A High Resolution, Hard X-ray Bio-imaging Facility at SSRL
J.C. Andrews, S. Brennan, C. Patty, K. Luening, P. Pianetta, E. Almeida, M.C.H. van der Meulen, M. Feser, J. Gelb, J. Rudati, A. Tkachuk and W.B. Yun Synchrotron Radiation News 21, 17-26 (MAY/JUN 2008)

The new X-ray imaging facility at the Stanford Synchrotron Radiation Laboratory, bsed on an Xradia nano-XCT full-field transmission x-ray microscope, can provide complementary and unique capabilities to the current microscopy methods for studying complex biological systems.

Molecular and Structural Basis of Cytokine Receptor Pleiotropy in the Interleukin-4/13 System
S.L. LaPorte, Z.S. Juo, J. Vaclavikova, L.A. Colf, X. Qi, N.M. Heller, A.D. Keegan and K.C. Garcia, Cell 132(2), 259-272 (APR 2008)

Interleukin-4 and Interleukin-13, cytokines critical to the development of T cell-mediated humoral immune responses, exert their actions through different combinations of shared receptors. In this issue, LaPorte et al. report the crystal structures of the three cytokine-receptor ternary complexes that comprise the IL-4/IL-13 system, providing molecular insight into the manner in which signaling specificity is attained by these cytokines in spite of binding similar receptor complexes. Significantly, the study reveals different assembly properties and signaling potencies of the receptor complex in response to each cytokine, suggesting that the extracellular cytokine-receptor interactions are modulating intracellular membrane-proximal signaling events. The multiplicity of signaling pathways and receptor complex combinations in the IL-4/13 system is depicted in schematic fashion based on a 2007 Madrid metro map. The starting and ending points of each pathway are the individual cytokines, receptors, or ternary signaling complexes. The intermediate binary complexes are depicted as "hubs" in the metro system, where signaling can proceed in either of two opposing directions. For instance, the IL-4/IL-4Ra complex can utilize the "Blue line" to form the type I complex, or the "Red line" to form the type II complex. As with most modern metro system maps, individual lines (pathways) are represented by different colors. The width of the "tracks" are related to the strength of the binding. The cover image is based on an initial concept by Heather Deacon (Biographica), Sherry LaPorte, and Chris Garcia and was redrawn by the Cell art department.

Structure of Human Synaptotagmin 1 C2AB in the Absence of Ca²⁺ Reveals a Novel Domain Association
K. L. Fuson, M. Montes, J. J. Robert and R. B. Sutton, Biochemistry 46, 13041 (DEC 2007)

Rendering of human synaptotagmin1 C2AB. The C2B domain is presented as the blue-colored domain, while the "inactive" C2A domain is colored green. This C2A-C2B association may represent a novel regulatory mechanism for Ca2+-dependent exocytosis.

Asymmetric Distribution of Metals in the Xenopus laevis Oocyte: A Synchrotron X-ray Fluorescence Microprobe Study
B.G.Gh. Popescu, Z.R. Belak, K. Ignatyev, N. Ovsenek and H. Nichol, Biochem. Cell Biol. 85, 537-542 (DEC 2007)

Inset image: Synchrotron X-ray fluorescence mapping reveals the asymetric distribution of iron in an intact Xenopus laevis oocyte. Image: Popescu et. al, University of Saskatchewan Background image: Human breast cancer cells MCF-7 immunostained for microtubules (alpha-tubulin) in green and transcription factor Sp1 in red, with nuclei in blue. Image provided by Shihua He, University of Manitoba.

Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution
P.D. Jadzinsky, G. Calero, C.J. Ackerson, D.A. Bushnell and R.D. Kornberg, Science. 318, 430 (OCT 2007)

Structure of a gold nanoparticle in which the central atoms are packed in a decahedron, surrounded by additional layers of gold atoms in unanticipated geometries. Gold atoms, gold; sulfur atoms, blue; carbon atoms, white; oxygen atoms, red; the superimposed red mesh depicts the electron-density distribution determined by x-ray crystallography. Image: Pablo D. Jadzinsky and Guillermo Calero.

