Interaction of calcium ions with α-elastin: An equilibrium dialysis,circular dichroism,and microcalorimetric study

The interaction of calcium ions with α-elastin has been investigated by equilibrium dialysis, CD, and microcalorimetric techniques. Consistent with literature data, it was found that the interaction in water is very poor. In trifluoroethanol (TFE), equilibrium dialysis experiments showed that calcium ions bind to ～-elastin with an association constant of ～250 L × mol^?1. Such a figure is not consistent with highly specific, highly selective binding. It was also found that the CD response is not directly proportional to the amount of bound calcium but depends on the protein concentration. From microcalorimetric experiments it was found that the heat effect relative to the binding process is of the order of 1.9 kcal/g ion. From this figure and from the binding constant, a positive ΔS value of about 17 e.u. was evaluated, leading to the conclusion that the binding process is entropy driven. From microcalorimetric measurements a ΔH of 1.5 kcal/residue was found for the calcium-induced conformational transition of the protein.     

Cholesterol‐β1AR interaction versus cholesterol‐β2AR interaction

Two 8‐µs all‐atom molecular dynamics simulations have been performed on the two highly homologous G protein‐coupled receptor (GPCR) subtypes, β₁‐ and β₂‐adrenergic receptors, which were embedded in a lipid bilayer with randomly dispersed cholesterol molecules. During the simulations, cholesterol molecules accumulate to different surface regions of the two receptors, suggesting the subtype specificity of cholesterol–β‐adrenergic receptor interaction and providing some clues to the physiological difference of the two subtypes. Meanwhile, comparison between the two receptors in interacting with cholesterols shed some new light on general determinants of cholesterol binding to GPCRs. Our results indicate that although the concave surface, charged residues and aromatic residues are important, neither of these stabilizing factors is indispensable for a cholesterol interaction site. Different combinations of these factors lead to the diversified binding modes of cholesterol binding to the receptors. Our long‐time simulations, for the first time, revealed the pathway of a cholesterol molecule entering the consensus cholesterol motif (CCM) site, and the binding process of cholesterol to CCM is accompanied by a side chain flipping of the conserved Trp4.50. Moreover, the simulation results suggest that the I‐/V‐/L‐rich region on the extracellular parts of helix 6 might be an alternatively conserved cholesterol‐binding site for the class‐A GPCRs. Proteins 2014; 82:760–770. © 2013 Wiley Periodicals, Inc.     

Mechanisms involved in α6β1-integrin-mediated Ca signalling

Contact of Jurkat T-lymphocytes with the extracellular matrix (ECM) protein laminin resulted in long-lasting α6β1-integrin-mediated Ca²⁺ signalling. Both Ca²⁺ release from thapsigargin-sensitive Ca²⁺ stores and capacitative Ca²⁺ entry via Ca²⁺ channels sensitive to SKF 96365 constitute important parts of this process. Inhibition of α6β1-integrin-mediated Ca²⁺ signalling by (1) the src kinase inhibitor PP2, (2) the PLC inhibitor U73122, and (3) the cyclic adenosine diphosphoribose (cADPR) antagonist 7-deaza-8-Br-cADPR indicate the involvement of src tyrosine kinases and the Ca²⁺-releasing second messengers d-myo-inositol 1,4,5-trisphosphate (InsP₃) and cADPR.     

Conformational study of poly(α,β-L-aspartic acid)

The conformation of several samples of poly(α,β-L -Asp) with a molar fraction of β-bonds ranging from 0.1 to 0.55 was investigated by means of ir and CD spectroscopy and potentiometric titration and compared with the results obtained previously with poly(α-L -Asp). All samples investigated underwent a conformational change induced by changes in their degree of ionization: unpronounced ir absorption of amide V at 650 cm^?1 was shifted to 620 cm^?1 and substantially increased on deionization; CD spectra changed with the degree of ionization, passing through an isosbestic point; and the pattern of the titration curves was more complex than that of a simple polyelectrolyte. The conformation developing with the decreasing degree of ionization may be considered to be α-helix, as deduced according to the analogous behavior of other polypeptides. The extent of the conformational change in the individual samples depends on the molar fraction of β-bonds: the higher it is, the lower is the helix-forming ability of the sample.     

