Development of α-helical calpain probes by mimicking a natural protein-protein interaction

We have designed a highly specific inhibitor of calpain by mimicking a natural protein-protein interaction between calpain and its endogenous inhibitor calpastatin. To enable this goal we established a new method of stabilizing an α-helix in a small peptide by screening 24 commercially available cross-linkers for successful cysteine alkylation in a model peptide sequence. The effects of cross-linking on the α-helicity of selected peptides were examined by CD and NMR spectroscopy, and revealed structurally rigid cross-linkers to be the best at stabilizing α-helices. We applied this strategy to the design of inhibitors of calpain that are based on calpastatin, an intrinsically unstable polypeptide that becomes structured upon binding to the enzyme. A two-turn α-helix that binds proximal to the active site cleft was stabilized, resulting in a potent and selective inhibitor for calpain. We further expanded the utility of this inhibitor by developing irreversible calpain family activity-based probes (ABPs), which retained the specificity of the stabilized helical inhibitor. We believe the inhibitor and ABPs will be useful for future investigation of calpains, while the cross-linking technique will enable exploration of other protein-protein interactions.

Matrix Gla protein inhibits ectopic calcification by a direct interaction with hydroxyapatite crystals

Mice lacking the gene encoding matrix gla protein (MGP) exhibit massive mineral deposition in blood vessels and die soon after birth. We hypothesize that MGP prevents arterial calcification by adsorbing to growing hydroxyapatite (HA) crystals. To test this, we have used a combined experimental-computational approach. We synthesized peptides covering the entire sequence of human MGP, which contains three sites of serine phosphorylation and five sites of γ-carboxylation, and studied their effects on HA crystal growth using a constant-composition autotitration assay. In parallel studies, the interactions of these sequences with the {100} and {001} faces of HA were analyzed using atomistic molecular dynamics (MD) simulations. YGlapS (amino acids 1-14 of human MGP) and SK-Gla (MGP43-56) adsorbed rapidly to the {100} and {001} faces and strongly inhibited HA growth (IC(50) = 2.96 μg/mL and 4.96 μg/mL, respectively). QR-Gla (MGP29-42) adsorbed more slowly and was a moderate growth inhibitor, while the remaining three (nonpost-translationally modified) peptides had little or no effect in either analysis. Substitution of gla with glutamic acid reduced the adsorption and inhibition activities of SK-Gla and (to a lesser extent) QR-Gla but not YGlapS; substitution of phosphoserine with serine reduced the inhibitory potency of YGlapS. These studies suggest that MGP prevents arterial calcification by a direct interaction with HA crystals that involves both phosphate groups and gla residues of the protein. The strong correlation between simulated adsorption and measured growth inhibition indicates that MD provides a powerful tool to predict the effects of proteins and peptides on crystal formation.

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.

Rational Design of Peptide Inhibitors of the Sarcoplasmic Reticulum Calcium Pump†

The sequence of phospholamban (PLB) is practically invariant among mammalian species. The hydrophobic transmembrane domain has 10 leucine and 8 isoleucine residues. Two roles have been proposed for the leucines; one subset stabilizes PLB oligomers, while a second subset physically interacts with SERCA. On the basis of the sequence of the PLB transmembrane domain, we chemically synthesized a series of peptides and tested their ability to regulate SERCA in reconstituted membranes. In all, eight peptides were studied: a peptide corresponding to the null-cysteine transmembrane domain of PLB (TM-Ala-PLB), two polyleucine peptides (Leu18 and Leu24), polyalanine peptides containing 4, 7, and 12 leucine residues (Leu4, Leu7, and Leu12, respectively), and a polyalanine peptide containing the 9 leucine residues present in the transmembrane domain of PLB with and without the essential Asn34 residue (Asn1Leu9 and Leu9, respectively). With the exception of Leu18, co-reconstitution of the peptides revealed effects on the apparent calcium affinity of SERCA. The TM-Ala-PLB peptide possessed approximately 70% of the inhibitory function of wild-type PLB. The remaining peptides exhibited significant inhibitory activity decreasing in the following order: Leu12, Leu9, Leu24, Leu7, and Leu4. Replacing Asn34 of PLB in the Leu9 peptide resulted in superinhibition of SERCA. On the basis of these observations, we conclude that a partial requirement for SERCA inhibition is met by a simple hydrophobic surface on a transmembrane alpha-helix. In addition, the superinhibition observed for the Asn34-containing peptide suggests that the model peptides mimic the inhibitory properties of PLB. A model is presented in which surface complementarity around key amino acid positions is enhanced in the interaction with SERCA.

Total Chemical Synthesis of Human Psoriasin by Native Chemical Ligation†

Human psoriasin (S100A7), a member of the S100 family of calcium-binding proteins, is richly expressed in keratinocytes of patients suffering from psoriasis. To date, the exact physiological function of psoriasin abundant in many human cell types remains unclear. A recent report by Schröder and colleagues suggests that psoriasin, purified from human stratum corneum extracts, selectively kills Escherichia coli by sequestering Zn(2+) ions essential for bacterial growth, indicative of an important role in innate immune defense against microbial infection. We chemically synthesized the N-terminally acetylated psoriasin of 100 amino acid residues using solid phase peptide synthesis in combination with native chemical ligation. More than 140 mg of highly pure and correctly folded synthetic psoriasin was obtained from a single synthesis on a 0.25 mmol scale. Analysis of synthetic psoriasin by size exclusion chromatography showed that the protein forms a homodimer in solution. Circular dichroism analysis indicated that the alpha-helicity of psoriasin increases by more than 20% in the presence of CaCl(2) or ZnCl(2), suggesting a metal ion binding induced conformational change. Circular dichroism based titration further established that the synthetic protein binds two Ca(2+) and two Zn(2+) ions per dimer, in agreement with the published structural findings. Importantly, the ability of the synthetic protein to kill E. coli and the inhibition of the killing by ZnCl(2) is comparable to that of psoriasin isolated from its natural source. The robust synthetic access to large quantities of human psoriasin should facilitate studies of its biological functions as well as its mode of action.

