Portal de Periódicos da CAPES

Paper Alert

2001; Elsevier BV; Volume: 9; Issue: 1 Linguagem: Inglês

10.1016/s0969-2126(00)00563-3

1878-4186

Robert Liddington, Christin Frederick, Jane Clarke, Sophie Jackson,

Cell death mechanisms and regulation

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. □ Structure of Bax: coregulation of dimer formation and intracellular localization. Suzuki M., Youle R. J., and Tjandra, N. (2000). Cell 103, 645–654. Apoptosis is stimulated by the insertion of Bax from the cytosol into mitochondrial membranes. The solution structure of Bax, including the putative transmembrane domain at the C terminus, was determined in order to understand the regulation of its subcellular location. Bax consists of nine α helices where the assembly of helices α1 through α8 resembles that of the apoptosis inhibitor Bcl-xL. The C-terminal α9 helix occupies the hydrophobic pocket proposed previously to mediate heterodimer formation and bioactivity of opposing members of the Bcl-2 family. The Bax structure shows that the orientation of helix α9 provides simultaneous control over its mitochondrial targeting and dimer formation. November 10, 2000, Cell □ Solution structure of the interacting domains of the Mad–Sin3 complex: implications for recruitment of a chromatin-modifying complex. Brubaker, K., Cowley, S. M., Huang, K., Loo, L., Yochum, G. S., Ayer, D. E., Eisenman, R. N., and Radhakrishnan, I. (2000). Cell 103, 655–665. Gene-specific targeting of the Sin3 corepressor complex by DNA-bound repressors is an important mechanism of gene silencing in eukaryotes. The Sin3 corepressor specifically associates with a diverse group of transcriptional repressors, including members of the Mad family, that play crucial roles in development. The NMR structure of the complex formed by the PAH2 domain of mammalian Sin3A with the transrepression domain (SID) of human Mad1 reveals that both domains undergo mutual folding transitions upon complex formation, generating an unusual left-handed four-helix bundle structure and an amphipathic α helix, respectively. The SID helix is wedged within a deep hydrophobic pocket defined by two PAH2 helices. Structure-function analyses of the Mad–Sin3 complex provide a basis for understanding the underlying mechanism(s) that lead to gene silencing. November 10, 2000, Cell □ Structure of the molecular chaperone prefoldin: unique interaction of multiple coiled coil tentacles with unfolded proteins. Siegert, R., Leroux, M. R., Scheufler, C., Hartl, F. U., and Moarefi, I. (2000). Cell 103, 621–632. Prefoldin (GimC) is a hexameric molecular chaperone complex built from two related classes of subunits and present in all eukaryotes and archaea. Prefoldin interacts with nascent polypeptide chains and, in vitro, can functionally substitute for the Hsp70 chaperone system in stabilizing nonnative proteins for subsequent folding in the central cavity of a chaperonin. The authors present the crystal structure and characterization of the prefoldin hexamer from the archaeum Methanobacterium thermoautotrophicum. Prefoldin has the appearance of a jellyfish: its body consists of a double β barrel assembly with six long tentacle-like coiled coils protruding from it. The distal regions of the coiled coils expose hydrophobic patches and are required for multivalent binding of nonnative proteins. November 10, 2000, Cell □ Crystal and solution structures of an HslUV protease–chaperone complex. Sousa, M. C., Trame, C. B., Tsuruta, H., Wilbanks, S. M., Reddy, V. S., and McKay, D. B. (2000). Cell 103, 633–643. HslUV is a “prokaryotic proteasome” composed of the HslV protease and the HslU ATPase, a chaperone of the Clp/Hsp100 family. The 3.4 Å crystal structure of an HslUV complex is presented. Two hexameric ATP-binding rings of HslU bind intimately to opposite sides of the HslV protease; the HslU “intermediate domains” extend outward from the complex. The solution structure of HslUV, derived from small angle X-ray scattering data under conditions where the complex is assembled and active, agrees with this crystallographic structure. When the complex forms, the C-terminal helices of HslU distend and bind between subunits of HslV, and the apical helices of HslV shift substantially, transmitting a conformational change to the active-site region of the protease. November 