function of the Na,K-ATPase enzyme complex in specific tissues and specific physiological states. In heart and skeletal muscle sarcolemma, FXYD1 is also the principal substrate of hormone-regulated phosphorylation by c-AMP dependent protein kinase A and by protein kinase C, which phosphorylate the protein at conserved Ser residues in its cytoplasmic domain, altering its Na,K-ATPase regulatory activity. FXYD1 adopts an L-shaped alpha-helical structure with the transmembrane helix loosely connected to a cytoplasmic amphipathic helix that rests on the membrane surface. In this paper we describe NMR experiments showing that neither PKA phosphorylation at Ser68 nor the physiologically relevant phosphorylation mimicking mutation Ser68Asp induces major changes in the protein conformation. The results, viewed in light of a model of FXYD1 associated with the Na,K-ATPase alpha and beta subunits, indicate that the effects of phosphorylation on the Na,K-ATPase regulatory activity of FXYD1 could be due primarily to changes in electrostatic potential near the membrane surface and near the Na(+)/K(+) ion binding site of the Na,K-ATPase alpha subunit.

Structural constraints on the transmembrane and juxtamembrane regions of the phospholamban pentamer in membrane bilayers: Gln29 and Leu52

The Ca2+-ATPase of cardiac muscle cells transports Ca2+ ions against a concentration gradient into the sarcoplasmic reticulum and is regulated by phospholamban, a 52-residue integral membrane protein. It is known that phospholamban inhibits the Ca2+ pump during muscle contraction and that inhibition is removed by phosphorylation of the protein during muscle relaxation. Phospholamban forms a pentameric complex with a central pore. The solid-state magic angle spinning (MAS) NMR measurements presented here address the structure of the phospholamban pentamer in the region of Gln22-Gln29. Rotational echo double resonance (REDOR) NMR measurements show that the side chain amide groups of Gln29 are in close proximity, consistent with a hydrogen-bonded network within the central pore. 13C MAS NMR measurements are also presented on phospholamban that is 1-13C-labeled at Leu52, the last residue of the protein. pH titration of the C-terminal carboxyl group suggests that it forms a ring of negative charge on the lumenal side of the sarcoplasmic reticulum membrane. The structural constraints on the phospholamban pentamer described in this study are discussed in the context of a multifaceted mechanism for Ca2+ regulation that may involve phospholamban as both an inhibitor of the Ca2+ ATPase and as an ion channel.

Intrinsically Disordered Proteins in the Neurodegenerative Processes: Formation of Tau Protein Paired Helical Filaments and Their Analysis

1. Several intrinsically disordered proteins (IDPs) play principal role in the neurodegenerative processes of various types. Among them, alpha-synuclein is involved in Parkinson's disease, prion protein in transmissible spongiform encephalopathies, and tau protein in Alzheimer's disease (AD) and related tauopathies. Neuronal damage in AD is accompanied by the presence of tau protein fibrils composed of paired helical filaments (PHF). 2. Tau protein represents a typical IDP. IDPs do not exhibit any stable secondary structure in the free form, but they are able to fold after binding to targets and contain regions with large propensity to adopt a defined type of secondary structure. Binding-folding event at tau protein leading to PHF generation is believed to happen in the course of tauopathies. 3. Detailed molecular topology of PHF formation is unknown. There are evidences about the cross-beta structure in PHF core; however the precise arrangement of the tau polypeptide chain is unclear. In this review we summarize current attempts at in vitro PHF reconstruction and the development of methods for PHF structure determination. The emphasis is put on the monoclonal antibodies used as structural molecular probes for research on the role of IDPs in pathogenesis of neurodegenerative diseases.