The effects of Ca2+ binding on the dynamic properties of a designed Ca2+-binding protein

The effects of Ca(2+) binding on the dynamic properties of Ca(2+)-binding proteins are important in Ca(2+) signaling. To understand the role of Ca(2+) binding, we have successfully designed a Ca(2+)-binding site in the domain 1 of rat CD2 (denoted as Ca.CD2) with the desired structure and retained function. In this study, the backbone dynamic properties of Ca.CD2 have been investigated using (15)N spin relaxation NMR spectroscopy to reveal the effect of Ca(2+) binding on the global and local dynamic properties without the complications of multiple interactive Ca(2+) binding and global conformational change. Like rat CD2 (rCD2) and human CD2 (hCD2), residues involved in the recognition of the target molecule CD48 exhibit high flexibility. Mutations N15D and N17D that introduce the Ca(2+) ligands increase the flexibility of the neighboring residues. Ca(2+)-induced local dynamic changes occur mainly at the residues proximate to the Ca(2+)-binding pocket or the residues in loop regions. The beta-strand B of Ca.CD2 that provides two Asp for the Ca(2+) undergoes an S(2) decrease upon the Ca(2+) binding, while the DE-loop that provides one Asn and one Asp undergoes an S(2) increase. Our study suggests that Ca(2+) binding has a differential effect on the rigidity of the residues depending on their flexibility and location within the secondary structure.

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.

Protein Dissection Experiments Reveal Key Differences in the Equilibrium Folding of α-Lactalbumin and the Calcium Binding Lysozymes

The alpha-lactalbumins and c-type lysozymes have virtually identical structure but exhibit very different folding behavior. All alpha-lactalbumins form a well populated molten globule state, while most of the lysozymes do not. alpha-Lactalbumin consists of two subdomains, and the alpha-subdomain is considerably more structured in the molten globule state than the beta-subdomain. Constructs derived from the alpha-subdomain of human alpha-lactalbumin containing the A, B, D, and 3(10) helices are known to form a molten globule state in the absence of the rest of the protein (Demarest, S. et al. (1999) J. Mol. Biol. 294, 213-221). Here we reported comparative studies of constructs derived from the same regions of canine and equine lysozymes. These proteins form two of the most stable molten globule states among all the lysozymes. A construct containing the A, B, D, and 3(10) helices of equine lysozyme is partially helical but is less structured than the corresponding human alpha-lactalbumin peptide. Addition of the C-helix leads to a construct that is still less structured and less stable than the alpha-lactalbumin construct. The corresponding construct from canine lysozyme is also less structured and less stable than the alpha-lactalbumin peptide. Thus, molten globule formation in human alpha-lactalbumin can be driven by the isolated alpha-subdomain, while more extensive interactions are required to generate a stable molten globule in the two lysozymes. The stability of the canine and equine lysozyme constructs is similar, indicating that the extraordinary stability of the canine lysozyme molten globule is not due to an unusually stable isolated alpha-subdomain.

The beta- and gamma-CH2 of B27-WT's Leu11 and Ile18 side chains play a direct role in calpain inhibition

Uncontrolled activation of calpain has been linked to tissue damage after neuronal and cardiac ischemias, traumatic spine and brain injuries, and multiple sclerosis and Alzheimer's disease. In vivo, the activity of calpain is regulated by its endogenous inhibitor calpastatin. The pathological role of calpain has been attributed to an imbalance between the activities of the protease and its inhibitor. Thus, it is possible that by reimposing functional control on the protease, the progression of calpain-mediated diseases could be slowed or eliminated. B27-WT is a 27-residue peptide (DPMSSTYIEELGKREVTIPPKYRELLA) derived from calpastatin that was previously shown to be a potent inhibitor of mu- and m-calpain. Recently, we identified two hot spots (Leu(11)-Gly(12) and Thr(17)-Ile(18)-Pro(19)) within which the amino acid residues that are key to B27-WT's bioactivity are clustered. In the work described here, the most critical residues of B27-WT, Leu(11) and Ile(18), were further probed to determine the nature of their interaction with calpain. Our results demonstrate that the side chains of both residues interact with hydrophobic pockets in calpain and that each of these interactions is indispensable for effective inhibition of calpain. Direct interactions involving the beta- and gamma-CH(2)- of the Leu(11) and Ile(18) side chains, respectively, rather than the degree of side chain branching or hydrophobicity, seemed to play a significant role in the peptide's ability to inhibit calpain. Furthermore, the minimum peptide sequence that still retained the calpain-inhibitory potency of B27-WT was found to be MSSTYIEELGKREVTIPPKYRELL.