10, 2000, Cell □ X-ray structures of the universal translation initiation factor IF2/eIF5B: conformational changes on GDP and GTP binding. Roll-Mecak, A., Cao, C., Dever, T. E., and Burley, S. K. (2000). Cell 103, 781–792. X-ray structures of the universal translation initiation factor IF2/eIF5B have been determined in three states: free enzyme, inactive IF2/eIF5B·GDP, and active IF2/eIF5B·GTP. The “chalice-shaped” enzyme is a GTPase that facilitates ribosomal subunit joining and Met-tRNAi binding to ribosomes in all three kingdoms of life. The conserved core of IF2/eIF5B consists of an N-terminal G domain (I) plus an EF-Tu-type β barrel (II), followed by a novel αβα-sandwich (III) connected via an α helix to a second EF-Tu-type β barrel (IV). Structural comparisons reveal a molecular lever, which amplifies a modest conformational change in the Switch 2 region of the G domain induced by Mg2+/GTP binding over a distance of 90 Å from the G domain active center to domain IV. November 22, 2000, Cell □ Structural basis for double-sieve discrimination of L-valine from L-isoleucine and L-threonine by the complex of tRNAVal and Valyl-tRNA synthetase. Fukai, S., Nureki, O., Sekine, S.-I., Shimada, A., Tao, J., Vassylyev, D. G., and Yokoyama, S. (2000). Cell 103, 793–803. Valyl-tRNA synthetase (ValRS) strictly discriminates the cognate L-valine from the larger L-isoleucine and the isosteric L-threonine by the tRNA-dependent “double sieve” mechanism. The authors have determined the 2.9 Å crystal structure of a complex of Thermus thermophilus ValRS, tRNAVal, and an analog of the Val-adenylate intermediate. The analog is bound in a pocket, where Pro41 allows accommodation of the valine and threonine moieties but precludes the isoleucine moiety (the first sieve), on the aminoacylation domain. The editing domain, which hydrolyzes incorrectly synthesized Thr-tRNAVal, is bound to the 3′ adenosine of tRNAVal. A contiguous pocket was found to accommodate the threonine, but not the valine moiety (the second sieve). Furthermore, another threonine-binding pocket for Thr-adenylate hydrolysis was suggested on the editing domain. November 22, 2000, Cell □ Crystal structure of the Xrcc4 DNA repair protein and implications for end joining. Junop, M. S., Modesti, M., Guarné, A., Ghirlando, R., Gellert, M., and Yang, W. (2000). EMBO J. 19, 5962–5970. XRCC4 carries out non-homologous DNA end joining (NHEJ) in all eukaryotes and, in particular, V(D)J recombination in vertebrates. The crystal structure of a functional fragment of Xrcc4 reveals a long dumb-bell-like tetramer. Each of the N-terminal globular head domains consists of a β sandwich and a potential DNA-binding helix-turn-helix motif. The C-terminal stalk comprising a single α helix >120 Å in length interacts with DNA ligase IV. The structure suggests a possible mode of coupling ligase IV association with DNA binding for effective ligation of DNA ends. November 15, 2000, The EMBO Journal □ The structural basis for the recognition of acetylated histone H4 by the bromodomain of histone acetyltranferase Gcn5p. Owen, D. J., Ornaghi, P., Yang, J.-C., Lowe, N., Evans, P. R., Ballario, P., Neuhaus, D., Filetici, P., and Travers, A. A. (2000). EMBO J. 19, 6141–6149. The bromodomain is an ∼ 110 amino acid module found in histone acetyltransferases. The crystal structure of the S. cerevisiae Gcn5p bromodomain complexed with a peptide corresponding to residues 15–29 of histone H4 acetylated at the ζ-N of Lys16 has been determined. The structure shows that the primary interaction is with the N-acetyl lysine which binds in a cleft; specificity is provided by the interaction of the amide nitrogen of a conserved asparagine with the oxygen of the acetyl carbonyl group. Additional sidechain binding occurs in a shallow depression. These findings suggest that the Gcn5p bromodomain may discriminate between different acetylated lysine residues depending on the context in which they are displayed. November 1, 2000, The EMBO Journal □ Solution structure of the phosphoryl transfer complex between the signal transducing proteins HPr and IIAGlucose of the Escherichia coli phosphoenolpyruvate: sugar phosphotransferase system. Wang, G., Louis, J. M., Sondej, M., Seok, Y.