Martha M. Teeter

Structure of the hydrophobic protein crambin determined directly from the anomalous scattering of sulphur

The highly ordered crystal structure of crambin has been solved at 1.5 Å resolution directly from the diffraction data of a native crystal at a wavelength remote from the sulphur absorption edge. The molecule has three disulphide bridges among its 46 amino acid residues, of which 46% are in helices and 17% are in a β-sheet. Crambin is shown to be an amphipathic protein, inasmuch as its six charged groups are segregated from hydrophobic surface elements. Phasing methods used here will also apply elsewhere.

Crystal structure of C-phycocyanin from Cyanidium caldarium provides a new perspective on phycobilisome assembly.

The crystal structure of the light-harvesting protein phycocyanin from the cyanobacterium Cyanidium caldarium with novel crystal packing has been solved at 1.65-A resolution. The structure has been refined to an R value of 18.3% with excellent backbone and side-chain stereochemical parameters. In crystals of phycocyanin used in this study, the hexamers are offset rather than aligned as in other phycocyanins that have been crystallized to date. Analysis of this crystals unique packing leads to a proposal for phycobilisome assembly in vivo and for a more prominent role for chromophore beta-155. This new role assigned to chromophore beta-155 in phycocyanin sheds light on the numerical relationships among and function of external chromophores found in phycoerythrins and phycoerythrocyanins.

Solution of a Protein Crystal Structure with a Model Obtained from NMR Interproton Distance Restraints

Model calculations were performed to test the possibility of solving crystal structures of proteins by Patterson search techniques with three-dimensional structures obtained from nuclear magnetic resonance (NMR) interproton distance restraints. Structures for crambin obtained from simulated NMR data were used as the test system; the root-mean-square deviations of the NMR structures from the x-ray structure were 1.5 to 2.2 � for backbone atoms and 2.0 to 2.8 � for side-chain atoms. Patterson searches were made to determine the orientation and position of the NMR structures in the unit cell. The correct solution was obtained by comparing the rotation function results of several of the NMR structures and the average structure derived from them. Conventional refinement techniques reduced the R factor from 0.43 at 4 � resolution to 0.27 at 2 � resolution without inclusion of water molecules. The partially refined structure has root-mean-square backbone and side-chain atom deviations from the x-ray structure of 0.5 and 1.3 �, respectively.

Highly ordered crystals of the plant seed protein crambin.

Crystals of crambin, a plant seed protein of molecular weight 5000, diffract X-rays strongly to the interplanar spacing limit of 0.88 A. These diffraction data should allow a definition of atomic structure that is on a par with that typically obtained from crystals of small organic molecules. The crystals are in space group P21 and have unit cell dimensions a = 41.1 A, b = 18.7 A, c = 22·7 A, and β = 90.6 °. The asymmetric unit contains one protein molecule.

Assemblies of Alzheimer’s peptides Aβ25–35 and Aβ31–35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction

Alzheimers beta amyloid protein (A beta) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimers disease (AD). The assemblies constituted from residues 25-35 (A beta 25-35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by A beta 25-35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of beta-sheet structure. Moreover, the peptides A beta 31-35 (Ile-Ile-Gly-Leu-Met) and tachykinin analogue A beta(Phe(31))31-35 (Phe-Ile-Gly-Leu-Met) gave powder diffraction patterns to 2.8A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a=9.36 A, b=15.83 A, and c=20.10 A for the native A beta 31-35 peptide, and a=9.46 A, b=16.22 A, and c=11.06 A for the peptide having the Ile31Phe substitution. The initial model was a beta strand where the hydrogen bonding, chain, and intersheet directions were placed along the a, b, and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that A beta is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like A beta 31-35 domain is a turn exposed at the A beta oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.

Refinement of purothionins reveals solute particles important for lattice formation and toxicity. Part 2: structure of beta-purothionin at 1.7 A resolution.

The crystal structure of beta-purothionin (beta-PT) has been determined at 1.7 A resolution. beta-PT and previously solved alpha(l)-PT belong to a family of membrane-active plant toxins homologous to crambin. (beta-PT crystallizes in the same space group as alpha(l)-PT (1422) but with the c axis 3 A longer than (alpha(l)-PT. The unit-cell dimensions of beta-PT crystals are a = b = 53.94 and c = 72.75 A. Two data sets were collected on a multiwire area detector, each with R(sym) around 6.0%, and were merged to get a single data set at 1.7 A, (R(merge) = 9.6%). The X-ray structure of alpha(l)-PT was used to build a starting model for beta-PT. The beta-PT model was refined using the program PROLSQ from 10 to 1.7 A resolution to an R-factor of 19.8% with very good geometry. The final structure contains 439 atoms including 337 protein atoms, 77 waters, two acetates, two glycerols and one phosphate. The high-resolution structure of the beta-PT agreed well with that of the lower resolution alpha(l)-PT structure only after the latter was extensively rerefined. Both refinements revealed phosphate and glycerol molecules which are important in lattice formation. The binding of phosphate and glycerol molecules to purothionins (PT) was confirmed by NMR and was implicated in the biological activity of toxins. Modeling of phospholipid binding to PT based on glycerol and phosphate-binding site could shed light on the lytic toxicity of this protein-toxin family. Although the structures of (alpha(l)-PT and beta-PT preserve the overall fold of crambin, they exhibit key differences that are directly relevant to the toxicity of thionins.

Crystal structure of small protein crambin at 0.48 A resolution

With the development of highly brilliant and extremely intense synchrotron X-ray sources, extreme high-resolution limits for biological samples are now becoming attainable. Here, a study is presented that sets the record in crystallographic resolution for a biological macromolecule. The structure of the small protein crambin was determined to 0.48 Å resolution on the PETRA II ring before its conversion to a dedicated synchrotron-radiation source. The results reveal a wealth of details in electron density and demonstrate the possibilities that are potentially offered by a high-energy source. The question now arises as to what the true limits are in terms of what can be seen at such high resolution. From what can be extrapolated from the results using crystals of crambin, this limit would be at approximately 0.40 Å, which approaches that for smaller compounds.