Electron Paramagnetic Resonance Reveals a Large-Scale Conformational Change in the Cytoplasmic Domain of Phospholamban upon Binding to the Sarcoplasmic Reticulum Ca-ATPase

We used EPR spectroscopy to probe directly the interaction between phospholamban (PLB) and its regulatory target, the sarcoplasmic reticulum Ca-ATPase (SERCA). Synthetic monomeric PLB was prepared with a single cytoplasmic cysteine at residue 11, which was then spin labeled. PLB was reconstituted into membranes in the presence or absence of SERCA, and spin label mobility and accessibility were measured. The spin label was quite rotationally mobile in the absence of SERCA, but became more restricted in the presence of SERCA. SERCA also decreased the dependence of spin label mobility on PLB concentration in the membrane, indicating that SERCA reduces PLB-PLB interactions. The spin label MTSSL, attached to Cys11 on PLB by a disulfide bond, was stable at position 11 in the absence of SERCA. In the presence of SERCA, the spin label was released and a covalent bond was formed between PLB and SERCA, indicating direct interaction of one or more SERCA cysteine residues with Cys11 on PLB. The accessibility of the PLB-bound spin label IPSL to paramagnetic agents, localized in different phases of the membrane, indicates that SERCA greatly reduces the level of interaction of the spin label with the membrane surface. We propose that the cytoplasmic domain of PLB associates with the lipid surface, and that association with SERCA induces a major conformational change in PLB in which the cytoplasmic domain is drawn away from the lipid surface by SERCA.

Poly-arginine-fused calpastatin peptide, a living cell membrane-permeable and specific inhibitor for calpain

Calpain, a Ca2+-dependent neutral protease, is highly related to the pathogenesis of a variety of disorders and its inhibitors offer potential for therapeutic intervention. General calpain inhibitors, however, have the disadvantage of a lack of specificity or poor cellular permeability or oxidization under physiological conditions. Here, we developed a membrane-permeable specific calpain inhibitor by fusing calpastatin peptide (CS) and 11 poly-arginine peptides (11R). The 11R-fused CS (11R-CS) effectively penetrated across the plasma membrane of living neurons and significantly inhibited calpain activity in the cells.

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.

Rational design of a calcium-binding protein

Calcium ions play key roles as structural components in biomineralization and as a second messenger in signaling pathways. We have introduced a de novo designed calcium-binding site into the framework of a non-calcium-binding protein, domain 1 of CD2. The resulting protein selectively binds calcium over magnesium with calcium-binding affinity comparable to that of natural extracellular calcium-binding proteins (K(d) of 50 microM). This experiment is the first successful metalloprotein design that has a high coordination number (seven) metal-binding site constructed into a beta-sheet protein. Our results demonstrate the feasibility of designing a single calcium-binding site into a host protein, taking into account only local properties of a calcium-binding site obtained by a survey of natural calcium-binding proteins and chelators. The resulting site exhibits strong metal selectivity, suggesting that it should now be feasible to understand and manipulate signaling processes by designing novel calcium-modulated proteins with specifically desired functions and to affect their stability.

Helical structure of phospholamban in membrane bilayers

The regulation of calcium levels across the membrane of the sarcoplasmic reticulum involves the complex interplay of several membrane proteins. Phospholamban is a 52 residue integral membrane protein that is involved in reversibly inhibiting the Ca(2+) pump and regulating the flow of Ca ions across the sarcoplasmic reticulum membrane during muscle contraction and relaxation. The structure of phospholamban is central to its regulatory role. Using homonuclear rotational resonance NMR methods, we show that the internuclear distances between [1-(13)C]Leu7 and [3-(13)C]Ala11 in the cytoplasmic region, between [1-(13)C]Pro21 and [3-(13)C]Ala24 in the juxtamembrane region and between [1-(13)C]Leu42 and [3-(13)C]Cys46 in the transmembrane domain of phospholamban are consistent with alpha-helical secondary structure. Additional heteronuclear rotational-echo double-resonance NMR measurements confirm that the secondary structure is helical in the region of Pro21 and that there are no large conformational changes upon phosphorylation. These results support the model of the phospholamban pentamer as a bundle of five long alpha-helices. The long extended helices provide a mechanism by which the cytoplasmic region of phospholamban interacts with residues in the cytoplasmic domain of the Ca(2+) pump.

Conversion of phospholamban into a soluble pentameric helical bundle

Although membrane proteins and soluble proteins may achieve their final folded states through different pathways, it has been suggested that the packing inside a membrane protein could maintain a similar fold if the lipid-exposed surface were redesigned for solubility in an aqueous environment. To test this idea, the surface of the transmembrane domain of phospholamban (PLB), a protein that forms a stable helical homopentamer within the sarcoplasmic reticulum membrane, has been redesigned by replacing its lipid-exposed hydrophobic residues with charged and polar residues. CD spectra indicate that the full-length soluble PLB is highly alpha-helical. Small-angle X-ray scattering and multiangle laser light scattering experiments reveal that this soluble variant of PLB associates as a pentamer, preserving the oligomeric state of the natural protein. Mutations that destabilize native PLB also disrupt the pentamer. However, NMR experiments suggest that the redesigned protein exhibits molten globule-like properties, possibly because the redesign of the surface of this membrane protein may have altered some native contacts at the core of the protein or possibly because the core is not rigidly packed in wild-type PLB. Nonetheless, our success in converting the membrane protein PLB into a specific soluble helical pentamer indicates that the interior of a membrane protein contains at least some of the determinants necessary to dictate folding in an aqueous environment. The design we successfully used was based on one of the two models in the literature; the alternative design did not give stable, soluble pentamers. This suggests that surface redesign can be employed in gaining insights into the structures of membrane proteins.