-J., Peterkofsky, A., and Clore, G. M. (2000). EMBO J. 19, 5635–5649. The solution structure of the second protein–protein complex of the Escherichia coli phosphoenolpyruvate: sugar phosphotransferase system, that between histidine-containing phosphocarrier protein (HPr) and glucose-specific enzyme IIAGlucose (IIAGlc), has been determined by NMR spectroscopy, including the use of dipolar couplings to provide long-range orientational information and newly developed rigid body minimization and constrained/restrained simulated annealing methods. A protruding convex surface on HPr interacts with a complementary concave depression on IIAGlc. Comparisons with the structures of the enzyme I–HPr and IIAGlc–glycerol kinase complexes reveal how similar binding surfaces can be formed with underlying backbone scaffolds that are structurally dissimilar. November 1, 2000, The EMBO Journal □ The structure of the mRNA export factor TAP reveals a cis arrangement of a non-canonical RNP domain and an LRR domain. Liker, E., Fernandez, E., Izaurralde, E., and Conti, E. (2000). EMBO J. 19, 5587–5598. Human TAP is implicated in mRNA nuclear export and is used by simian type D retroviruses to export their unspliced genomic RNA to the cytoplasm of the host cell. The crystal structure of the minimal TAP fragment that binds the constitutive transport element (CTE) of retroviral RNAs consists of a ribonucleoprotein (RNP) domain, which is not identifiable by its sequence, and a leucine-rich repeat (LRR) domain. The structural and biochemical properties of the domains point to a remarkable similarity with the U2B″(RNP)-U2A′ (LRR) spliceosomal heterodimer. November 1, 2000, The EMBO Journal □ The molecular basis of FHA domain:phosphopeptide binding specificity and implications for phospho-dependent signaling mechanisms. Durocher, D., Taylor, I. A., Sarbassova, D., Haire, L. F., Westcott, S. L., Jackson, S. P.,. Smerdon, S. J., and Yaffe, M. B. (2000). Mol. Cell 6, 1169–1182. Forkhead-associated (FHA) domains are a class of ubiquitous signaling modules that appear to function through interactions with phosphorylated target molecules. The authors have used oriented peptide library screening to determine the optimal phosphopeptide binding motifs recognized by several FHA domains, including those within a number of DNA damage checkpoint kinases, and determined the X-ray structure of Rad53p-FHA1, in complex with a phosphothreonine peptide. The structure reveals a striking similarity to the MH2 domains of Smad tumor suppressor proteins and reveals a mode of peptide binding that differs from SH2, 14-3-3, or PTB domain complexes. November 2000, Molecular Cell □ Crystal structure of eukaryotic DNA ligase-adenylate illuminates the mechanism of nick sensing and strand joining. Odell, M., Sriskanda, V., Shuman, S., and Nikolov, D. B. (2000). Mol. Cell 6, 1183–1193. Chlorella virus DNA ligase is the smallest eukaryotic ATP-dependent ligase known; it has an intrinsic nick-sensing function and suffices for yeast cell growth. The authors report the 2.0 Å crystal structure of the covalent ligase-AMP reaction intermediate. The conformation of the adenosine nucleoside and contacts between the enzyme and the ribose sugar have undergone a significant change compared to complexes of T7 ligase with ATP or mRNA capping enzyme with GTP. The conformational switch allows the 3′ OH of AMP to coordinate directly the 5′ PO4 of the nick. The structure explains why nick sensing is restricted to adenylated ligase and why the 5′ phosphate is required for DNA binding. November 2000, Molecular Cell □ Crystal structure of yeast Esa1 suggests a unified mechanism for catalysis and substrate binding by histone acetyltransferases. Yan, Y., Barlev, N. A., Haley, R. H., Berger, S. L., and Marmorstein, R. (2000). Mol. Cell 6, 1195–1205. Esa1 is the catalytic subunit of the NuA4 histone acetylase (HAT) complex that acetylates histone H4, and is a member of the MYST family of HAT proteins that includes the MOZ oncoprotein and the HIV-1 Tat interacting protein Tip60. The authors report the crystal structure of the HAT domain of Esa1 bound to coenzyme A. Esa1 contains a central core domain harboring a putative catalytic base, and flanking domains that are implicated in histone binding. Comparisons with the Gcn5/PCAF and Hat1 proteins suggest a unified mechanism of catalysis and histone binding by HAT proteins, whereby a structurally conserved core domain