Structures of GRP94-Nucleotide Complexes Reveal Mechanistic Differences between the hsp90 Chaperones
D.E. Dollins, J.J. Warren, R.M. Immormino and D.T. Gewirth, Mol. Cell 28, 41 (OCT 2007)

The high-resolution structure of the mammalian endoplasmic reticulum Hsp90 chaperone GRP94 with bound ATP. The extent to which a common mechanism applies to all Hsp90 chaperones has been controversial. In cytosolic Hsp90s, ATP binding results in N-terminal dimerization that is critical for ATP hydrolysis and subsequent chaperone function. Dollins et al. show that nucleotide-bound GRP94 adopts a conformation that precludes N-terminal dimerization yet demonstrate GRP94-catalyzed ATPase activity using kinetic analyses, suggesting that nucleotide binding is not the major driving force for catalytically productive conformational changes. The differences between GRP94 and Hsp90 were further localized to their N-terminal domains, which adopt different conformations in response to nucleotide binding.

Ferroelectric Self-assembled PbTiO₃ Perovskite Nanostructures onto (100)SrTiO₃ Substrates from a Novel Microemulsion Aided Sol-Gel Preparation Method
M.L. Cazada, M. Torres, L.E. Fuentes-Cobas, A. Mehta, J. Ricote and L. Pardo, Nanotechnology 37, 375603 (SEP 2007)

Structure and Orientation of the Mn₄Ca Cluster in Plant Photosystem II Membranes Studied by Polarized Range-extended X-ray Absorption Spectroscopy
Y. Pushkar, J. Yano, P. Glatzel, J. Messinger, A. Lewis, K. Sauer, U. Bergmann and V.K. Yachandra, J. Biol. Chem. 282, 7198 (MAR 2007)

The photosynthetic water-oxidizing Mn₄Ca cluster in photosystem II is located on the luminal side of the thylakoid membrane of chloroplasts in the leafs of plants. The details about the orientation and structure of the Mn₄Ca cluster (manganese is red, calcium is green, and oxygen is brown) in the membrane are described in the article.

Structural and Kinetic Evidence for an Extended Hydrogen Bonding Network in Catalysis of Methyl Group Transfer: Role of an Active Site Asparagine Residue in Activation of Methyl Transfer by Methyltransferases
T.I. Doukov, H. Hemmi, C.L. Drennan and S.W. Ragsdale, J. Biol. Chem. 282, 6609 (MAR 2007)

How to activate a tertiary amine? Nature has put an asparagine residue instead of an acid around the contact point of the protein and the N5 methyltetrahydrofolate. The authors suggest that an extended hydrogen bond network is responsible for the protonation of N5 of the folate and the asparagine 199 plays an important role in stabilizing a transition state or high energy intermediate for methyl transfer.

Description of the Ground-State Covalencies of the Bis(dithiolato) Transition-Metal Complexes from X-ray Absorption Spectroscopy and Time-Dependent Density-Functional Calculations
K. Ray, S. DeBeer George, E.I. Solomon, K. Wieghardt and F. Neese Chemistry - A European Journal 13(2), 2783-2797 (FEB 2007)

A new methodology based on quasi-relativistic time-dependent density functional theory (TD-DFT) has been applied in order to interpret experimental covalencies from the sulfur K-edge pre-edge intensities obtained for a series of transition-metal dithiolene complexes.

Vaccinia Virus N1L Protein Resembles a B Cell Lymphoma-2 (Bcl-2) Family Protein
M. Aoyagi, D. Zhai, C. Jin, A.E. Aleshin, B. Stec, J.C. Reed and R.C. Liddington, Protein Science 16, 118 (JAN 2007)

The N1L protein of vaccinia and variola (Smallpox) viruses is critical for virus survival and propagation within host cells. The crystal structure of N1L (upper stereo pair) reveals a striking homology to host proteins of the Bcl-2 family (compared in lower pair), which regulate apoptosis, or programmed cell death, a host defense mechanism against pathogens. In vitro binding studies support a direct role for N1L in modulating apoptosis, a finding that should provide new leads for the development of antiviral therapies and vaccines to treat a future Smallpox outbreak.

Elemental Compositions of Comet 81P/Wild 2 Samples Collected by Stardust
G.J. Flynn et al., Science 314, 1731 (DEC 2006)

A large particle of comet dust collected by the NASA Stardust mission generated a carrot-shaped track in a 3-cm-deep silica tile as it was captured. Like the thousands of other particles returned by the mission, this one decelerated from high speed inside the silica aerogel.