Total chemical synthesis of human matrix Gla protein

Human matrix Gla protein (MGP) is a vitamin K-dependent extracellular matrix protein that binds Ca2+ ions and that is involved in the prevention of vascular calcification. MGP is a 10.6-kD protein (84 amino acids) containing five gamma-carboxyglutamic acid (Gla) residues and one disulfide bond. Studies of the mechanism by which MGP prevents calcification of the arterial media are hampered by the low solubility of the protein (<10 microg/mL). Because of solubility problems, processing of a recombinantly expressed MGP-fusion protein chimera to obtain MGP was unsuccessful. Here we describe the total chemical synthesis of MGP by tBoc solid-phase peptide synthesis (SPPS) and native chemical ligation. Peptide Tyr1-Ala53 was synthesized on a derivatized resin yielding a C-terminal thioester group. Peptide Cys54-Lys84 was synthesized on Lys-PAM resin yielding a C-terminal carboxylic acid. Subsequent native chemical ligation of the two peptides resulted in the formation of a native peptide bond between Ala53 and Cys54. Folding of the 1-84-polypeptide chain in 3 M guanidine (pH 8) resulted in a decrease of molecular mass from 10,605 to 10,603 (ESI-MS), representing the loss of two protons because of the formation of the Cys54-Cys60 internal disulfide bond. Like native MGP, synthetic MGP had the same low solubility when brought into aqueous buffer solutions with physiological salt concentrations, confirming its native like structure. However, the solubility of MGP markedly increased in borate buffer at pH 7.4 in the absence of sodium chloride. Ca2+-binding to MGP was confirmed by analytical HPLC, on which the retention time of MGP was reduced in the presence of CaCl2. Circular dichroism studies revealed a sharp increase in alpha-helicity at 0.2 mM CaCl2 that may explain the Ca2+-dependent shift in high-pressure liquid chromatography (HPLC)-retention time of MGP. In conclusion, facile and efficient chemical synthesis in combination with native chemical ligation yielded MGP preparations that can aid in unraveling the mechanism by which MGP prevents vascular calcification.

Total chemical synthesis and chemotactic activity of human S100A12 (EN-RAGE)

Human S100A12 (extracellular newly identified RAGE (receptor for advanced glycosylation end products)-binding protein), a new member of the S100 family of EF-hand calcium-binding proteins, was chemically synthesised using highly optimised 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate/tert-butoxycarbonyl in situ neutralisation solid-phase chemistry. Circular dichroism studies indicated that CaCl(2) decreased the helical content by 27% whereas helicity was marginally increased by ZnCl(2). The propensity of S100A12 to dimerise was examined by electrospray ionisation time-of-flight mass spectrometry which clearly demonstrated the prevalence of the non-covalent homodimer (20890 Da). Importantly, synthetic human S100A12 in the nanomolar range was chemotactic for neutrophils and macrophages in vitro.

Molecular mechanism of NPF recognition by EH domains

Eps15 homology (EH) domains are protein interaction modules that recognize Asn-Pro-Phe (NPF) motifs in their biological ligands to mediate critical events during endocytosis and signal transduction. To elucidate the structural basis of the EH-NPF interaction, the solution structures of two EH-NPF complexes were solved using NMR spectroscopy. The first complex contains a peptide representing the Hrb C-terminal NPFL motif; the second contains a peptide in which an Arg residue substitutes the C-terminal Leu. The NPF residues are almost completely embedded in a hydrophobic pocket on the EH domain surface and the backbone of NPFX adopts a conformation reminiscent of the Asx-Pro type I beta-turn motif. The residue directly following NPF is crucial for recognition and is required to complete the beta-turn. Five amino acids on the EH surface mediate specific recognition of this residue through hydrophobic and electrostatic contacts. The complexes explain the selectivity of the second EH domain of Eps15 for NPF over DPF motifs and reveal a critical aromatic interaction that provides a conserved anchor for the recognition of FW, WW, SWG and HTF ligands by other EH domains.

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

Chemical synthesis, functional reconstitution, and electron paramagnetic resonance (EPR) have been used to analyze the structure and function of phospholamban (PLB), a 52-residue integral membrane protein that regulates the calcium pump (Ca-ATPase) in cardiac sarcoplasmic reticulum (SR). PLB exists in equilibrium between monomeric and pentameric forms, as observed by SDS-PAGE, EPR, and fluorescence. It has been proposed that inhibition of the pump is due primarily to the monomeric form, with both pentameric stability and inhibition dependent primarily on the transmembrane (TM) domain. To test these hypotheses, we have studied the physical and functional properties of a synthetic null-cysteine PLB analogue that is entirely monomeric on SDS-PAGE, and compared it with the synthetic null-cysteine TM domain (residues 26-52). The TM domain was found to be primarily oligomeric on SDS-PAGE, and boundary lipid spin label analysis in lipid bilayers verified that the isolated TM domain is more oligomeric than the full-length parent molecule. These results indicate that the stability of the PLB pentamer is due primarily to attractive interactions between hydrophobic TM domains, overcoming the repulsive electrostatic interactions between the cationic cytoplasmic domains (residues 1-25). When reconstituted into liposomes containing the Ca-ATPase, the null-cysteine TM domain had the same inhibitory function as that of the full-length parent molecule. We conclude that the TM domain of PLB is sufficient for inhibitory function, the oligomeric stability of PLB does not determine its inhibitory activity, and the three Cys residues in the TM domain are not required for inhibitory function.