Essential role for Pro21 in phospholamban for optimal inhibition of the Ca-ATPase

We have investigated the functional role of the flexible hinge region centered near the sequence TIEMP(21), which connects the N-terminal cytosolic and C-terminal membrane-spanning helical domains of phospholamban (PLB). Specifically, we ask if the conformation of this region is important to attain optimal inhibitory interactions with the Ca-ATPase. A genetically engineered PLB mutant was constructed in which Pro(21) was mutated to an alanine (P21A-PLB(C)); in this construct, all three transmembrane cysteines were substituted with alanines to stabilize the monomeric form of PLB, and a unique cysteine was introduced at position 24 near the hinge element (A24C), permitting the site-specific attachment of fluorescein-5-maleimide (FMal) to monitor structure changes. In agreement with prior measurements in cardiac SR microsomes, the calcium concentration associated with half-maximal activation (Ca(1/2)) of the Ca-ATPase, 290 +/- 10 nM, is shifted to 580 +/- 20 nM when co-reconstituted with PLB(C) (Pro21) as a result of a reduction in the cooperativity associated with the calcium-dependent structural transition. Kinetic simulations indicate that PLB(C) association with the Ca-ATPase results in a 75% reduction in the equilibrium constant associated with the formation of the second high-affinity calcium binding site. In comparison, there is a 43% reduction in KCa(1/2) upon reconstitution of the Ca-ATPase with P21A-PLB(C), which can be simulated by decreasing the equilibrium constant associated with the calcium-dependent structural activation by 50%. The diminished inhibitory action of P21A-PLB(C) is associated with alterations in the structure of the hinge element, as evidenced by the diminished solvent accessibility of FMal relative to the native structure. Likewise, increases in the alpha-helical content and decreases in the mobility of the carboxyl-terminal domain of P21A-PLB(C) are observed using circular dichroism and fluorescence spectroscopy. Collectively, these results indicate that the overall dimensions of the carboxyl-terminal domain of PLB are increased through a stabilization of secondary structural elements upon mutation in P21A-PLB(C) that result in a reduction in the ability of the amino-terminal cytosolic portion of PLB to productively inhibit the Ca-ATPase. Further, these results suggest that the unstructured characteristics of the flexible hinge region in PLB are critical for optimal inhibitory interactions with the Ca-ATPase and suggest its role as a conformational switch.

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 binds in a compact and ordered conformation to the Ca-ATPase

Mutagenesis and cross-linking measurements have identified specific contact interactions between the cytosolic and the transmembrane sequences of phospholamban (PLB) and the Ca-ATPase, and in conjunction with the high-resolution structures of PLB and the Ca-ATPase, have been used to construct models of the PLB-ATPase complex, which suggest that PLB adopts a more extended structure within this complex. To directly test these predictions, we have used fluorescence resonance energy transfer to measure the average conformation and heterogeneity between chromophores covalently bound to the transmembrane and cytosolic domains of PLB reconstituted in proteoliposomes. In the absence of the Ca-ATPase, the cytosolic domain of PLB assumes a wide range of structures relative to the transmembrane sequence, which can be described using a model involving a Gaussian distribution of distances with an average distance (Rav) of less than 21 A and a half-width (HW) of 36 A. This conformational heterogeneity of PLB is consistent with the 10 structures resolved by NMR for the C41F mutant of PLB in organic cosolvents. In contrast, PLB bound to the Ca-ATPase assumes a unique and highly ordered conformation, where Rav = 14.0 +/- 0.3 A and HW = 3.7 +/- 0.6 A. The small spatial separation between the bound chromophores on PLB is inconsistent with an extended conformation of bound PLB in current models. Thus, to satisfy known interaction sites of PLB and the Ca-ATPase, these findings suggest a reorientation of the nucleotide binding domain of the Ca-ATPase toward the bilayer surface to bring known PLB binding sites into close juxtaposition with residues near the amino-terminus of PLB. Induction of an altered conformation of the nucleotide binding domain of the Ca-ATPase by PLB binding is suggested to underlie the reduced calcium sensitivity associated with PLB inhibition of the pump.

Phosphorylation by cAMP-dependent protein kinase modulates the structural coupling between the transmembrane and cytosolic domains of phospholamban