Structural Basis of Transcription: Role of the Trigger Loop in Substrate Specificity and Catalysis
D. Wang, D.A. Bushnell, K.D. Westover, C.D. Daplan and R.D. Kornberg, Cell 127, 941 (DEC 2006)

X-ray crystal structures of RNA polymerase II (pol II) transcribing complexes reveal a key step in the transcription mechanism. The pol II "trigger loop" forms a network of interactions with a nucleoside triphosphate (NTP) in the active center. When base, sugar, and phosphates are all correct, a histidine residue of the trigger loop is aligned with the ß phosphate, facilitating nucleophilic attack by the RNA 3'-OH and phosphodiester bond formation. In this way, the trigger loop couples nucleotide selection to catalysis. The cover shows DNA in cyan, RNA in red, GTP in orange, Mg²⁺; ions in magenta, the trigger loop in gold, bridge helix in silver, and additional pol II residues (Rpb1-752, Rpb2-766, Rpb2-1020) in gray. Nucleophilic attack and phosphoanhydride bond breakage are indicated by white arrows.

FeMo Cofactor Maturation on NifEN and
Nitrogenase Fe Protein: A Molybdate/Homocitrate Insertase
Y. Hu, M.C. Corbett, A.W. Fay, J.A. Webber, K.O. Hodgson, B. Hedman and M.W. Ribbe, PNAS 103, 17119-17130 (NOV 2006)

The FeMo cofactor (spheres), which is the active site of substrate reduction in the Mo-nitrogenase (comprising Fe protein and MoFe protein), is located within the a-subunit of MoFe protein (tubes). The final steps of FeMoco biosynthesis involve the insertion of molybdenum and homocitrate into the NifEN-bound FeMoco precursor by Fe protein and the transfer of the cluster to its destined location in MoFe protein upon protein-protein interaction.

Type IV Pilus Structure by Cryo-Electron Microscopy and Crystallography: Implications for Pilus Assembly and Functions
L. Craig, N. Volkmann, A.S. Arvai, M.E. Pique, M. Yeager, E.H. Egelman and J.A. Tainer, Molecular Cell 23, 651 (SEP 2006)

Type IV pilus filaments on Gram-negative bacterial pathogens mediate motility, attachment, immune escape, and natural transformation. Key roles in bacterial virulence plus prominent cell surface exposure, as shown in this scanning electron micrograph of Neisseria gonorrhoea diplococci (background, courtesy of Charles Brinton), make pili attractive targets for vaccines and therapeutics.

Molecular and Electronic Structures of Oxo-bis(benzene-1,2-dithiolato)chromate(V) Monoanions. A Combined Experimental and Density Functional Study
R. Kapre, K. Ray, I. Sylvestre, T. Weyhermüller, S. DeBeer George, F. Neese and K. Wieghardt, Inorg. Chem 45, 3499 (MAY 2006)

The X-ray structures of two monoanionic oxobis(benzene-1,2-dithiolato)chromate(V) complexes show a remarkable folding about the S-S vectors of the dithiolate ligands, leading to a lower local C_s symmetry compared to the more intuitively expected C_2v symmetry. A detailed theoretical study (DFT, DKH2, and ZORA) in combination with spectroscopic data (UV-vis, MCD, EPR, and XAS) reveals that C_s instead of C_2v symmetry affords a stabilization due to strong S(3p)--> Cr(3d_x2-_y2) p donation.

Metallogenomics and Biological X-ray Absorption Spectroscopy
I. Ascone, R. Fourme, S. Hasnain and K. Hodgson, J. Synchrotron Radiat. 12, 1 (JAN 2005)

The most important steps of the metallogenomics pipeline (see Ascone, Fourme, Hasnain and Hodgson, pages 1-3). This is a composite image including figures from this issue. (Corbett et al., pages 28-34 and Scott et al., pages 19-22).

Lensless Imaging of Magnetic Nanostructures by X-ray Spectro-holography
S. Eisebitt, J. Lüning, W.F. Schlotter, M. Lörgen, O. Hellwig, W. Eberhardt and J. Stöhr, Nature 432, 885 (16 DEC 2004)

This cover shows a random magnetic domain pattern in a thin film, imaged by x-ray holography with spectroscopic contrast. Spatial resolution is 50 nm establishing it as a practical x-ray imaging technique today, with potential for ultra-fast snapshots using x-ray lasers in the future.