Plasma gelsolin facilitates interaction between β2 glycoprotein I and α5β1 integrin

Antiphospholipid syndrome (APS) is characterized by thrombosis and the presence of antiphospholipid antibodies (aPL) that directly recognizes plasma β₂‐glycoprotein I (β₂GPI). Tissue factor (TF), the major initiator of the extrinsic coagulation system, is induced on monocytes by aPL in vitro, explaining in part the pathophysiology in APS. We previously reported that the mitogen‐activated protein kinase (MAPK) pathway plays an important role in aPL‐induced TF expression on monocytes. In this study, we identified plasma gelsolin as a protein associated with β₂GPI by using immunoaffinity chromatography and mass spectrometric analysis. An in vivo binding assay showed that endogenous β₂GPI interacts with plasma gelsolin, which binds to integrin a₅β₁ through fibronectin. The tethering of β₂GPI to monoclonal anti‐β₂GPI autoantibody on the cell surface was enhanced in the presence of plasma gelsolin. Immunoblot analysis demonstrated that p38 MAPK protein was phosphorylated by monoclonal anti‐β₂GPI antibody treatment, and its phosphorylation was attenuated in the presence of anti‐integrin a₅β₁ antibody. Furthermore, focal adhesion kinase, a downstream molecule of the fibronectin‐integrin signalling pathway, was phosphorylated by anti‐β₂GPI antibody treatment. These results indicate that molecules including gelsolin and integrin are involved in the anti‐β₂GPI antibody‐induced MAPK pathway on monocytes and that integrin is a possible therapeutic target to modify a prothrombotic state in patients with APS.     

Molecular determinants of desensitization and assembly of the chimeric GABAA receptor subunits (α1/γ2) and (γ2/α1) in combinations with β2 and γ2

Two γ-aminobutyric acid_A (GABA_A) receptor chimeras were designed in order to elucidate the structural requirements for GABA_A receptor desensitization and assembly. The (α1/γ2) and (γ2/α1) chimeric subunits representing the extracellular N-terminal domain of α1 or γ2 and the remainder of the γ2 or α1 subunits, respectively, were expressed with β2 and β2γ2 in Spodoptera frugiperda (Sf-9) cells using the baculovirus expression system. The (α1/γ2)β2 and (α1/γ2)β2γ2 but not the (γ2/α1)β2 and (γ2/α1)β2γ2 subunit combinations formed functional receptor complexes as shown by whole-cell patch–clamp recordings and [³H]muscimol and [³H]flunitrazepam binding. Moreover, the surface immunofluorescence staining of Sf-9 cells expressing the (α1/γ2)-containing receptors was pronounced, as opposed to the staining of the (γ2/α1)-containing receptors, which was only slightly higher than background. To explain this, the (α1/γ2) and (γ2/α1) chimeras may act like α1 and γ2 subunits, respectively, indicating that the extracellular N-terminal segment is important for assembly. However, the (α1/γ2) chimeric subunit had characteristics different from the α1 subunit, since the (α1/γ2) chimera gave rise to no desensitization after GABA stimulation in whole-cell patch–clamp recordings, which was independent of whether the chimera was expressed in combination with β2 or β2γ2. Surprisingly, the (α1/γ2)(γ2/α1)β2 subunit combination did desensitize, indicating that the C-terminal segment of the α1 subunit may be important for desensitization. Moreover, desensitization was observed for the (α1/γ2)β2γ2 receptor with respect to the direct activation by pentobarbital. This suggests differences in the mechanism of channel activation for pentobarbital and GABA.     

Structural classification of αββ and ββα supersecondary structure units in proteins

We present a fully automatic structural classification of supersecondary structure units, consisting of two hydrogen-bonded β strands, preceded or followed by an α helix. The classification is performed on the spatial arrangement of the secondary structure elements, irrespective of the length and conformation of the intervening loops. The similarity of the arrangements is estimated by a structure alignment procedure that uses as similarity measure the root mean square deviation of superimposed backbone atoms. Applied to a set of 141 well-resolved nonhomologous protein structures, the classification yields 11 families of recurrent arrangements. In addition, fragments that are structurally intermediate between the families are found; they reveal the continuity of the classification. The analysis of the families shows that the α helix and β hairpin axes can adopt virtually all relative orientations, with, however, some preferable orientations; moreover, according to the orientation, preferences in the left/right handedness of the α–β connection are observed. These preferences can be explained by favorable side by side packing of the α helix and the β hairpin, local interactions in the region of the α–β connection or stabilizing environments in the parent protein. Furthermore, fold recognition procedures and structure prediction algorithms coupled to database-derived potentials suggest that the preferable nature of these arrangements does not imply their intrinsic stability. They usually accommodate a large number of sequences, of which only a subset is predicted to stabilize the motif. The motifs predicted as stable could correspond to nuclei formed at the very beginning of the folding process. Proteins 30:193–212, 1998. © 1998 Wiley-Liss, Inc.     