We have used frequency-domain fluorescence spectroscopy to investigate the structural linkage between the transmembrane and cytosolic domains of the regulatory protein phospholamban (PLB). Using an engineered PLB having a single cysteine (Cys(24)) derivatized with the fluorophore N-(1-pyrenyl)maleimide (PMal), we have used fluorescence resonance energy transfer (FRET) to measure the average spatial separation and conformational heterogeneity between PMal bound to Cys(24) in the transmembrane domain and Tyr(6) in the cytosolic domain near the amino terminus of PLB. In these measurements, PMal serves as a FRET donor, and Tyr(6) serves as a FRET acceptor following its nitration by tetranitromethane. The native structure of PLB is retained following site-directed mutagenesis and chemical modification, as indicated by the ability of the derivatized PLB to fully regulate the Ca-ATPase following their co-reconstitution. To assess how phosphorylation modulates the structure of PLB itself, FRET measurements were made following reconstitution of PLB in membrane vesicles made from extracted sarcoplasmic reticulum membrane lipids. We find that the cytosolic domain of PLB assumes a wide range of conformations relative to the transmembrane sequence, consistent with other structural data indicating the presence of a flexible hinge region between the transmembrane and cytosolic domains of PLB. Phosphorylation of Ser(16) by PKA results in a 3 A decrease in the spatial separation between PMal at Cys(24) and nitroTyr(6) and an almost 2-fold decrease in conformational heterogeneity, suggesting a stabilization of the hinge region of PLB possibly through an electrostatic linkage between phosphoSer(16) and Arg(13) that promotes a coil-to-helix transition. This structural transition has the potential to function as a conformational switch, since inhibition of the Ca-ATPase requires disruption of the secondary structure of PLB in the vicinity of the hinge element to permit association with the nucleotide binding domain at a site located approximately 50 A above the membrane surface. Following phosphorylation, the stabilization of the helical content in the hinge domain will disrupt this inhibitory interaction by reducing the maximal dimension of the cytosolic domain of PLB. Thus, stabilization of the structure of PLB following phosphorylation of Ser(16) is part of a switching mechanism, which functions to alter binding interactions between PLB and the nucleotide binding domain of the Ca-ATPase that modulates enzyme inhibition.

High-yield expression of isotopically labeled peptides for use in NMR studies

Fusion protein constructs of the 56 amino acid globular protein GB-1 with various peptide sequences, coupled with the incorporation of a histidine tag for affinity purification, have generated high-yield fusion protein constructs. Methionine residues were inserted into the constructs to generate pure peptides following CNBr cleavage, yielding a system that is efficient and cost effective for isotopic labeling of peptides for NMR studies and other disciplines such as mass spectroscopy. Six peptides of varying sequences and hydrophobicities were expressed using this GB-1 fusion protein technique and produced soluble fusion protein constructs in all cases. The ability to easily express and purify recombinant peptides in high yields is applicable for biomedical research and has medicinal and pharmaceutical applications.

Phosphorylation induces a conformational transition near the lipid-water interface of phospholamban reconstituted with the Ca-ATPase

We have measured conformational changes of phospholamban (PLB) induced both by its interaction with the SR Ca-ATPase and by phosphorylation of Ser-16 by cAMP-dependent protein kinase (PKA) using an engineered PLB having a single cysteine (Cys-24) derivatized with the fluorophore 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (ANSmal). This modified mutant PLB is fully functional when co-reconstituted with the affinity-purified Ca-ATPase in liposomes. ANSmal emission properties and its solvent accessibility indicate that Cys-24 is in an aqueous environment outside the membrane. Fluorescence quenching and time-resolved anisotropy measurements of ANSmal-PLB demonstrate distinct structures for PLB in the free and Ca-ATPase-bound state. Both solvent exposure and probe motions of ANSmal are enhanced upon interaction of PLB with the Ca-ATPase. This conformational transition entails conversion of free PLB in a conformation which is insensitive to one which is sensitive to the phosphorylation state of PLB. Upon phosphorylation of Ca-ATPase-bound PLB, a decreased level of solvent exposure of ANSmal is observed, suggesting that the amino acid sequence of PLB near the lipid-water interface acts as a conformational switch in response to the phosphorylation of PLB. A longer correlation time, resolved by anisotropy measurements, corresponding to polypeptide chain fluctuations, is substantially restricted by interaction of PLB with the Ca-ATPase. This restriction is not reversed by phosphorylation of PLB, indicating that the region around Cys-24 near the lipid-water interface does not undergo dissociation from the Ca-ATPase. These results suggest that the phosphorylation by PKA induces a redistribution of PLB-Ca-ATPase protein contacts to relieve the inhibitory effect of PLB for the activation of calcium transport.