SSRL Facility Update: SPEAR3, SPPS, LCLS
R. Hettel, J. Hastings and J. Galayda, SRN 17, 37 (SEP/OCT 2004)

Photograph showing the SPEAR3 ring shortly after the completion of the major installation activites in October 2003. (Photo: P. Ginter, 2003)

Crystal Structure of the Catalytic Core of Inosital 1,4,5-Trisphosphate 3-Kinase
G.J. Miller and J.H. Hurley, Molecular Cell 15, 703 (10 SEP 2004)

Agranoff's turtle, released from the confines of the lipid bilayer into the cytoplasmic sea, swims toward the catalytic core of the Ins(1,4,5)P3 3-kinase. While the structure of this inositol kinase resembles both protein and lipid kinase folds, a novel helical domain dictates its exquisitely specific reactivity for a soluble inositol phosphate.

Crystal Structure of the Catalytic Core of Inosital 1,4,5-Trisphosphate 3-Kinase
M.B. Austin, M.E. Bowman, J.-L. Ferrer, J. Schröder and J.P. Noel, Chemistry & Biology 11, 1179 (SEP 2004)

Resveratrol, an antifungal polyketide-derived stilbene natural product produced by grape and a few other plant species, has traditionally entered the human diet via the consumption of red wine. Linked to the health benefits of moderate wine consumption, resveratrol has indeed been shown in laboratory tests to possess an impressive repertoire of medicinal properties. Recently, resveratrol was also shown to significantly increase longevity. Although resveratrol production is quite rare in the plant kingdom, the iterative, multifunctional stilbene synthases that produce resveratrol are closely related to chalcone synthases, which are ubiquitous in higher plants. Austin et al. present the first crystal structures of stilbene synthases, along with the surprising molecular basis for these enzymes' divergent cyclization specificity. Aside from yielding mechanistic insight into polyketide cyclization in general, this discovery of an "aldol switch" also facilitates the mutagenic conversion of chalcone synthases into efficient resveratrol synthases. Wine photograph (image background) by Marc Lieberman.

Crystal Structure of a b-Catenin/APC Complex Reveals a Critical Role for APC Phosphorylation in APC Function
Y. Xing, W.K. Clements, I. Le Trong, T.R. Hinds, R. Stenkamp, D. Kimelman and W. Xu, Molecular Cell 15, 523 (27 AUG 2004)

In the foreground are three views (rotated around the long axis) of the structure of ß-catenin bound to a phosphorylated APC fragment. In the background is a representation of the crypts and villi of the colon, which become cancerous when ß-catenin degradation is no longer properly regulated due to mutations in APC.

Selenium Biotransformations in an Insect Ecosystem: Effects of Insects on Phytoremediation
D.B. Vickerman, J.T. Trumble, G.N. George, I.J. Pickering and H. Nichol, Environ. Sci. Technol. 38, 3581 (JUL 2004)

Danel Vickerman of the University of California at Riverside photographed this beet armyworm, which harbors an immature wasp parasitoid larva, on a sprig of alfalfa. An X-ray absorption spectroscopy study at SSRL was used to study the biotransformation of selenium as it moves through the ecosystem - from irrigation water through the plant and armyworm and finally to the parasitoid.

Pore Morphologies in Disordered Nanoporous Thin Films
J.A. Hedstrom, M.F. Toney, E. Huang, H.-C. Kim, W. Volksen, T. Magbitang and R.D. Miller, Langmuir 20, 1535 (2 MAR 2004)

The complicated pore morphology of many nanoporous materials affects the materials" properties, but is often difficult to accurately characterize. A method of generating representative 3D morphologies using SAXS data has been developed and applied to nanoporous methyl silsesquioxane (MSSQ) films that were generated by the incorporation of a sacrificial polymeric component (porogen) into the matrix and subsequent removal by thermolysis. Shown here are representative pore morphologies as a function of porogen loading; the yellow represents the MSSQ matrix and the pore surface is highlighted in red as seen through the side of a cube. This visualization of the pore topology permits a determination of the transition from closed pores (5 and 10% in this case) to interconnected pores (15%) to a bicontinuous morphology (>25%). The methodology that has been developed will be valuable for characterization of a variety of nanoscale, two-phase materials, including polymer. (image by Mike Toney)

Structural Rationale for the Broad Neutralization of HIV-1 by Human Monoclonal Antibody 447-52D
R.L. Stanfield, M.K. Gorny, C. Williams, S. Zolla-Pazner and I.A. Wilson, Structure 12, 193 (FEB 2004)

The HIV-1 envelope protein gp120 mutates rapidly in response to immunological pressure in order to escape antibody recognition and neutralization. Sequence variability in gp120 is a key obstacle in the development of an HIV-1 vaccine that would be effective against the primary viruses encountered worldwide. The crystal structure of HIV-1 neutralizing antibody 447-52D, in complex with a gp120 peptide, reveals how the immune system solves this problem through antibody binding to the peptide backbone of one of the HIV-1 hypervariable loops. Furthermore, specificity for gp120 is achieved through insertion of the highly conserved tip of the otherwise hypervariable V3 loop into a pocket in the antibody-combining site.

