Portal de Periódicos da CAPES

Paper Alert

2001; Elsevier BV; Volume: 9; Issue: 2 Linguagem: Inglês

10.1016/s0969-2126(01)00569-x

1878-4186

Robert Liddington, Christin Frederick, Sophie Jackson,

Wnt/β-catenin signaling in development and cancer

A selection of interesting papers that were published in the month before our press date in major journals most likely to report significant results in structural biology, protein and RNA folding. □ Crystal structure of β-catenin/Tcf complex. Thomas A. Graham, Carole Weaver, Feng Mao, David Kimelman, and Wenqing Xu (2000). Cell 103, 885–896. The Wnt signaling pathway plays critical roles in embryonic development and tumorigenesis. Stimulation of the Wnt pathway results in the accumulation of a nuclear β-catenin–Tcf complex, activating Wnt target genes. A crystal structure of β-catenin bound to the β-catenin-binding domain of Tcf3 (Tcf3–CBD) has been determined. The Tcf3–CBD forms an elongated structure with three binding modules that runs antiparallel to β-catenin along the positively charged groove formed by the armadillo repeats. Structure-based mutagenesis defines three sites in β-catenin that are critical for binding Tcf3–CBD and are differentially involved in binding APC, cadherin, and Axin. December 8, 2000, Cell □ Crystal structure and functional analysis of Ras binding to its effector phosphoinositide 3-kinase γ. Michael E. Pacold, Sabine Suire, Olga Perisic, Samuel Lara-Gonzalez, Colin T. Davis, Edward H. Walker, Phillip T. Hawkins, Len Stephens, John F. Eccleston, and Roger L. Williams (2000). Cell 103, 931–943. Ras activation of phosphoinositide 3-kinase (PI3K) is important for the survival of transformed cells. The authors have determined a crystal structure of a PI3Kγ/Ras·GMPPNP complex. A critical loop in the Ras-binding domain positions Ras so that it uses its switch I and switch II regions to bind PI3Kγ. Mutagenesis shows that interactions with both regions are essential for binding PI3Kγ. Ras also forms a direct contact with the PI3Kγ catalytic domain. The complex with Ras shows a change in the PI3K conformation that might represent an allosteric component of Ras activation and interactions that are likely to be shared by PI3Kα. December 8, 2000, Cell □ The three-dimensional map of microsomal glutathione transferase 1 at 6 Å resolution. I. Schmidt-Krey, K. Mitsuoka, T. Hirai, K. Murata, Y. Cheng, Y. Fujiyoshi, R. Morgenstern, and H. Herbert (2000). EMBO J. 9, 6311–6316. Microsomal glutathione transferase 1 (MGST1) is representative of a superfamily of membrane proteins. The authors propose that this superfamily consists of a new class of membrane proteins built on a common left-handed four-helix bundle motif within the membrane, as determined by electron crystallography of MGST1 at 6 Å resolution. The three-dimensional structure is distinct from that of soluble glutathione transferases. December 1, 2000, The EMBO Journal □ The cellular receptor to human rhinovirus 2 binds around the 5-fold axis and not in the canyon: a structural view. Elizabeth A. Hewat, Emmanuelle Neumann, James F. Conway, Rosita Moser, Bernhard Ronacher, Thomas C. Marlovits, and Dieter Blaas (2000). EMBO J. 19, 6317–6325. Human rhinovirus serotype 2 (HRV2) belongs to the minor group of HRVs that bind to members of the LDL receptor family, including the very low density lipoprotein receptor (VLDL-R). Structures of the complex between HRV2 and soluble fragments of the VLDL-R determined to 15 Å resolution by cryo-electron microscopy are described. The receptor fragments, which include the first three ligand-binding repeats of the VLDL-R (V1–3), bind to the small star-shaped dome on the icosahedral fivefold axis. This is in sharp contrast to the major group of HRVs where the receptor site for ICAM-1 is located at the base of a depression around each fivefold axis. Homology models of the three domains of V1–3 were used to explore the virus–receptor interaction. December 1, 2000, The EMBO Journal □ Crystal structure of the peptidyl-cysteine decarboxylase EpiD complexed with a pentapeptide substrate. Michael Blaesse, Thomas Kupke, Robert Huber, and Stefan Steinbacher (2000). EMBO J. 19, 6299–6310. The crystal structures of the flavoprotein EpiD and EpiD H67N in complex with the substrate pentapeptide DSYTC reveals Rossmann-type monomers arranged as a dodecamer of 23 point symmetry with trimers disposed at the vertices of a tetrahedron. Oligomer formation is essential for binding of the flavin mononucleotide and substrate, which is buried by an otherwise disordered substrate recognition clamp. The unusual substrate-recognition mode and type of chemical reaction performed provide insight into a novel family of flavoproteins. December 1, 2000, The EMBO Journal □ Crystal structure of Hsc20, a J-type co-chaperone from Escherichia coli. Jill R. Cupp-Vickery and Larry E. Vickery (2000). J. Mol. Biol. 304, 835–845. Hsc20 is a 20 kDa J-protein that regulates the ATPase activity and peptide-binding specificity of Hsc66, an hsp70 class molecular chaperone. The crystal structure of Hsc20 from Escherichia coli has been determined to a resolution of 1.8 Å. The overall structure of Hsc20 consists of two distinct domains, an N-terminal J-domain containing residues 1–75 connected by a short loop to a C-terminal domain containing residues 84–171. The structure of the J-domain, involved in interactions with Hsc66, resembles the α-topology of J-domain fragments of E. coli DnaJ and human Hdj1, previously determined by solution NMR methods. The C-terminal domain, implicated in binding and targeting proteins to Hsc66, consists of a three-helix bundle in which two helices comprise an antiparallel coiled-coil. The two domains make contact through an extensive hydrophobic interface (∼650 Å2), suggesting that their relative orientations are fixed. Thus, Hsc20, in addition to its role in the regulation of the ATPase activity of Hsc66, might also function as a rigid scaffold to facilitate positioning of the protein substrates targeted to Hsc66. December 15, 2000, Journal of Molecular Biology □ Structure of membrane-bound annexin A5 trimers: a hybrid cryo-EM-X-ray crystallography study. Frank Oling, Jana Sopkova-de Oliveira Santos, Natalia Govorukhina, Christine Mazères-Dubut, Wilma Bergsma-Schutter, Gert Oostergetel, Wilko Keegstra, Olivier Lambert, Anita Lewit-Bentley, and Alain Brisson (2000). J. Mol. Biol. 304, 561–573. Annexins constitute a family of phospholipid- and Ca2+-binding proteins involved in a variety of membrane-related processes. Several annexins, including annexin A5, self-organize at the surface of lipid membranes into two-dimensional ordered arrays. The structure of annexin A5 trimers in membrane-bound two-dimensional crystals was determined by cryo-electron microscopy in a new crystal form. A two-dimensional projection map was obtained at 6.5 Å resolution, revealing protein densities within each of the four domains characteristic of annexins. This structure is essentially identical to that in three-dimensional crystals, except for small local changes attributed to membrane binding. As a consequence, and contary to the common view, annexin A5 molecules maintain their bent shape and do not flatten upon membrane binding. December 4, 2000, Journal of Molecular Biology □ Rapid determination of protein folds using residual dipolar couplings. C. Andrew Fowler, Fang Tian, Hashim M. Al-Hashimi, and James H. Prestegard (2000). J. Mol. Biol. 304, 447–460. Over the next few years, various genome projects will sequence many new genes and yield many new gene products. A rapid determination of a protein fold, even at low resolution, can aid in the identification of function and expedite the determination of structure at higher resolution. Recently devised NMR methods of measuring residual dipolar couplings provide one route to the determination of a fold. When combined with constraints involving loops connecting elements or other