mediates catalysis, and sequence variability within a structurally related N- and C-terminal scaffold determines substrate specificity. November 2000, Molecular Cell □ Structural basis for nucleotide exchange and competition with tRNA in the yeast elongation factor complex eEF1A:eEF1Bα. Gregers Rom Andersen, Lise Pedersen, Louis Valente, Ishita Chatterjee, Terri Goss Kinzy, Morten Kjeldgaard, and Jens Nyborg (2000). Mol. Cell 6, 1261–1266. The crystal structure of a complex between the protein biosynthesis elongation factor eEF1A (formerly EF-1α) and the catalytic C terminus of its exchange factor, eEF1Bα (formerly EF-1β), was determined to 1.67 Å resolution. One end of the nucleotide exchange factor is buried between the switch 1 and 2 regions of eEF1A and destroys the binding site for the Mg2+ ion associated with the nucleotide. The second end of eEF1Bα interacts with domain 2 of eEF1A in the region hypothesized to be involved in the binding of the CCA-aminoacyl end of the tRNA. The competition between eEF1Bα and aminoacylated tRNA may be a central element in channeling the reactants in eukaryotic protein synthesis. The recognition of eEF1A by eEF1Bα is very different from that observed in the prokaryotic EF-Tu:EF-Ts complex. Recognition of the switch 2 region in nucleotide exchange is, however, common to the elongation factor complexes and those of Ras:Sos and Arf1:Sec7. November 2000, Molecular Cell □ Structural basis for the activation of 20S proteasomes by 11S regluators. Whitby, F. G., Masters, E. I., Kramer, L., Knowlton, J. R., Yao, Y., Wang, C. C., and Hill, C. P. (2000). Nature 408, 115–120. Most of the non-lysosomal proteolysis that occurs in eukaryotic cells is performed by the 20S proteasome. Substrates access the active sites, which are sequestered in an internal chamber, by traversing a narrow opening (α annulus) that is blocked in the unliganded 20S proteasome by N-terminal sequences of α subunits. 11S regulators (also called PA26, PA28 and REG) are heptamers that stimulate 20S proteasome peptidase activity in vitro and may facilitate product release in vivo. The co-crystal structure of yeast 20S proteasome with the 11S regulator from Trypanosoma brucei (PA26) is reported. The PA26 C-terminal tails insert into pockets on the 20S proteasome, and PA26 activation loops induce conformational changes in α subunits that open the gate separating the proteasome interior from the intracellular environment. November 2, 2000, Nature □ Structure and assembly of the Alu domain of the mammalian signal recognition particle. Weichenrieder, O., Wild, K., Strub, K., and Cusack, S. (2000). Nature 408, 167–173. The Alu domain of the mammalian signal recognition particle (SRP) comprises the heterodimer of proteins SRP9 and SRP14 bound to the 5′ and 3′ terminal sequences of SRP RNA. It retards the ribosomal elongation of signal-peptide-containing proteins before their engagement with the translocation machinery in the endoplasmic reticulum. The authors report two crystal structures of the heterodimer SRP9/14 bound either to the 5′ domain or to a construct containing both 5′ and 3′ domains. SRP9/14 binds strongly to the conserved core of the 5′ domain, which forms a U-turn connecting two helical stacks. The Alu domain structure is probably conserved in other cytoplasmic ribonucleoprotein particles and retroposition intermediates. November 9, 2000, Nature □ Insights into SCF ubiquitin ligases from the structure of the Skp1–Skp2 complex. Schulman, B. A., Carrano, A. C., Jeffrey, P. D., Bowen, Z., Kinnucan, E. R. E., Finnin, M. S., Elledge, S. J., Harper, J. W., Pagano, M., and Pavletich, N. P. (2000). Nature 408, 381–386. F-box proteins are members of a large family that regulates the cell cycle, the immune response and signalling cascades by targeting proteins for ubiquitination. F-box proteins are the substrate-recognition components of SCF (Skp1-Cullin-F-box protein) ubiquitin–protein ligases. They bind the SCF constant catalytic core by means of the F-box motif interacting with Skp1, and they bind substrates through their variable protein–protein interaction domains. The crystal structure of the human F-box protein Skp2 bound to Skp1 shows that Skp1 recruits the F-box protein through a bipartite interface involving both the F-box and the substrate-recognition domain. The structure raises the possibility that different Skp1 family members evolved to function with different subsets of F-box proteins, and suggests that the F-box protein may not only recruit substrate, but may also position it optimally for the ubiquitination reaction. November 16, 2000, Nature □ The structure of the central stalk in bovine F1-ATPase at 2.4 Å resolution. Gibbons, C., Montgomery, M. G., Leslie, A. G. W., and Walker, J. E. (2000). 