Towards Automated Data Collection at the Stanford Synchrotron Radiation Laboratory
H. van den Bedem, M.D. Miller and G. Wolf, SRN 16, 15 (NOV/DEC 2003)

Abstract view of the sample cassette that forms part of the user cassette kit designed for SSRL's robotic sample changing systems on the macromolecular crystallography beamlines.

Structure of Actin Cross-linked with a-Actinin: A Network of Bundles
O. Pelletier, E. Pokidysheva, L.S. Hirst, N. Bouxsein, Y. Li and C.R. Safinya, Phys. Rev. Lett 91, 148102
(3 OCT 2003)

Illustration of a proposed structure of a filamentous actin bundle at a branching site, showing how the protein a-actinin (red) induces a networklike structure in the F-actin protein by forming cross links between its filaments. The model is based on a synchrotron x-ray scattering study which indicates that the a-actinin forms a disordered square lattice within the bundle.

Structural Characterization and Comparison of RGD Cell-adhesion Recognition Sites Engineered into Streptavidin
I. Le Trong, T.C. McDevitt, K.E. Nelson, P.S. Stayton and R.E. Stenkamp, Acta Cryst. D 59, 828 (MAY 2003)

Atomic displacement parameters (50% ellipsoids) for a streptavidin subunit containing an Arg-Gly-Asp (RGD) sequence from fibronectin.

Electronic Structure Contributions to Electron-Transfer Reactivity in Iron-Sulfur Active Sites:
1. Photoelectron Spectroscopic Determination of Electronic Relaxation, p. 679
2. Reduction Potentials, p. 689
3. Kinetics of Electron Transfer, p. 696
P. Kennepohl and E.I. Solomon, Inorg. Chem. 42 (10 FEB 2003)

Redox processes are commonly assumed to occur without a significant change in electronic structure on oxidation (the frozen orbital approximation). Photoelectron spectroscopy has been used to quantitatively evaluate the changes in electronic structure that occur upon oxidation through the presence of satellite peaks, which gain intensity due to this electronic relaxation. For high-spin iron sites as in Rubredoxin, electronic relaxation is observed to be extremely large and its influence on redox properties (E⁰, l , H_DA, and ET pathways) has been evaluated.

Angle-resolved Photoemission Studies of the Cuprate Superconductors
A. Damascelli, Z. Hussain and Z.-X. Shen, Rev. Mod. Phys. 75, 473 (APR 2003)

Crystal Structures of Reversible Ketone-based Inhibitors of the Cysteine Protease Cruzain
L. Huang, L.S. Brinen and J.A. Ellman, Bioorg. Med. Chem. 11, 21 (2 JAN 2003)

The crystal structures of two hydroxymethyl ketone inhibitors complexed to the cysteine protease cruzain have been determined at 1.1 and 1.2 Å resolution, respectively. These high resolution crystal structures provide the first structures of non-covalent inhibitors bound to cruzain. A series of compounds were prepared and tested based upon the structures providing further insight into the key binding interactions.

Looking at Trace Impurities on Silicon Wafers with Synchrotron Radiation
K. Baur, S. Brennan, P. Pianetta and R. Opila, Anal. Chem. A-Pages 74, 609 (1 DEC 2002)

Checking silicon wafers for minute impurities at the Stanford Synchrotron Radiation Laboratory.

High Resolution 3D X-ray Diffraction Microscopy
J. Miao, T. Ishikawa, B. Johnson, E. H. Anderson, B. Lai and K.O. Hodgson, Phys. Rev. Lett. 89, 088303 (19 AUG 2002)

High-resolution x-ray diffraction pattern of a buried nanostructure; using a series of such patterns, 3D images of nonperiodic structures are reconstructed with a 50-nm resolution. The technique used could lead to x-ray diffraction microscopy capable of imaging a single biomolecule at near-atomic resolution.