short-range experimental distance information, a fold is produced. This approach is illustrated by fold determination of 15N-labeled Escherichia coli acyl carrier protein using a limited set of 15N-1H and 1H-1H dipolar couplings and a related protein, 13C, 15N-labeled NodF protein, using a more extended data set. November 23, 2000, Journal of Molecular Biology □ Crystal structure of human GM2-activator protein with a novel β-cup topology. Christine Schubert Wright, Su-Chen Li, and Fraydoon Rastinejad (2000). J. Mol. Biol. 304, 411–422. GM2 activator protein (GM2-AP) acts as a cofactor in the sequential degradation of gangliosides. It has been postulated that GM2-AP extracts single GM2 molecules from membranes and presents them in soluble form to β-hexosaminidase A for cleavage and conversion to GM3. The high affinity of GM2-AP for GM2 is based on specific recognition of the oligosaccharide moiety as well as the ceramide lipid tail. The crystal structure reveals a previously unobserved fold whose main feature is an eight-stranded cup-shaped antiparallel β-pleated sheet. The striking feature of the GM2-AP structure is an accessible central hydrophobic cavity rather than a buried hydrophobic core. The dimensions of this cavity are suitable for binding 18-carbon lipid acyl chains. December 1, 2000, Journal of Molecular Biology □ Crystal structure of the spliceosomal 15.5 kDa protein bound to a U4 snRNA fragment. Ivan Vidovic, Stephanie Nottrott, Klaus Hartmuth, Reinhard Lührmann, and Ralf Ficner (2000). Mol. Cell 6, 1331–1342. The authors have determined the crystal structure of a spliceosomal RNP complex comprising the 15.5 kDa protein of the human U4/U6·U5 tri-snRNP and the 5′ stem-loop of U4 snRNA. The protein interacts almost exclusively with a purine-rich (5+2) internal loop within the 5′ stem-loop, giving an unusual RNA fold characterized by two tandem sheared G–A base pairs, a high degree of purine stacking, and the accommodation of a single RNA base, rotated out of the RNA chain, in a pocket of the protein. The complex provides a structure of an important entity in the pre-mRNA splicing apparatus, implies a model for the complex of the 15.5 kDa protein with box C/D snoRNAs, and suggests a general recognition principle in a novel family of RNA-binding proteins. December 2000, Molecular Cell □ Modulation of host signaling by a bacterial mimic: structure of the Salmonella effector SptP bound to Rac1. C. Erec Stebbins, and Jorge E. Galán (2000). Mol. Cell 6, 1449–1460. Salmonella spp. utilize a specialized protein secretion system to deliver a battery of effector proteins into host cells. Several of these effectors stimulate Cdc42- and Rac1-dependent cytoskeletal changes that promote bacterial internalization. These potentially cytotoxic alterations are rapidly reversed by the effector SptP, a tyrosine phosphatase and GTPase-activating protein (GAP) that targets Cdc42 and Rac1. The 2.3 Å resolution crystal structure of an SptP–Rac1 transition state complex reveals an unusual GAP architecture that mimics host functional homologs. The phosphatase domain possesses a conserved active site but distinct surface properties. Binding to Rac1 induces a dramatic stabilization in SptP of a four-helix bundle that makes extensive contacts with the switch I and switch II regions of the GTPase. December 2000, Molecular Cell □ Crystal structure of the hexameric traffic ATPase of the Helicobacter pylori type IV secretion system. Hye-Jeong Yeo, Savvas N. Savvides, Andrew B. Herr, Erich Lanka, and Gabriel Waksman (2000). Mol. Cell 6, 1461–1472. The type IV secretion system of Helicobacter pylori consists of 10–15 proteins responsible for transport of the transforming protein CagA into target epithelial cells. Secretion of CagA crucially depends on the hexameric ATPase, HP0525, a member of the VirB11-PulE family. The authors present the crystal structure of a binary complex of HP0525 bound to ADP. Each monomer consists of two domains formed by