Characterization and quantitation of phospholamban and its phosphorylation state using antibodies

Quantitative immunoassays to discriminate and quantitate phospholamban and its phosphorylation states in heart homogenates were developed using known amounts of protein determined by amino acid analysis. Synthetic 1-52 phospholamban, the hydrophilic 1-25 peptide, and 1-25 phosphopeptides containing P-Ser(16), P-Thr(17), and dually phosphorylated (P-Ser(16), P-Thr(17)) were used to calibrate immunoblot systems. In addition, synthetic 1-52 peptide was phosphorylated using cAMP-dependent protein kinase (P-Ser(16)) or Ca(2+)-calmodulin protein kinase (P-Thr(17)) and then separated from unphosphorylated 1-52 by HPLC prior to quantitation. Further, canine cardiac sarcoplasmic reticulum was phosphorylated in vitro using [gamma-(32)P]-ATP with cAMP-dependent protein kinase and/or Ca(2+)-calmodulin-dependent protein kinase as well as sequential phosphorylation in both orders to assess the veracity of antibody recognition of phosphorylated forms. Western blots proved useful in characterizing the reactivity of the different antibodies to phospholamban and phosphorylated phospholamban, but were inefficient for accurate quantitation and problems with antibody recognition of dually phosphorylated phospholamban were found. mAb 1D11 recognized all forms of phospholamban, polyclonal antibodies 285 and PS-16 were highly selective for P-Ser(16) phospholamban but had diminished reactivity to diphosphorylated (P-Ser(16), P-Thr(17)) phospholamban, and polyclonal antibody PT-17, although selective for P-Thr(17) phospholamban, generated very weak signals on Western blots and reacted poorly with diphosphorylated phospholamban. Results in quantitative immunodot blot experiments were even more compelling. None of the phosphorylation specific antibodies reacted with the diphospho 1-25 phospholamban peptide. Transgenic mouse hearts expressing varying levels of PLB and ferret heart biopsy samples taken before and after isoproterenol perfusion were analyzed. In all samples containing phospholamban, a basal level of Ser(16) phosphorylation (about 4% of the total PLB population) and a lesser amount of Thr(17) phosphorylation was observed. Upon isoproterenol perfusion, Ser(16) phosphorylation increased only to 17% of the total phospholamban population with a similar change in Thr(17) phosphorylation. This suggests that phospholamban phosphorylation may serve as an electrostatic switch that dissociates inactive calcium pump complexes into catalytically active units. Thus, direct correlations between phospholamban phosphorylation state and contractile parameters may not be valid.

In vitro disulfide-coupled folding of guanylyl cyclase-activating peptide and its precursor protein

Guanylyl cyclase-activating peptide II (GCAP-II), an endogenous ligand of particulate guanylyl cyclase C (GC-C), is processed from the precursor protein and circulates in human blood. GCAP-II consists of 24 amino acid residues and contains two disulfide bridges. The correct disulfide paring of GCAP-II is an absolute requirement for its biological activity. This study shows that the folding of the peptide from the reduced form yields a peptide with the native disulfide paring as a minor product and with non-native ones as major products, regardless of the presence or absence of reduced and oxidized glutathione. The results suggest that GCAP-II does not possess sufficient information to permit the adoption of the native conformation and to effectively form the correct disulfide pairing and, as a result, that GCAP-II is correctly folded by assistance of a factor(s) such as an intra- or intermolecular chaperone. We studied whether a peptide in the pro-leader sequence of the precursor protein (proGCAP-II) contains sufficient information to facilitate the folding of GCAP-II. For this purpose, we prepared proGCAP-II in Escherichia coli by a recombinant technique and examined the disulfide-coupled folding of proGCAP-II from the reduced form. proGCAP-II was quantitatively recovered with the correctly folded structure from the reduced form both in the presence and in the absence of reduced and oxidized glutathione. The protein contains only disulfide linkages at the same positions as the mature form of proGCAP-II, GCAP-II, and the biologically active isomer of GCAP-II in the molecule. These results provide evidence that the propeptide of proGCAP-II is a critical factor in the formation of the correct disulfide paring in the folding of the protein.

Biochemical and biophysical comparison of native and chemically synthesized phospholamban and a monomeric phospholamban analog

Phospholamban (PLB) was rapidly isolated from canine cardiac sarcoplasmic reticulum using immunoaffinity chromatography and prepared by solid phase peptide synthesis. The two proteins are indistinguishable when analyzed by SDS-polyacrylamide gel electrophoresis and exhibit pentameric oligomeric states. They are similarly detected on Western blots, are phosphorylation substrates, have identical amino acid compositions that directly reflect their predicted values, yield the same internal amino acid sequences upon CNBr digestion, and have molecular mass values agreeing with the expected value (approximately 6123 Da). Native and synthetic PLB reduced the calcium sensitivity of Ca2+ATPase, which is reversed by anti-PLB antibody. A Cys-to-Ser PLB analog, where the cysteines (36, 41, and 46) were substituted by serines, is monomeric on SDS-polyacrylamide gel electrophoresis, can be phosphorylated, and is recognized by polyclonal antisera. PLB migrates with a sedimentation coefficient of 4.8 S in sedimentation velocity ultracentrifugation experiments, whereas Cys-to-Ser PLB does not sediment, consistent with a monomeric state. Circular dichroism spectral analysis of PLB indicates about 70% alpha-helical structure, whereas Cys-to-Ser PLB manifests only about 30%. Because the physiochemical properties of native and synthetic PLB appear identical, the more readily available synthetic protein should be suitable for more extensive structural studies.

Engineering magnesium selectivity in the helix-loop-helix calcium-binding motif

Engineering magnesium selectivity into the helix-loop-helix (hlh) cation binding site is relatively unstudied in the calmodulin superfamily of calcium-regulated proteins, which include parvalbumin, oncomodulin, troponin C, calbindin, and calmodulin. Studies using a 33-residue synthetic peptide model of the hlh cation binding motif have indicated that magnesium will induce structural change in those peptide motifs containing three or four acid residues in chelating positions with a single-acid-pair on the Z-axis. Decreasing the cation binding cavity size in Z-axis acid-paired motifs through replacement of chelating residues in the +Z or -X metal ion coordinating positions in the loop region by glutamic acid has been successful in decreasing the calcium ion affinity. The same changes did not create or enhance magnesium binding in the 33-residue model hlh cation binding motif.