Blood Platelets Contain and Secrete Laminin-8 (α4β1γ1) and Adhere to Laminin-8 via α6β1 Integrin

Laminins, a family of heterotrimeric proteins with cell adhesive/signaling properties, are characteristic components of basement membranes of vasculature and tissues. In the present study, permeabilized platelets were found to react with a monoclonal antibody to laminin γ1 chain by immunofluorescence. In Western blot analysis of platelet lysates, several monoclonal antibodies to γ1 and β1 laminin chains recognized 220- to 230-kDa polypeptides, under reducing conditions, and a structure with much slower electrophoretic mobility under nonreducing conditions. Immunoaffinity purification on a laminin β1 antibody–Sepharose column yielded polypeptides of 230, 220, 200, and 180 kDa from platelet lysates. In the purified material, mAbs to β1 and γ1 reacted with the two larger polypeptides, while affinity-purified rabbit antibodies to laminin α4 chain recognized the smallest polypeptide. Identity of the polypeptides was confirmed by microsequencing. One million platelets contained on average 1 ng of laminin (approximately 700 molecules per cell), of which 20–35% was secreted within minutes after stimulation with either thrombin or phorbol ester. Platelets adhered to plastic surfaces coated with the purified platelet laminin, and this process was largely inhibited by antibodies to β1 and α6 integrin chains. We conclude that platelets contain and, following activation, secrete laminin-8 (α4β1γ1) and that the cells adhere to the protein by using α6β1 integrin.     

β-Turn conformations in crystal structures of model peptides containing α,α-Di-n-propylglycine and α,α-Di-n-butylglycine

The crystal state conformations of three peptides containing the α,α-dialkylated residues. α,α-di-n-propylglycine (Dpg) and α,α-di-n-butylglycine (Dbg), have been established by x-ray diffraction. Boc-Ala-Dpg-Alu-OMe (I) and Boc-Ala-Dbg-Ala-OMe (III) adopt distorted type II β-turn conformations with Ala (1) and Dpg/Dbg (2) as the corner residues. In both peptides the conformational angles at the Dxg residue (I: ? = 66.2°, ψ = 19.3°; III: ? = 66.5°. ψ = 21.1°) deviate appreciably from ideal values for the i + 2 residue in a type II β-turn. In both peptides the observed (N…O) distances between the Boc CO and Ala (3) NH groups are far too long (1: 3.44 Å: III: 3.63 Å) for an intramolecular 4 → 1 hydrogen bond. Boc-Ala-Dpg-Ata-NHMe (II) crystallizes with two independent molecules in the asymmetric unit. Both molecules HA and HB adopt consecutive β-turn (type III-III in HA and type III-I in IIB) or incipient 3₁₀-helical structures, stabilized by two intramolecular 4 → 1 hydrogen bonds. In all four molecules the bond angle N-C^α-C′ (τ) at the Dxg residues are ≥ 110°. The observation of conformational angles in the helical region of ?,ψ space at these residues is consistent with theoretical predictions. © 1995 John Wiley & Sons, Inc.     

A simple method for preparation of valency hybrid hemoglobins, (αβ)2 and (αβ)2

The improved methods for the preparation of valency hybrid hemoglobins, (α³⁺β²⁺)₂ and (α²⁺β³⁺)₂ were presented. The (α³⁺β²⁺)₂ valency hybrid was separated from the solutions of partially reduced methemoglobin with ascorbic acid, by using CM 32 column chromatography. The (α²⁺β³⁺)₂ valency hybrid was also isolated from hemoglobin solutions, which were partially oxidized with ferricyanide, by chromatography on CM 32 column. These valency hybrid hemoglobins were found to be single on isoelectric focusing electrophoresis. Present procedures are very simple and are suitable for the bulk preparation of (α³⁺β²⁺)₂ and (α²⁺β³⁺)₂ valency hybrids.     

Synthesis, and carbon-13 n.m.r. study, of O-α- -rhamnopyranosyl-(1→3)-O-α- -rhamnopyranosyl-(1→2)- -rhamnopyranose and O-α- -rhamnopyranosyl-(1→3)-O-α- -rhamnopyranosyl (1→3)- -rhamnopyranose, constituents of bacterial, cell-wall polysaccharides