the N- and C-terminal halves of the sequence. ADP is bound at the interface between the two domains. In the hexamer, the N- and C-terminal domains form two rings, which together form a chamber open on one side and closed on the other. December 2000, Molecular Cell □ Structure of the AAA ATPase p97. Xiaodong Zhang, Anthony Shaw, Paul A. Bates, Richard H. Newman, Brent Gowen, Elena Orlova, Michael A. Gorman, Hisao Kondo, Pawel Dokurno, John Lally, Gordon Leonard, Hemmo Meyer, Marin van Heel, and Paul S. Freemont (2000). Mol. Cell 6, 1473–1484. p97, an abundant hexameric ATPase of the AAA family, is involved in homotypic membrane fusion. It is thought to disassemble SNARE complexes formed during the process of membrane fusion. The authors report two structures: a crystal structure of the N-terminal and D1 ATPase domains of murine p97 at 2.9 Å resolution, and a cryoelectron microscopy structure of full-length rat p97 at 18 Å resolution. Together, these structures show that the D1 and D2 hexamers pack in a tail-to-tail arrangement, and that the N domain is flexible. A comparison with NSF D2 (ATP complex) reveals possible conformational changes induced by ATP hydrolysis. [Similar results are reported in the same issue by Rouiller et al., Mol. Cell (2000) 6, 1485–1490.] December 2000, Molecular Cell □ Structure of the bacteriophage φ29 DNA packaging motor. Alan A. Simpson, Yizhi Tao, Petr G. Leiman, Mohammed O. Badasso, Yongning He, Paul J. Jardine, Norman H. Olson, Marc C. Morais, Shelley Grimes, Dwight L. Anderson, Timothy S. Baker, and Michael G. Rossmann (2000). Nature 408, 745–750. Motors generating mechanical force, powered by the hydrolysis of ATP, translocate double-stranded DNA into preformed capsids (proheads) of bacterial viruses and certain animal viruses. The crystal structure of the head–tail connector—the central component of the φ29 DNA packaging motor—was fitted into the electron densities of the prohead and of the partially packaged prohead as determined using cryo-electron microscopy. The results suggest that the prohead plus dodecameric connector, prohead RNA, viral ATPase, and DNA comprise a rotary motor with the head-prohead RNA-ATPase complex acting as a stator, the DNA acting as a spindle, and the connector as a ball-race. The helical nature of the DNA converts the rotary action of the connector into translation of the DNA. December 7, 2000, Nature □ Crystal structure of Rac1 in complex with the guanine nucleotide exchange region of Tiam1. David K. Worthylake, Kent L. Rossman, and John Sondek (2000). Nature 408, 682–688. The principal guanine nucleotide exchange factors for Rho family G proteins contain tandem Dbl-homology (DH) and pleckstrin-homology (PH) domains that catalyze nucleotide exchange and the activation of G proteins. The crystal structure of the DH and PH domains of the T-lymphoma invasion and metastasis factor 1 (Tiam1) protein in complex with its cognate Rho family G protein, Rac1, reveals that the two switch regions of Rac1 are stabilized in conformations that disrupt both magnesium binding and guanine nucleotide interaction. The PH domain of Tiam1 does not contact Rac1, and the position and orientation of the PH domain are markedly altered relative to the structure of the uncomplexed, GTPase-free DH/PH element from Sos1. December 7, 2000, Nature □ Structural basis for binding of Smac/DIABLO to the XIAP BIR3 domain. Zhihong Liu, Chaohong Sun, Edward T. Olejniczak, Robert P. Meadows, Stephen F. Betz, Thorsten Oost, Julia Herrmann, Joe C. Wu, and Stephen W. Fesik (2000). Nature 408, 1004–1008. The inhibitor-of-apoptosis proteins (IAPs) regulate programmed cell death by inhibiting members of the caspase family of enzymes. Recently, a mammalian protein called Smac (also named DIABLO) was identified that binds to the IAPs and promotes caspase activation. The solution structure of the BIR3 domain of X-linked IAP (XIAP) in complex with a nine-residue peptide derived from the N terminus of