Solution structure of the cytoplasmic domain of phopholamban: phosphorylation leads to a local perturbation in secondary structure

Peptides representing the N-terminal domain (Ia) of the cardiac sarcoplasmic reticulum protein phospholamban (residues 1-25 [PLB(1-25)] and a phosphorylated form [pPLB(1-25)]) were synthesized and their conformations examined using circular dichroism and nuclear magnetic resonance spectroscopy. In aqueous solution, both PLB(1-25) and pPLB(1-25) adopt a primarily disordered conformation. In 30% trifluoroethanol/10 mM phosphate, PLB(1-25) exhibits a CD spectrum consistent with 60% helical structure. This value decreases to 27% for the phosphorylated peptide. CD spectra in 2% SDS indicate 40% alpha-helix for PLB(1-25) and 20% for pPLB(1-25). Full chemical shift assignments were obtained by conventional homonuclear NMR methodologies for both PLB(1-25) and pPLB(1-25) in 30% trifluoroethanol/water and 300 mM SDS. The solution structure of PLB(1-25) in 30% TFE/water was determined from distance geometry calculations using 54 NOE distance constraints and 17 torsion angle constraints. In the family of 20 calculated conformers, the root mean square deviation from the mean structure is 0.79 A for backbone heavy atoms of residues 1-17. The structure comprises a regular alpha-helix extending from M1 to S16 with the remaining C-terminal residues disordered. The calculated structure is supported by analysis of C alpha H secondary shifts which are significantly negative for residues 1-16. Chemical shift degeneracy is substantially more extensive in the phospho form and precludes a direct comparison of calculated structures. However, the magnitudes of upfield secondary shifts are decreased by 20% in residues 1-11 and are not significantly helical for residues 12-16 according to the criteria of Wishart et al. [(1992) Biochemistry 31, 1647-1651]. 3JHN alpha coupling constants measured for I12, R13, A15, and S16 also suggest that residues 12-16 undergo a local unwinding of the helix upon phosphorylation. Similar results are obtained for PLB(1-25) and pPLB(1-25) in 300 mM perdeuterated sodium dodecyl sulfate except that differences in backbone dynamics for the helical and nonhelical regions of the peptide are evident in the DQF-COSY line shapes for fingerprint cross-peaks. This disruption of structure at the C-terminus of the helix suggests a model for phosphorylation-induced dissociation of the PLB/Ca(2+)-ATPase complex.

Conformation of the cytoplasmic domain of phospholamban by NMR and CD

Nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy have been used to characterize the conformation of the putative cytoplasmic domain of phospholamban (PLB), an oligomeric membrane-bound protein which regulates the activity of the cardiac sarcoplasmic reticulum Ca(2+)-dependent ATPase. In aqueous solution the 25-residue peptide adopts a number of rapidly interconverting conformers with no secondary structural type obviously predominating. However, in trifluoroethanol (TFE) the conformation, while still highly dynamic, is characterized by a high proportion of helical structures. Evidence for this is provided by alpha CH chemical shifts and low NH chemical shift temperature coefficients, small NH-alpha CH intraresidue scalar coupling constants, a substantial number of distinctive interresidue nuclear Overhauser effects (NOEs) [dNN(i, i + 1), d alpha N(i, i + 3), d alpha beta(i, i + 3) and d alpha N(i, i + 4)] and characteristic CD bands at 190 (positive), 206 (negative) and 222 nm (negative). The helicity is interrupted around Pro-21. The activity of PLB is regulated by phosphorylation at either Ser-16 or Thr-17. CD shows that phosphorylation at Ser-16 by the cAMP-activated protein kinase causes about an 11% decrease in alpha-helical content in TFE.

A novel peptide designed for sensitization of terbium (III) luminescence

Several synthetic peptides, modelled from a Ca(2+)-binding loop of the EF-hand family of proteins, were prepared containing cysteine residues. The peptide, GDKNADGFICFEEL, was labelled covalently at the cysteine residue (loop position 9) with iodoacetamidosalicylic acid. This novel conjugate is a metal-binding loop containing a salicylic acid side chain that could not only chelate Tb3+ in conjunction with the other chelating groups in the sequence, but could also sensitize Tb3+ luminescence. The loop had a high Tb3+ affinity, with stoichiometric binding observed under experimental conditions. The luminescence from the Tb(3+)-peptide complex was more than 10-fold greater than the luminescence reported from a related peptide which contained Trp as the Tb3+ donor at loop position 7. This peptide has significant potential for use in lanthanide-based time-resolved luminescence immunoassays.

Gelsolin immunoreactivity in corneal amyloid, wound healing, and macular and granular dystrophies

Immunohistologic studies of tissue sections obtained from patients with type 1 or type 2 lattice corneal dystrophy, polymorphic amyloid degeneration, or gelatinous amyloid degeneration were performed by using a monoclonal antibody raised to a chymotryptic fragment inclusive of the carboxy-terminal half of plasma gelsolin, and also with a series of polyclonal antibodies specific for synthetic peptides corresponding to immunogenic epitopes of gelsolin. These epitopes are parts of sequences at the amino- and carboxy-terminal ends of gelsolin, as well as adjacent to and inclusive of the codon 187 mutant 7-11 kD fragment that has been shown to be the subunit protein of amyloid fibrils occurring systemically in patients affected by Finnish type familial amyloidosis. These antibodies were also tested on tissue sections obtained from patients with granular and macular corneal dystrophy, corneal wounds, and normal control corneas. Specificity of staining was established by absorption with gelsolin purified from plasma, or the appropriate synthetic peptide. Gelsolin immunoreactivity was detected in the conjunctival and skin amyloid in familial amyloidosis by using familial amyloid (Finnish type) antibody. In other types of corneal amyloid, including lattice dystrophy type 1, immunoreactivity with gelsolin and synthetic peptides was observed adjacent to the deposits, but rarely within them. In macular dystrophy, variable staining of the deposits could result from the association of subunit proteins with glycosaminoglycans.