O-α- -Rhamnopyranosyl-(1→3)- -rhamnopyranose (19) and O-α- -rhamnopyranosyl-(1→2)- -rhamnopyranose were obtained by reaction of benzyl 2,4- (7) and 3,4-di-O-benzyl-α- -rhamnopyranoside (8) with 2,3,4-tri-O-acetyl-α- -rhamnopyranosyl bromide, followed by deprotection. The per-O-acetyl α-bromide (18) of 19 yielded, by reaction with 8 and 7, the protected derivatives of the title trisaccharides (25 and 23, respectively), from which 25 and 23 were obtained by Zemplén deacetylation and catalytic hydrogenolysis, With benzyl 2,3,4-tri-O-benzyl-β- -galactopyranoside, compound 18 gave an ≈3:2 mixture of benzyl 2,3,4-tri-O-benzyl-6-O-[2,4-di-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α- -rhamnopyranosyl)-α- -rhamnopyranosyl]-β- -galactopyranoside and 4-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α- -rhamnopyranosyl)-β- -rhamnopyranose 1,2-(1,2,3,4-tetra-O-benzyl-β- -galactopyranose-6-yl (orthoacetate). The downfield shift at the α-carbon atom induced by α- -rhamnopyranosylation at HO-2 or -3 of a free α- -rhamnopyranose is 7.4-8.2 p.p.m., ≈1 p.p.m. higher than when the (reducing-end) rhamnose residue is benzyl-protected (6.6-6.9 p.p.m.). α- -Rhamnopyranosylation of HO-6 of gb- -galactopyranose deshields the C-6 atom by 5.7 p.p.m. The 1 2-orthoester ring structure [O₂,C(me)OR] gives characteristic resonances at 24.5 ±0.2 p.p.m. for the methyl, and at 124.0 ±0.5 p.p.m. for the quaternary, carbon atom.     

Na4‐αM2+α/2(P2O7)2 (2/3 ≤ α ≤ 7/8, M = Fe,Fe0.5Mn0.5, Mn): A Promising Sodium Ion Cathode for Na‐ion Batteries

A new polyanion‐based compound, Na_3.12M_2.44(P₂O₇)₂ (M = Fe, Fe_0.5Mn_0.5, Mn) is synthesized and examined as a cathode for Na ion batteries. Off‐stoichiometric synthesis induces the formation of a Na‐rich phase, Na_3.32Fe_2.34(P₂O₇)₂ ‐ a member of the solid solution series Na_4‐αFe_2+α/2(P₂O₇)₂ (2/3 ≤ α ≤ 7/8) ‐ which delivers a reversible capacity of about 85 mA h g^?1 at ca. 3 V vs. Na/Na⁺ and exhibits very stable cycle performance. Above all, it shows fast kinetics for Na ions, delivering an almost constant 72% reversible capacity at rates between C/10 and 10C without the necessity for nanosizing or carbon coating. We attribute this to the spacious channel size along the a‐axis, along with a single phase transformation upon de/sodiation.     

Engineered cystine knot peptides that bind αvβ3, αvβ5, and α5β1 integrins with low‐nanomolar affinity

There is a critical need for compounds that target cell surface integrin receptors for applications in cancer therapy and diagnosis. We used directed evolution to engineer the Ecballium elaterium trypsin inhibitor (EETI‐II), a knottin peptide from the squash family of protease inhibitors, as a new class of integrin‐binding agents. We generated yeast‐displayed libraries of EETI‐II by substituting its 6‐amino acid trypsin binding loop with 11‐amino acid loops containing the Arg‐Gly‐Asp integrin binding motif and randomized flanking residues. These libraries were screened in a high‐throughput manner by fluorescence‐activated cell sorting to identify mutants that bound to α_vβ₃ integrin. Select peptides were synthesized and were shown to compete for natural ligand binding to integrin receptors expressed on the surface of U87MG glioblastoma cells with half‐maximal inhibitory concentration values of 10–30 nM. Receptor specificity assays demonstrated that engineered knottin peptides bind to both α_vβ₃ and α_vβ₅ integrins with high affinity. Interestingly, we also discovered a peptide that binds with high affinity to α_vβ₃, α_vβ₅, and α₅β₁ integrins. This finding has important clinical implications because all three of these receptors can be coexpressed on tumors. In addition, we showed that engineered knottin peptides inhibit tumor cell adhesion to the extracellular matrix protein vitronectin, and in some cases fibronectin, depending on their integrin binding specificity. Collectively, these data validate EETI‐II as a scaffold for protein engineering, and highlight the development of unique integrin‐binding peptides with potential for translational applications in cancer. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Design and characterization of a model αβ peptide