Atomic Structures of Human Dihydrofolate Reductase Complexed with NADPH and Two Lipophilic Antifolates at 1.09 Å and 1.05 Å Resolution.
A.E. Klon, A. Héroux, L.J. Ross, V. Pathak, C.A. Johnson, J.R. Piper and D.W. Borhani, J. Mol. Biol. 320, 677 (12 July 2002)

Atomic resolution crystal structure of human DHFR demonstrates the relative order (inhibitor SRI-9439, thermal ellipsoids colored from blue to red, 8-17 Å2) and disorder (cofactor NADPH, 10-60 Å2) of the DHFR ligands.

Structural Flexibility, an Essential Component of the Allosteric Activation in Escherichia coli Glucosamine-6-phosphate Deaminase
E. Rudino-Pinera, S. Morales-Arrieta, S. P. Rojas-Trejo and E. Horjales, Acta Cryst. D 58, 10 (JAN 2002)

The molecular surface of the homohexameric allosteric enzyme glucosamine-6-phosphate deaminase from E. coli, showing aproaching views of the active and allosteric sites during the alosteric transition process, where the role of the structural flexibility is crucial. The protein surface is colored according to the behaviour during the allosteric transition: cyan, the internal zone; yellow the external zone and magenta, the active-site lid.

Crystal Structure of ATP Sulfurylase from Penicillium chrysogenum: Insights into the Allosteric Regulation of Sulfate Assimilation
I.J. MacRae, I.H. Segel and A.J. Fisher, Biochemistry 40, 6795 (12 JUN 2001)

(cover appeared on the Vol. 41, Issue 13, 2 APR 2002 cover)

From NEXAFS Spectroscopy to New Flat Panel Displays: A Story of Basic Science with Technological Impact
J. Stöhr, M.G. Samant and J. Lüning, SRN 14, 32 (NOV/DEC 2001)

The role of NEXAFS spectroscopy has played in the development of new flat panel displays is a story of basic science with technological impact.

Oligomerization and Ligand Binding in a Homotetrameric Hemoglobin: Two High-resolution Crystal Structures of Hemoglobin Bart's (g₄), a Marker for a-Thalassemia
R.D. Kidd, H.M. Baker, A.J. Mathews, T. Brittain and E.N. Baker, Protein Sci. 10, 1739 (10 SEP 2001)

Structure of the carbonmonoxy form of hemoglobin Bart's (g₄). Ribbon diagram of the CO-g₄ tetramer.

Structure of MsbA from E. coli: A Homolog of the Multidrug Resistance ATP Binding Cassette (ABC) Transporters
C. Chang and C.B. Roth, Science 293, 1793 (7 SEP 2001)

Crystal structure of the multidrug resistance ABC transporter homolog MsbA from E. coli as viewed from the plane of the lipid bilayer. This class of integral membrane proteins transports hydrophobic molecules such as lipids and drugs across the cell membrane bilayer. The structure of MsbA reveals three domains, which include the transmembrane (red), intracellular (dark blue), and nucleotide-binding (teal) domains. The structure can help elucidate the mechanism underlying multidrug resistance in the treatment of cancer and infectious diseases.

Structural Basis of Transcription: An RNA Polymerase II Elongation Complex at 3.3 Å Resolution
A.L. Gnatt, P. Cramer, J. Fu, D.A. Bushnell and R.D. Kornberg, Science 292, 1876 (8 JUN 2001)

The enzyme RNA polymerase II in the act of transcribing a gene. X-ray crystal structure comprises the protein (gray, except for orange "clamp" and green "bridge" helix), DNA (blue template strand, green nontemplate strand), and RNA (red). The pink sphere is an active center Mg²⁺ ion. Double-stranded DNA enters from the right and unwinds before the active center. The unwound nontemplate DNA strand is obscured by motion or disorder.

Nucleotide Binding by the Histidine Kinase CheA
A.M. Bilwes, C.M. Quezada, L.R. Croal, B.R. Crane and M.I. Simon,
8, 353 (APR 2001)

The nucleotide binding domain of CheA in complex with two different ATP analogs, ADPCP (electron density) and TNP-ATP (stick model). CheA is the protein kinase component of a bacterial signal transduction system that regulates the swimming behavior of bacteria.