Smac, reveals that the peptide binds across the third β strand of the BIR3 domain in an extended conformation with only the first four residues contacting the protein. These results suggest how Smac may relieve IAP inhibition of caspase-9 activity. [Similar results are presented by Wu et al. Nature 408, 1008–1012.] December 21/28, 2000, Nature □ Mad1–Sin3B interaction involves a novel helical fold. Christian A. E. M. Spronk, Marco Tessari, Anita M. Kaan, Jacobus F. A. Jansen, Michiel Vermeulen, Hendrik G. Stunnenberg, and Geerten W. Vuister (2000). Nat. Struct. Biol. 7, 1100–1104. Sin3A or Sin3B are components of a corepressor complex that mediates repression by transcription factors such as the helix-loop-helix proteins Mad and Mxi. Members of the Mad/Mxi family of repressors play important roles in the transition between proliferation and differentiation by down-regulating the expression of genes that are activated by the proto-oncogene product Myc. The authors report the solution structure of the second paired amphipathic helix (PAH) domain (PAH2) of Sin3B in complex with a peptide comprising the N-terminal region of Mad1. This complex exhibits a novel interaction fold for which they propose the name “wedged helical bundle.” Four α helices of PAH2 form a hydrophobic cleft that accommodates an amphipathic Mad1 α helix, providing a basis for determining the principles of protein interaction and selectivity involving PAH domains. December 2000, Nature Structural Biology □ Crystal structure of a nucleosome core particle containing the variant histone H2A.Z. Robert K. Suto, Michael J. Clarkson, David J. Tremethick, and Karolin Luger (2000). Nat. Struct. Biol. 7, 1121–1124. Incorporation of histone variants, such as H2A.Z, into nucleosomes is thought to establish structurally distinct chromosomal domains correlated with the activation of transcription within chromatin. The authors report the 2.6 Å crystal structure of a nucleosome core particle containing the histone variant H2A.Z. The overall structure is similar to that of the previously reported 2.8 Å nucleosome structure containing major histone proteins. However, distinct localized changes result in the subtle destabilization of the interaction between the (H2A.Z–H2B) dimer and the (H3–H4)2 tetramer. Moreover, H2A.Z nucleosomes have an altered surface that includes a metal ion. This altered surface may lead to changes in higher order structure, and/or could result in the association of specific nuclear proteins with H2A.Z. Incorporation of H2A.Z and H2A within the same nucleosome is unlikely, due to significant changes in the interface between the two H2A.Z–H2B dimers. December 2000, Nature Structural Biology □ Crystal structures of ribosome anti-association factor IF6. Caroline M. Groft, Roland Beckmann, Andrej Sali, and Stephen K. Burley (2000). Nat. Struct. Biol. 7, 1156–1164. Ribosome anti-association factor eIF6 (originally named according to translation initiation terminology as eukaryotic initiation factor 6) binds to the large ribosomal subunit, thereby preventing inappropriate interactions with the small subunit during initiation of protein synthesis. The authors have determined the X-ray structures of two IF6 homologs, Methanococcus jannaschii archaeal aIF6 and Sacchromyces cerevisiae eIF6, revealing a phylogenetically conserved 25 kDa protein consisting of five quasi-identical α/β subdomains arrayed about a fivefold axis of pseudosymmetry. Yeast eIF6 prevents ribosomal subunit association. December 2000, Nature Structural Biology □ Novel DNA binding domain and genetic regulation model of Bacillus subtilis transition state regulator AbrB. Jeffrey L. Vaughn, Victoria Feher, Stephen Naylor, Mark A. Strauch, and John Cavanagh (2000). Nat. Struct. Biol. 7, 1139–1146. The authors have determined the NMR solution structure of the novel DNA-binding domain of the Bacillus subtilis transition state regulator AbrB. Comparisons of the AbrB DNA-binding domain with DNA-binding