7, 1055–1061. The central stalk in ATP synthase, made of γ, δ, and ϵ subunits in the mitochondrial enzyme, is the key rotary element in the enzyme's catalytic mechanism. The γ subunit penetrates the catalytic αβ3 domain and protrudes beneath it, interacting with a ring of c subunits in the membrane that drives rotation of the stalk during ATP synthesis. In other crystals of F1-ATPase, the protrusion was disordered, but with crystals of F1-ATPase inhibited with dicyclohexylcarbodiimide the complete structure was revealed. The δ and ϵ subunits interact with a Rossmann fold in the γ subunit, forming a foot. In ATP synthase, this foot interacts with the c-ring and couples the transmembrane proton motive force to catalysis in the αβ3 domain. (Rodgers and Wilce also report the structure of the γϵ complex of ATP synthase in the same issue [Nat. Struct. Biol. 7, 1051–1054].) November 2000, Nature Structural Biology □ Structure of a dioxygen reduction enzyme from Desulfovibrio gigas. Carlos Frazão, Gabriela Silva, Cláudio M. Gomes, Pedro Matias, Ricardo Coelho, Larry Sieker, Sofia Macedo, Ming Y. Liu, Solange Oliveira, Miguel Teixeira, António V. Xavier, Claudina Rodrigues-Pousada, Maria A. Carrondo, and Jean Le Gall (2000). Nat. Struct. Biol. 7, 1041–1045. Desulfovibrio gigas is a strict anaerobe that contains a well-characterized metabolic pathway that enables it to survive transient contacts with oxygen. The terminal enzyme in this pathway, rubredoxin:oxygen oxidoreductase (ROO), reduces oxygen to water in a direct and safe way. The crystal structure of ROO shows that each monomer of this homodimeric enzyme consists of a novel combination of two domains, a flavodoxin-like domain and a Zn-β-lactamase-like domain that contains a di-iron center for dioxygen reduction. This is the first structure of a member of a superfamily of enzymes widespread in strict and facultative anaerobes, indicating its broad physiological significance. November 2000, Nature Structural Biology □ Point mutations alter the mechanical stability of immunoglobulin modules. Li, H., Carrion-Vazquez, M., Oberhauser, A. F., Marszalek, P. E., and Fernandez, J. M. (2000). Nat. Struct. Biol. 7, 1117–1120. Immunoglobulin-like modules are common components of proteins that play mechanical roles in cells such as muscle elasticity and cell adhesion. Mutations in these proteins may affect their mechanical stability and thus may compromise their function. Using single molecule atomic force microscopy (AFM) and protein engineering, the authors demonstrate that point mutations in two β strands of an immunoglobulin module in human cardiac titin alter the mechanical stability of the protein, resulting in mechanical phenotypes. The results demonstrate a previously unrecognized class of phenotypes that may be common in cell adhesion and muscle proteins. December 2000, Nature Structural Biology □ An evolutionary bridge to a new protein fold. Cordes, M. H. J., Burton, R. E., Walsh, N. P., McKnight, C. J., and Sauer, R. T. (2000). Nat. Struct. Biol. 7, 1129–1132. Arc repressor bearing the N11L substitution (Arc-N11L) is an evolutionary intermediate between the wild-type protein, in which the region surrounding position 11 forms a β sheet, and a double mutant “switch Arc,” in which this region is helical. The authors show that Arc-N11L is able to adopt either the wild type or mutant conformations. Exchange between these structures occurs on the millisecond time scale in a dynamic equilibrium in which the relative populations of each fold depend on temperature, solvent conditions, and ligand binding. The N11L mutation serves as an evolutionary bridge from the β sheet to the helical fold because in the mutant, leucine is an integral part of the hydrophobic core of the new structure but can also occupy a surface position in the wild-type structure. Conversely, the polar Asn11 sidechain serves as a negative design