CD and nmr spectroscopy were used to compare the conformational properties of two related peptides. One of the peptides, Model AB, was designed to adopt a helix-turn-extended strand (αβ) tertiary structure in water that might be stabilized by hydrophobic interactions between two leucine residues in the amino-terminal segment and two methionine residues in the carboxyl terminal segment. The other peptide, AB Helix, has the same amino acid sequence as Model AB except that it lacks the-Pro-Met-Thr-Met-Thr-Gly segment at the carboxyl-terminus. Although the carboxyl-terminal segment of Model AB was found to be unstructured, its presence increases the number of residues in a helical conformation, shifts the pKas of three ionizable side chains by 1 pH unit or more compared to an unstructured peptide, stabilizes the peptide as a monomer in high concentrations of ammonium sulfate, increases the conformational stability of residues at the terminal ends of the helix, and results in many slowly exchanging amide protons throughout the entire backbone of the peptide. These results suggest that interactions between adjacent segments in a small peptide can have significant structure organizing effects. Similar kinds of interactions may be important in determining the structure of early intermediates in protein folding and may be useful in the de novo design of independently folding peptides. © 1995 John Wiley & Sons, Inc.     

Analysis of Ca2+/Mg2+ selectivity in α‐lactalbumin and Ca2+‐binding lysozyme reveals a distinct Mg2+‐specific site in lysozyme

The triggering of Ca²⁺ signaling pathways relies on Ca²⁺/Mg²⁺ specificity of proteins mediating these pathways. Two homologous milk Ca²⁺‐binding proteins, bovine α‐lactalbumin (bLA) and equine lysozyme (EQL), were analyzed using the simplest “four‐state” scheme of metal‐ and temperature‐induced structural changes in a protein. The association of Ca²⁺/Mg²⁺ by native proteins is entropy‐driven. Both proteins exhibit strong temperature dependences of apparent affinities to Ca²⁺ and Mg²⁺, due to low thermal stabilities of their apo‐forms and relatively high unfavorable enthalpies of Mg²⁺ association. The ratios of their apparent affinities to Ca²⁺ and Mg²⁺, being unusually high at low temperatures (5.3–6.5 orders of magnitude), reach the values inherent to classical EF‐hand motifs at physiological temperatures. The comparison of phase diagrams predicted within the model of competitive Ca²⁺ and Mg²⁺ binding with experimental data strongly suggests that the association of Ca²⁺ and Mg²⁺ ions with bLA is a competitive process, whereas the primary Mg²⁺ site of EQL is different from its Ca²⁺‐binding site. The later conclusion is corroborated by qualitatively different molar ellipticity changes in near‐UV region accompanying Mg²⁺ and Ca²⁺ association. The Ca²⁺/Mg²⁺ selectivity of Mg²⁺‐site of EQL is below an order of magnitude. EQL exhibits a distinct Mg²⁺‐specific site, probably arising as an adaptation to the extracellular environment. Proteins 2010. © 2010 Wiley‐Liss, Inc.     

Aggregation of poly(γ-benzyl-α,L-glutamate)

The aggregation of poly(γ-benzyl-α,L -glutamate) and its enantiomer in toluene has been investigated by following the viscosity as a function of temperature, concentration, molecular weight, molecular-weight distribution, helix chirality, and shear rate. The temperature and concentration data for a 138,000-molecular-weight sample was fitted to an open, reversible end-to-end aggregation model. The aggregation numbers resulting from this fit were consistent with the sudden onset in non-Newtonian flow resulting from only a 0.2-wt% increase in concentration. The association equilibrium constant was then used to predict viscosity for comparison with other data, in particular, the effect of molecular weight and molecular-weight distribution. A mixture of right-and left-handed helices showed the aggregation was not chiral selective. The stiffness of end-to-end aggregated (hydrogen-bonded) molecules differed little from their covalent counterparts, at least below a molecular weight of ～10⁶. We conclude that polybenzylglutamate aggregation in toluene can be described by an open end-to-end aggregation model.     

A study of the reactivity of (methyl 2-acetamidoacrylate)-tricarbonyliron(0) leading to a novel synthesis of β,β,β-trialkyl α-amino acids

(Methyl 2-acetamidoacrylate)tricarbonyliron(0) (3) reacts with 2 equivalents of methyllithium to give methyl N-acetylalaninate (4) and 2-acetamido-4-oxopentanoate (5) when the reaction is quenched with trifluoroacetic acid. Production of methyl N-acetylalaninate is dependent only on the presence of trifluoroacetic acid, and the ratio of 4 to 5 generated in these reactions is related to the quantity of trifluoroacetic acid used to quench them. Addition of two equivalents of methyllithium followed by tertiary haloalkanes gives protected β,β,β-trialkyl α-amino acids which may be hydrolysed to give tert-leucine (13) and the new α-amino acids 2-amino-3,3-dimethylpentanoic acid (14) and 2-amino-3,3-dimethylhexanoic acid (15).