proteins of known structure show that it is a member of a completely novel class of DNA-recognition fold that employs a dimeric topology for cellular function. This new DNA-binding conformation is referred to as the looped-hinge helix fold. Sequence homology investigations show that this DNA-binding topology is found in other disparately related microbes. December 2000, Nature Structural Biology □ Crystal structure of the bacterial protein export chaperone SecB. Zhaohui Xu, John D. Knafels, and Kae Yoshino (2000). Nat. Struct. Biol. 7, 1172–1177. SecB is a bacterial molecular chaperone involved in mediating translocation of newly synthesized polypeptides across the cytoplasmic membrane of bacteria. The crystal structure of SecB from Haemophilus influenzae shows that the molecule is a tetramer organized as a dimer of dimers. Two long channels run along the side of the molecule. These are bounded by flexible loops and lined with conserved hydrophobic amino acids, which define a suitable environment for binding non-native polypeptides. The structure also reveals an acidic region on the top surface of the molecule, several residues of which have been implicated in binding to SecA, its downstream target. December 2000, Nature Structural Biology □ Crystal structure of an intracellular protease from Pyrococcus horikoshii at 2-Å resolution. Xinlin Du, In-Geol Choi, Rosalind Kim, Weiru Wang, Jaru Jancarik, Hisao Yokota, and Sung-Hou Kim (2000). Proc. Natl. Acad. Sci. USA 97, 14079–14084. The intracellular protease from Pyrococcus horikoshii (PH1704) and PfpI from Pyrococcus furiosus are members of a class of intracellular proteases that have no sequence homology to any other known protease family. The authors report the crystal structure of PH1704 at 2.0 Å resolution. The protease is tentatively identified as a cysteine protease based on the presence of cysteine (residue 100) in a nucleophile elbow motif. In the crystal, PH1704 forms a hexameric ring structure and the active sites are formed at the interfaces between three pairs of monomers. December 19, 2000, Proceedings of the National Academy of Science □ Structural characterization of the human respiratory syncytial virus fusion protein core. Xun Zhao, Mona Singh, Vladimir N. Malashkevich, and Peter S. Kim (2000). Proc. Natl. Acad. Sci. USA 97, 14172–14177. Human respiratory syncytial virus (HRSV) is a major cause of a number of severe respiratory diseases, including bronchiolitis and pneumonia, in infants and young children. Two heptad-repeat regions within the sequence of HRSV F, a glycoprotein essential for viral entry, were predicted to form trimer-of-hairpins-like structures, similar to those found in the fusion proteins of several enveloped viruses. The hairpin structure probably brings the viral and cellular membranes into close apposition, thereby facilitating membrane fusion and subsequent viral entry. An HRSV N/C complex comprised of peptides, denoted HR-N and HR-C, corresponding to the heptad-repeat regions from the N-terminal and C-terminal segments of the HRSV F protein, respectively, was crystallized and its X-ray structure determined. The complex is a six-helix bundle in which the HR-N peptides form a three-stranded, central coiled coil, and the HR-C peptides pack in an antiparallel manner into hydrophobic grooves on the coiled-coil surface. There is remarkable structural similarity between the HRSV N/C complex and the fusion protein core of other viruses, including HIV-1 gp41. December 19, 2000, Proceedings of the National Academy of Science □ Topology to geometry in protein folding: β-lactoglobulin. Ariel Fernandez, Andres Colubri, and R. Stephen Berry (2000). Proc. Natl. Acad. Sci. USA 7, 14062–14066. Evolution of protein structure from random coil to native is first represented topologically by its time-dependent sequences of discretized Ramachandran basins occupied by successive backbone residues. Introducing energetic and entropic criteria at each instant