element in wild type Arc because it cannot be incorporated into the core of the mutant fold. December 2000, Nature Structural Biology □ The structural basis for red fluorescence in the tetrameric GFP homolog DsRed. Wall, M. A., Socolich, M., and Ranganathan, R. (2000). Nat. Struct. Biol. 7, 1133–1138. Green fluorescent protein (GFP) has rapidly become a standard tool for investigating a variety of cellular activities, and has served as a model system for understanding spectral tuning in chromophoric proteins. Distant homologs of GFP in reef coral and anemone display two new properties of the fluorescent protein family: dramatically red-shifted spectra, and oligomerization to form tetramers. The authors report the 1.9 Å crystal structure of DsRed, a red fluorescent protein from Discosoma coral. DsRed monomers show similar topology to GFP, but additional chemical modification to the chromophore extends the conjugated π-system and probably accounts for the red-shifted spectra. Oligomerization of DsRed occurs at two chemically distinct protein interfaces to assemble the tetramer. The DsRed structure reveals the chemical basis for the functional properties of red fluorescent proteins and provides the basis for rational engineering of this subfamily of GFP homologs. December 2000, Nature Structural Biology □ Crystal structure of the bacterial protein export chaperone SecB. Xu, Z., Knafels, J. D., and Yoshino, K. (2000). Nat. Struct. Biol. 7, 1172–1177. SecB is a bacterial molecular chaperone involved in mediating the 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 nonnative 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 yeast initiation factor 4A, a DEAD-box RNA helicase. Caruthers, J. M., Johnson, E. R., and McKay, D. B. (2000). Proc. Natl. Acad. Sci. USA 97, 13080–13085. The eukaryotic translation initiation factor 4A (eIF4A) is a member of the DEA(D/H)-box RNA helicase family, a diverse group of proteins that couples an ATPase activity to RNA binding and unwinding. Previous work has provided the structure of the N-terminal, ATP-binding domain of eIF4A. Extending those results, the authors have solved the structure of the C-terminal domain of eIF4A with data to 1.75 Å resolution; it has a parallel αβ topology that superimposes, with minor variations, on the structures and conserved motifs of the equivalent domain in other, distantly related helicases. Using data to 2.8 Å resolution and molecular replacement with the refined model of the C-terminal domain, the authors have completed the structure of full-length eIF4A; it is a “dumbbell” structure consisting of two compact domains connected by an extended linker. Interactions with ATP and the DEA(D/H) motif provide a mechanism for coupling ATP binding and hydrolysis with conformational changes that modulate RNA binding. November 2000, Proceedings of the National Academy of Science □ Dynamics and folding of single two-stranded coiled-coil peptides studied by fluorescent energy transfer confocal microscopy. Talaga, D. S., Lau, W. L., Roder, H., Tang, J., Jia, Y., DeGrado, W. F., and Hochstrasser, R. M. (2000). Proc. Natl. Acad. Sci. USA 97, 13021–13026. The authors report single-molecule measurements on the folding and unfolding conformational equilibrium distributions and dynamics of a disulfide crosslinked version of the two-stranded coiled coil from GCN4. The peptide has a fluorescent donor and acceptor at the N termini of its two chains and a cysteine disulfide near its C terminus. Thus, folding brings the two N termini of the two chains close together, resulting in an enhancement of fluorescent resonant energy transfer. End-to-end distance distributions have thus been characterized under conditions where the peptide is nearly fully folded (0 M urea), unfolded (7.4 M urea), and in dynamic exchange between folded and unfolded states (3.0 M urea). The distributions have been compared for the peptide freely diffusing in solution and deposited onto aminopropyl silanized glass. As the urea concentration is increased, the mean end-to-end distance shifts to longer distances both in free solution and on the modified surface. The widths of these distributions indicate that the molecules are undergoing millisecond conformational fluctuations. Under all three conditions, these fluctuations gave nonexponential correlations on 