Structural Basis of the Antagonism between Inorganic Mercury and Selenium in Mammals
G. Gailer, G.N. George, I.J. Pickering, S. Madden, R.C. Prince, E.Y. Yu, M. Bonner Denton, H.S. Younis and H. Vasken Aposhian, Chem. Res. Toxicol. 13, 1135 (NOV 2000)

Schematic of a possible mechanism underlying the mutual detoxification of mercury and selenium.

The 1.0 Å Crystal Structure of Ca²⁺-bound Calmodulin: An Analysis of Disorder and Implications for Functionally Relevant Plasticity
M.A. Wilson and A.T. Brunger, J. Mol. Biol. 301, 1237 (SEP 2000)

Ribbon and ADP representations of CaM.

A Transient Interaction between Two Phosporelay Proteins Trapped in a Crystal Lattice Reveals the Mechanism of Molecular Recognition and Phosphotransfer in Signal Transduction
J. Zapf, U. Sen, Madhusudan, J.A. Hoch and K.I. Varughese, Structure 8, 851 (15 AUG 2000)

Spo0F-Spo0B associate and loop movements in Spo0F. View of the Spo0B-Spo0F complex down the axis of the four-helix bundle. The two protomers of the Spo0B dimer are shown in blue and dark green. The two Spo0F molecules are shown in magenta. The sites of phosporylation, His30 of Spo0B and Asp54 of Spo0F, are in close proximity for phosphoryltransfer and are colored red.

Effect of Framework Polymerization of the Phase Stability of Periodic Silica/Surfactant Nanostructured Composites
A.F. Gross, E.J. Ruiz and S.H. Tolbert, J. Phys. Chem. B 104, 5448 (15 JUN 2000)

The transition of hexagonal cylinders breaking open and becoming lamellar layers is illustrated. The transition propagates from one pore to another in a direction perpendicular to the axis of the pore. A transition of the sort illustrated here would have an n parameter of 1.

Architecture of RNA Polymerase II and Implications for the Transcription Mechanism
P. Cramer, D.A. Bushnell, J. Fu, A.L. Gnatt, B. Maier-Davis, N.E. Thompson, R.R. Burgess, A.M. Edwards, P.R. David and R.D. Kornberg, Science 288, 640 (28 APR 2000)

Two views of a backbone model of RNA polymerase II, the central enzyme of gene expression, derived from x-ray crystallography. DNA, depicted as a blue helix, was placed in the structure on the basis of results from electron crystallography. The direction of transcription is from right to left in the view at the upper left and from back to front in the view at the lower right. A pink sphere identifies a metal ion at the active center.

(FEB 2000)

Crystal Structures of the Toxoplasma gondii Hypoxanthine-guanine Phosphoribosyltransferase-GMP and -IMP Complexes: Comparison of Purine Binding Interactions with the XMP Complex
A. Heroux, E.L. White, L.J. Ross and D.W. Borhani, Biochemistry 38, 14485 (2 NOV 1999) and
Crystal Structure of Toxoplasma gondii Hypoxanthine-guanine Phosphoribosyltransferase with XMP, Pyrophosphate, and Two Mg²⁺ Ions Bound: Insights into the Catalytic Mechanism
A. Heroux, E.L. White, L.J. Ross, R.L. Davis and D.W. Borhani, Biochemistry 38, 14495 (2 NOV 1999)

Motions that accompany HGPRT catalysis.

Crystallization of Truncated Human Apolipoprotein A-I in Novel Conformation
D.W. Borhani, J.A. Engler and C.G. Brouillete, 55, 1578 (SEP 1999)

Molecular-packing model, shown projected along the a axis, for an apo D(1-43)A-I extended helical rod. Each monomer is coloured from red at the N-terminus to blue at the C-terminus.

X-ray Crystal Structures of 70S Ribosome Functional Complexes
J.H. Cate, M.M. Yusupov, G.Zh. Yusupova, T.N. Earnest and H.F. Noller, Science 285, 2095 (24 SEP 1999)

(Upper left) X-ray crystal structure of the Thermus thermophilus 70S ribosome at 7.8 Å resolution. Transfer RNAs (green, blue, and red) bound to the aminoacyl, peptidyl, and exit sites, respectively, occupy the cavity between the 30S (light blue) and 50S (gray) ribosomal subunits. (Lower right) The two ribosomal subunits are moved apart to show the relative orientations of the three transfer RNAs. (note: much of the data collection was done at the ALS)