of observation transforms the description from a structurally ambiguous topological representation to an unambiguous geometric picture of the folding process. The method is applied with success to the folding of β-lactoglobulin, which was traditionally perplexing because of its reputed nonhierarchical folding pattern. This molecule passes through a stage, ∼0.1 μs duration, of transient, “flickering” α-helical structure, until a bit of tertiary structure forms that stabilizes the system long enough to allow it to pass to its native β sheet. December 19, 2000, Proceedings of the National Academy of Science □ Aβ amyloid fibrils possess a core structure highly resistant to hydrogen exchange. Indu Kheterpal, Shaolian Zhou, Kelsey D. Cook, and Ronald Wetzel (2000). Proc. Natl. Acad. Sci. USA 97, 13597–13601. The authors describe experiments designed to characterize the secondary structure of amyloid fibrils of the Alzheimer's amyloid plaque peptide Aβ, using hydrogen-deuterium exchange measurements evaluated by mass spectrometry. The results show that 50% of the amide protons of the polypeptide backbone of Aβ(1–40) resist exchange in aqueous, neutral pH buffer even after more than 1000 h of incubation at room temperature. The authors attribute this extensive, strong protection to hydrogen-bonding by residues in core regions of β-sheet structure within the fibril. The backbone amide hydrogens exchange at variable rates, suggesting different degrees of protection within the fibril. These data suggest that it is unlikely that the entire Aβ sequence is involved in hydrogen-bonded secondary structure within the amyloid fibril. Future studies using the methods described here should reveal further details of Aβ fibril structure and assembly. These methods also should be amenable to studies of other amyloid fibrils and protein aggregates. December 5, 2000, Proceedings of the National Academy of Science □ Protein folding and unfolding in microseconds to nanoseconds by experiment and simulation. Ugo Mayor, Christopher M. Johnson, Valerie Daggett, and Alan R. Fersht (2000). Proc. Natl. Acad. Sci. USA 97, 13518–13522. The Engrailed Homeodomain protein has the highest refolding and unfolding rate constants directly observed to date. Temperature jump relaxation measurements gave a refolding rate constant of 37,500 s–1 in water at 25°C, rising to 51,000 s–1 around 42°C. The unfolding rate constant was 1100 s–1 in water at 25°C and 205,000 s–1 at 63°C. The unfolding half-life is extrapolated to be 7.5 ns at 100°C, which allows real-time molecular dynamics unfolding simulations to be tested on this system at a realistic temperature. Preliminary simulations did indeed conform to unfolding on this time scale. Further, similar transition states were observed in simulations at 100°C and 225°C, suggesting that high-temperature simulations provide results applicable to lower temperatures. December 5, 2000, Proceedings of the National Academy of Science □ Cross-talk and ammonia channeling between active centers in the unexpected domain arrangement of glutamate synthase. Claudia Binda, Roberto T. Bossi, Soichi Wakatsuki, Steffi Arzt, Alessandro Coda, Bruno Curti, Maria A. Vanoni, and Andrea Mattevi (2000). Structure 8, 1299–1308. The 3.0 Å crystal structure of the dimeric 324 kDa core protein of a bacterial glutamate synthase was solved by the MAD method, using the very weak anomalous signal of the two 3Fe–4S clusters present in the asymmetric unit. The 1472 amino acids of the monomer fold into a four-domain architecture. The two catalytic domains have canonical Ntn-amidotransferase and FMN binding (β/α)8 barrel folds, respectively. The other two domains have an unusual “cut (β/α)8 barrel” topology and an unexpected novel β-helix structure. Channeling of the ammonia intermediate is brought about by an internal tunnel of 31 Å length, which runs from the site of L-glutamine hydrolysis to the site of L-glutamate synthesis. December 15, 2000, Structure