1–100 ms time scale. A component of the correlation decay that was sensitive to the concentration of urea corresponded to that measured by bulk relaxation kinetics. The trajectories provided effective intramolecular diffusion coefficients as a function of the end-to-end distances for the folded and unfolded states. Single-molecule folding studies provide information concerning the distributions of conformational states in the folded, unfolded, and dynamically interconverting states. November 21, 2000, Proceedings of the National Academy of Science □ Allosteric effects of Pit-1 DNA sites on long-term repression in cell type specification. Scully, K. M., Jacobson, E. M., Jepsen, K., Lunyak, V., Viadiu, H., Carrière, C., Rose, D. W., Hooshmand, F., Aggarwal, A. K., and Rosenfeld, M. G. (2000). Science 290, 1127–1131. Reciprocal gene activation and restriction during cell type differentiation from a common lineage is a hallmark of mammalian organogenesis. A key question, then, is whether a critical transcriptional activator of cell type-specific gene targets can also restrict expression of the same genes in other cell types. The authors show that whereas the pituitary-specific POU domain factor Pit-1 activates growth hormone gene expression in one cell type, the somatotrope, it restricts its expression from a second cell type, the lactotrope. Comparison of two Pit-1 binding site cocrystal structures, with prolactin and growth hormone respectively, demonstrates that the distinction depends on a two-base pair spacing in accommodation of the bipartite POU domains on a conserved growth hormone promoter site. The allosteric effect on Pit-1, in combination with other DNA-binding factors, results in the recruitment of a corepressor complex, including nuclear receptor corepressor N-CoR, which, unexpectedly, is required for active long-term repression of the growth hormone gene in lactotropes. November 10, 2000, Science □ Crystal structure of the human acyl protein thioesterase I from a single X-ray data set to 1.5 Å. Devedjiev, Y., Dauter, Z., Kuznetsov, S. R., Jones, T. L. Z., and Derewenda, Z. S. (2000). Structure 8, 1137–1146. The crystal structure of the human putative Gα-regulatory protein acyl thioesterase (hAPT1) was solved with a single data set collected from a crystal containing the wild-type protein. The phases were calculated to 1.8 Å resolution based on anomalous scattering from Br− ions introduced in the cryoprotectant solution in which the crystal was soaked for 20 seconds. November 15, 2000, Structure □ Solution structures of two CCHC zinc fingers from the FOG family protein U-shaped that mediate protein–protein interactions. Liew, C. K., Kowalski, K., Fox, A. H., Newton, A., Sharpe, B. K., Crossley, M., and Mackay, J. P. (2000). Structure 8, 1157–1166. NMR spectroscopy was used to determine the first structures of two FOG family zinc fingers that are involved in protein–protein interactions: fingers 1 and 9 from U-shaped. These fingers resemble classical TFIIIA-like zinc fingers, with the exception of an unusual extended portion of the polypeptide backbone prior to the fourth zinc ligand. [15N,1H]-HSQC titrations were used to define the GATA binding surface of USH-F1, and comparison with other FOG family proteins indicates that the recognition mechanism is conserved across species. November 15, 2000, Structure □ Structure of the Tie2 RTK domain self-inhibition by the nucleotide binding loop, activation loop, and C-terminal tail. Shewchuk, L. M., Hassell, A. M., Ellis, B., Holmes, W. D., Davis, R., Horne, E. L., Kadwell, S. H., McKee, D. D., and Moore, J. T. (2000). Structure 8, 1105–1113. Tie2 (also known as Tek) is an endothelium-specific receptor tyrosine kinase involved in angiogenesis. The crystal structure contains the catalytic core, the kinase insert domain (KID), and the C-terminal tail. The overall fold is similar to that observed in other kinase structures; however, the Tie2 nucleotide-binding loop is in an inhibitory conformation, which is not seen in other kinase structures, while its activation loop adopts an “activated-like” conformation in the absence of phosphorylation. Conformational changes in the nucleotide-binding loop, activation loop, C helix and the C-terminal tail all appear to be required for ATP and substrate binding. November 15, 2000, Structure