Human Glutathione Transferase A4-4 Crystal Structures and Mutagenesis Reveal the Basis of High Catalytic Efficiency with Toxic Lipid Peroxidation Products
C.M. Bruns, I. Hubatsch, M. Ridderstrom, B. Mannervik and J.A. Tainer, J. Mol. Biol. 288, 427 (7 MAY 1999)

Specificity canyon of GST A4-4 with a model of the glutathione backbone (pink) and the HNE residue (green) are shown in the binding pocket, which is colored blue for Tyr212 and magenta around the other residues which are conserved in GST A4-4 relative to other alpha class GSTs (Phe111 and Val216). The hydroxyl group of Tyr212 (blue patch, center) makes a specific contact to the aldehydic oxygen atom of the substrate in this model.

Neutron-scattering Studies Reveal Further Details of the Ca²⁺/Calmodulin-dependent Activation Mechanism of Myosin Light Chain Kinase
J.K. Krueger, G. Zhi, J.T. Stull and J. Trewhella, Biochemistry 37, 13997 (15 SEP 1998)

Application of XAFS to Materials and Environmental Science
S.D. Conradson, Appl. Spectrosc. 52, 252A (JUL 1998)

Structural Basis for G*C Recognition in the DNA Minor Groove
C.L. Kielkopf, E.E. Baird, P.B. Dervan and D.C. Rees, Nat. Struct. Biol. 5, 104 (FEB 1998)

Conformational Substates in Enzyme Mechanism: The 120 K Structure of a-lytic Protease at 1.5 Å Resolution
S.D. Rader and D.A. Agard, Prot. Sci. (JUL 1997)

Correlated motions in the binding pocket of a-lytic protease. This subset of structures from the 16- conformation refinement shows correlation in atomic positions across the residues of the P1 binding pocket. The atoms from the same structure (e.g., Gly 216 C_a from structure 14 and Met 190 C_a from structure 14, leftmost structure) are shifted in the same direction relative to the mean position from each atom, suggesting that residues across the binding pocket are not moving independently. Atoms are colored according to atom type: yellows, C; dark blue, N; red, O; green, S.

Crystal Structures of Human DNA Polymerase b Complexed with Gapped and Nicked DNA: Evidence for an Induced Fit Mechanism
M.R. Sawaya, R. Prasad, S.H. Wilson, J. Kraut and H. Pelletier, Biochemistry 36, 11205 (16 SEP 1997)

2.2 Å crystal structure of human DNA polymerase beta (pol beta) complexed with gapped DNA substrate and ddCTP. DNA in every cell of the human body is spontaneously damaged more than 10,000 times every day. Pol beta fills in single nucleotide gaps in DNA that are formed when damaged nucleotides are excised by the base excision repair pathway. The most prominent feature in this structure is the 90 degree kink in the template strand, located in the polymerase's active site. Such a kink is suggested to enhance fidelity of the polymerase catalyzed reaction, nucleotidyl transfer.

First XAFS with a YB₆₆ Monochromator
M. Rowen, Z.U. Rek, J. Wong, G.N. George, I.J. Pickering, G.H.V. Via and G.E. Brown, Jr., SRN 6, 25 (NOV/DEC 1993)

Three-dimensional Structure of Myosin Subfragment-1: A Molecular Motor
I. Rayment, W.R. Rypniewski, K. Schmidt-Base, R. Smith, D.R. Tomchick, M.M. Benning, D.A. Winkelmann, G. Wesenberg, H.M. Holden, Science 261, 50 (2 JUL 1993)

A space-filling representation of all of the atoms in the current model of myosin S1.

X-ray Tomographic Study of Chemical Vapor Infiltration Processing of Ceramic Composites
J.H. Kinney, T.M. Breunig, T.L. Starr, D. Haupt, M.C. Nichols, S.R. Stock, M.D. Butts, and R.A. Saroyan, Science 260, 789 (7 MAY 1993)

Three-dimensional Structure of an Oncogene Protein: Catalytic Domain of Human c-H-ras p21
A.M. de Vos, L. Tong, M.V. Milburn, P.M. Matias, J. Jancarik, S. Noguchi, S. Nishimura, K. Miura, E. Ohtsuka and S.-H. Kim, Science 239, 888 (19 FEB 1988)

The backbone structure of human c-H-ras oncogene protein. The flow of the backbone is represented by a continuous ribbon using a program BSRIBBON. The guanosine diphosphate molecule is shown as a dot-surface using the program PSFRODO.