Energy Minimization for Tertiary Structure Prediction of Homologous Proteins: α1-purothionin and Viscotoxin A3 Models from Crambin

Homologous proteins may fold into similar three-dimensional structures. Spectroscopic evidence suggests this is true for the cereal grain thionins, the mistletoe toxins, and for crambin, three classes of plant proteins. We have combined primary sequence homology and energy minimization to predict the structures alpha 1-purothionin (from Durum wheat) and viscotoxin A3 (from Viscum album, European mistletoe) from the high resolution (0.945 A) crystal structure of crambin (from Crambe abyssinica). Our predictions will be verifiable because we have diffraction-quality crystals of alpha 1-purothionin whose structure we are have predicted. The potential energy minimizations for each protein were performed both with and without harmonic constraints to its initial backbone to explore the existence of local minima for the predicted proteins. Crambin was run as a control to examine the effects of the potential energy minimization on a protein with a well-known structure. Only alpha 1-purothionin which has one fewer residue in a turn region shows a significant difference for the two minimization paths. The results of these predictions suggest that alpha 1-purothionin and viscotoxin are amphipathic proteins, and this character may relate to the mechanism of action for these proteins. Both are mildly membrane-active and their amphipatic character is well suited for interaction with a lipid bilayer.

Full-matrix refinement of the protein crambin at 0.83 A and 130 K.

This paper describes the first successful full-matrix least-squares (FMLS) refinement of a protein structure. The example used is crambin which is a small hydrophobic protein (4.7 kDa, 46 residues). It proves the feasibility of refining such large molecules by this classic method, routinely applied to small molecules. The final structure with 381 non-H protein atoms (54 protein atoms in alternative positions), 367 H atoms, 162 water molecules (combined occupancy 93) and one disordered ethanol molecule converged to a standard unweighted crystallographic R-factor of R = 9.0% when refined against F with reflections stronger than F > 2sigma(F) and R = 9.5% when refined against F(2). The programs RFINE [Finger & Prince (1975). Natl Bur. Stand. (US) Tech. Note 854. A System of Fortran IV Computer Programs for Crystal Structure Computations] and SHELXL93 [Sheldrick (1993). SHELXL93. Program for Crystal Structure Refinement, Univ. of Göttingen, Germany] were used for FMLS refinement with the high-resolution low-temperature (0.83 A, 130 K) data set of a mixed-sequence form of crambin. A detailed analysis of the models obtained in FMLS and PROLSQ [restrained least squares or RLS; Teeter, Roe & Heo (1993). J. Mol. Biol. 230, 292-311] refinements with the same data set is presented. The differences between the models obtained by both FMLS and RLS refinements are systematic but negligible and advantages and shortcomings of both methods are discussed. The final structure has very good geometry, fully comparable to the geometry of other structures in this resolution range. Ideal values used in PROLSQ and those by Engh & Huber [Engh & Huber (1991). Acta Cryst. A47, 392-400] differ significantly from this refinement and we recommend a new standard. FMLS refinement constitutes a sensitive tool to detect and model disorder in highly refined protein structures. We describe the modeling of temperature factors by the TLS method [Schomaker & Trueblood (1968). Acta Cryst. B24, 63-76]. Rigid body-TLS refinements led to a better understanding of different modes of vibrations of the molecule. Refinements using F(2) or F protocols converged and reached slightly different minima. Despite theoretical support for F(2)-based refinement, we recommend refinement on structure factors.

Synthesis and characterization of various coordination modes of 1,1′-bis(diphenylphosphino)ferrocene in iron carbonyl complexes. X-Ray crystal structures of (η-BPPF)Fe(CO)4, (μ,η 2-BPPF)(Fe(CO)4)2, and (μ,η2-BPPF)Fe3(CO)10

Reactions of 1,1′-bis(diphenylphosphino)ferrocene (BPPF) with Fe2(CO)9 and Fe3(CO)12 produced a series of iron carbonyl derivatives: (η2-BPPF)Fe(CO)3 (1), (η2-BPPF)Fe(CO)4 (2), (μ,η2BPPF)(Fe(CO)4)2 (3), (η2-BPPF)Fe2(CO)7 (4), and (μ,η2BPPF)Fe3(CO)10 (5). The synthesis and characterization of 4 and 5 including X-ray crystal structures of 2, 3, and 5 confirmed various coordination modes of BPPF. Crystal data are as follows: (η1-BPPF)Fe(CO)4 (2): orthorhombic, space group Pbca, a = 26.27(1), b = 23.18(1), c = 10.938(8) A, V = 6661(7) A3, Z = 8; 3017 data with I > 3.0σ(I were refined to R = 0.057, Rw = 0.056; (μ,η2-BPPF)Fe3(CO)10 (5): monoclinic, space group P21 / n, a = 11.231(1) b = 21.043(4), c = 20.373(9) A, β = 97.373(9)° V = 4693(4) A3 V, Z = 4; 6082 data with I > 3.0σ(I) were refined to R = 0.054, Rw = 0.071; (μ,η2-BPPF)Fe2(CO)8 (3): monoclinic, space group C2/ c, a = 17.223(7), b = 14.97(2), c = 18.558(3) A, β = 108.39(3)°, V = 4541(6) A3, Z = 4; 3201 data with I > 3.0σ(I) were refined to R = 0.070, Rw = 0.139, as a result of difficulties with modeling electron density peaks associated with highly disordered solvent atoms.