Total synthesis and functional properties of the membrane-intrinsic protein phospholamban

The membrane-intrinsic protein phospholamban (PLN), the regulatory protein of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, was chemically synthesized. The synthesis was accomplished by double couplings and efficient capping procedures, thus eliminating hydrophobic failure sequences. The crude peptide was purified by high-performance liquid chromatographic ion exchange and gel permeation chromatography in chloroform-methanol mixtures. Ion spray mass spectroscopy showed that the product had the correct molecular mass. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis runs produced the typical monomer-pentamer structural pattern. A predominantly helical CD spectrum was obtained in 0.075% C12E8 (67.9% helix, 1.8% beta, 12.2% turn, 18.1% random coil). Synthetic PLN was phosphorylated in detergent solutions by protein kinase A with a stoichiometry close to 1:1 (Pi to PLN monomer). Reconstitution of the isolated skeletal muscle SR Ca2+ ATPase in phosphatidylcholine membranes in the presence of PLN using the freezing and thawing technique yielded a preparation with lower Ca(2+)-dependent ATPase activity. The inhibition was mainly due to a decrease in the affinity (Km(Ca)) of the ATPase for Ca2+ and was partially reversed by PLN phosphorylation with protein kinase A. By contrast, addition of PLN to diluted intact SR vesicles uncoupled the Ca(2+)-transport reaction, suggesting an ionophoric effect of PLN. Because this effect was observed at very high PLN-to-SR vesicle ratios and was not influenced by PLN phosphorylation, its biological function is doubtful.

Determination of the solution structure of a synthetic two-site calcium-binding homodimeric protein domain by NMR spectroscopy

The solution structure of a 34-residue synthetic calcium-binding peptide from site III of chicken troponin-C has been determined by 1H NMR spectroscopy. In solution and in the presence of calcium this peptide forms a symmetric two-site homodimeric calcium-binding domain (Shaw et al., 1990). The solution structure of this dimer was determined from the measurement of 470 NOEs from a 75-ms NOESY data set. For the dimer structure determination, the constraint list included 868 distance restraints, 44 phi angles, and 24 chi 1 and 2 chi 2 angles. Seven structures were calculated by restrained molecular dynamics using a procedure in which intramonomer distances were used first and then all distances, intra- and intermonomer, were input during further dynamics. The structures exhibited a fold very similar to the C-terminal domain of troponin-C comprised of a pair of helix-loop-helix calcium-binding sites. The rms deviation of these structures for backbone atoms between residues 97-122 and 97'-122' for the dimer was 0.82 A. The dimer structure was also calculated to be more symmetric than sites III and IV in troponin-C.

Toward the elucidation of the mechanism of attachment and entry of malaria sporozoites into cells: synthetic polypeptides from the circumsporozoite protein of Plasmodium falciparum bind Ca2+ and interact with model phospholipid membranes

Through the joint use of CD, Fourier transform ir (FTIR), and attenuated total reflectance FTIR we have found that synthetic polypeptide models of the Plasmodium falciparum circumsporozoite (CS) protein repeat domain bind calcium ions in helicogenic environments. Ca(2+)-(NANP)n complexes (n greater than or equal to 20) interact vectorially with model phospholipid membranes orienting their polypeptide axes preferentially along those of the lipid acyl chains. It is proposed that the P. falciparum CS protein central region, rather than acting as a molecular lure helping the parasite to evade host immune control, plays, as a specific Ca2+ macroligand, a critical functional role during attachment, invasion, and development of the malaria parasite in the hepatic cell.

Regulation of the skeletal sarcoplasmic reticulum Ca2+ pump by phospholamban in reconstituted phospholipid vesicles

Phospholamban is the regulator of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum (SR). The mechanism of regulation appears to involve inhibition by dephosphorylated phospholamban, and phosphorylation may relieve this inhibition. Fast-twitch skeletal muscle SR does not contain phospholamban, and it is not known whether the Ca(2+)-ATPase isoform from this muscle may be also subject to regulation by phospholamban in a similar manner as the cardiac isoform. To determine this we reconstituted the skeletal isoform of the SR Ca(2+)-ATPase with phospholamban in phosphatidylcholine proteoliposomes. Inclusion of phospholamban was associated with significant inhibition of the initial rates of Ca2+ uptake at pCa 6.0, and phosphorylation of phospholamban by the catalytic subunit of cAMP-dependent protein kinase reversed the inhibitory effects on the Ca2+ pump. Similar effects of phospholamban were also observed using phosphatidylcholine:phosphatidylserine proteoliposomes, in which the Ca2+ pump was activated by the negatively charged phospholipids (24). Regulation of the Ca(2+)-ATPase appeared to involve binding with the hydrophilic portion of phospholamban, as evidenced by cross-linking experiments, using a synthetic peptide that corresponded to amino acids 1-25 of phospholamban. These findings suggest that the fast-twitch isoform of the SR Ca(2+)-ATPase may be also regulated by phospholamban, although this regulator is not expressed in fast-twitch skeletal muscles.

Terbium luminescence in synthetic peptide loops from calcium-binding proteins with different energy donors

Fourteen 14-mer peptides corresponding to a consensus sequence of metal-binding loops from proteins of the calmodulin family were synthesized. The effect of varying both the position in the binding loop, and the type of aromatic side chains as energy donors for enhancement of terbium luminescence, was studied. It was concluded that tryptophan in loop position 7 gave optimal luminescence enhancement, and that the additional inclusion of a tyrosine in the loop at positions 2 or 4 could further boost emission from the bound terbium. In all further cases energy transfer from aromatic residues at positions other than 7 was markedly less efficient. These results suggest that the peptides assume a configuration which allows a hexadentate ligand structure around the bound terbium ion. This is consistent with a Dexter-type electron exchange model of energy transfer.

Novel peptides derived from a region of local homology between uteroglobin and lipocortin-1 inhibit platelet aggregation and secretion

The active site for uteroglobin inhibition of phospholipase A2 has been localized to a nonapeptide (P1) which is partially homologous to a nonapeptide (P2) in lipocortin, which also inhibits phospholipase A2. P1 and P2 share an identical tetrapeptide (P4) which is required for inhibition, although P4 alone does not inhibit this enzyme. We found the mechanism of inhibition of platelet aggregation and secretion by the nonapeptides and P4 varied depending on whether platelets were thrombin- or ADP-activated. All three peptides decrease thrombin esterolytic activity and thereby inhibit thrombin-induced platelet activation. P1 decreases ADP-induced aggregation and serotonin secretion by inhibiting phospholipase A2 whereas P4 decreases only aggregation by blocking fibrinogen binding to activated platelets. The P4 sequence in P1 may affect the interaction of P1 with platelets since the presence of P4 potentiates P1 inhibition of platelet activation.

[Synthesis of tetracontapeptide--a hypothetical evolutionary precursor of calcium-binding proteins. I. Synthesis of (1-9), (10-14), (15-20), (21-26), (29-33), (34-40) fragments]

Fragments (1-9), (10-14), (15-20), (21-26), (29-33) and (34-40) of a tetracontapeptide hypothetical ancestor of calcium-binding proteins were synthesised with the use of pentafluorophenyl esters. Formation of a succinimide derivative was detected during synthesis of fragment (15-20) containing Asp(OBzl)-Gly sequence. To avoid this side process, tert-butylprotecting group was used instead of benzyl group. alpha-Carboxyls of C-terminal amino acids were protected by phenacyl group.

[Synthesis of tetracontapeptide--a hypothetical evolutionary precursor of calcium-binding proteins. II. Condensation of fragments]

A tetracontapeptide was obtained by condensation of synthetic fragments (see the preceding paper) with the use of pentafluorophenyl esters as well as with carbodiimide and hydroxybenzotriazole. Racemization during fragment condensation was 1-2 per cent. Deblocking of the protected tetracontapeptide was carried out by treatment with trifluoroacetic acid and hydrogen fluoride with thioanisole. The obtained peptide was purified by gel-chromatography and HPLC.

Total sequential solid phase synthesis of rabbit skeletal troponin C calcium binding site III

The total sequential solid phase synthesis of AcA98STnC(90-123)amide is described. The yield is comparable to that obtained previously using fragment condensation on the solid support. HPLC purification results in a product which displays greater calcium induced conformational changes than the peptide prepared by the fragment condensation method. However, the former has a dissociation constant for calcium (28 microM) seven times higher than the latter (4 microM).

31P nuclear magnetic resonance spectroscopic evidence for ternary complex formation of fetal dentin phosphoprotein with calcium and inorganic orthophosphate ions

The single phosphoprotein of fetal calf dentin, having a molecular weight of approximately 94,000 and a phosphorus content of 8% (w/w), was examined by 31P NMR spectroscopy. The single resonance at 3.7 ppm at pH 10 and its chemical shift during acid titration established the phosphomonoester nature of the organic phosphorus moiety. During titration of the phosphoprotein with CaCl2 in the presence of inorganic orthophosphate ions, line broadening for the orthophosphate resonance was both phosphoprotein- and calcium-dependent, indicating ternary complex formation. The data indicate that the phosphoprotein of fetal calf dentin binds both calcium and inorganic orthophosphate ions and therefore has the requisite physical chemical properties necessary for it to facilitate the heterogeneous nucleation of a Ca-PO4 solid phase from solution during tissue mineralization.

Preparation of azidocalmodulin: a photoaffinity label for calmodulin-binding proteins

A photoaffinity label for calmodulin-binding proteins was prepared from 125I-labeled calmodulin (125I-calmodulin) and methyl-4-azidobenzimidate. Azidocalmodulin containing one azido group per calmodulin retained its ability to stimulate the CA2+-sensitive phosphodiesterase purified from bovine heart muscle. The concentrations of calmodulin and azidocalmodulin required for half-maximal stimulation of phosphodiesterase activity were 170 and 230 pM, respectively. Azido-125I-calmodulin was used to photoaffinity label troponin I, myosin light chain kinase, and the Ca2+-sensitive phosphodiesterase. Formation of crosslinked complexes required the presence of Ca2+ or Mn2+ and was inhibited by excess unmodified calmodulin. The calmodulin-binding subunits all formed 1:1 complexes with calmodulin, and the molecular weights of the crosslinked products obtained with troponin I, the phosphodiesterase, and myosin light chain kinase were 43,000, 79,000, and 116,000, respectively. Photolysis experiments using azido-125I-calmodulin and bovine cerebral cortex membranes or detergent-solubilized membranes resulted in formation of a limited number of specifically labeled polypeptides. Azido-calmodulin appears to be an appropriate photoaffinity label for the identification and characterization of calmodulin-binding subunits.