Ammonium is one of the most important nitrogen sources for bacteria, fungi, and plants, but it is toxic to animals. The ammonium transport proteins (methylamine permeases͞ammonium transporters͞rhesus) are present in all domains of life; however, functional studies with members of this family have yielded controversial results with respect to the chemical identity (NH 4 ؉ or NH 3) of the transported species. We have solved the structure of wild-type AmtB from Escherichia coli in two crystal forms at 1.8-and 2.1-Å resolution, respectively. Substrate transport occurs through a narrow mainly hydrophobic pore located at the center of each monomer of the trimeric AmtB. At the periplasmic entry, a binding site for NH 4 ؉ is observed. Two phenylalanine side chains (F107 and F215) block access into the pore from the periplasmic side. Further into the pore, the side chains of two highly conserved histidine residues (H168 and H318) bridged by a H-bond lie adjacent, with their edges pointing into the cavity. These histidine residues may facilitate the deprotonation of an ammonium ion entering the pore. Adiabatic free energy calculations support the hypothesis that an electrostatic barrier between H168 and H318 hinders the permeation of cations but not that of the uncharged NH 3. The structural data and energetic considerations strongly indicate that the methylamine permeases͞ammonium transporters͞rhesus proteins are ammonia gas channels. Interestingly, at the cytoplasmic exit of the pore, two different conformational states are observed that might be related to the inactivation mechanism by its regulatory partner.conformational change ͉ x-ray structure

The type II restriction endonuclease EcoRV was crystallized as a complex with the substrate DNA undecamer AAAGATATCTT (recognition sequence underlined). These crystals diffract to much better resolution (2 A) than was the case for the previously reported complex with the decamer GGGATATCCC [Winkler, F. K., Banner, D. W., Oefner, C., Tsernoglou, D., Brown, R. S., Heathman, S. P., Bryan, R. K., Martin, P. D., Petratos, K., & Wilson, K. S. (1993) EMBO J. 12, 1781-1795]. The crystal structure contains one dimer complex in the asymmetric unit and was solved by molecular replacement. The same kinked DNA conformation characteristic for enzyme-bound cognate DNA is observed. Crystals, soaked with Mg2+, show the essential cofactor bound at only one active site of the dimer, and the DNA is not cleaved. The Mg2+ has one oxygen from the scissile phosphodiester group and two carboxylate oxygens, one form Asp74 and one from Asp90, in its octahedral ligand sphere. The scissile phosphodiester group is pulled by 1 A toward the Mg2+. After substrate cleavage in solution, isomorphous crystals containing the enzyme--product--Mg2+ complex were obtained. In this structure, each of the 5'-phosphate groups is bound to two Mg2+. The kinked DNA conformation is essentially maintained, but the two central adenines, 3' to the cleavage sites, form an unusual cross-strand base stacking. The structures have been refined to R factors of 0.16 at 2.1-2.0 A resolution maintaining very good stereochemistry. On the basis of these structures and inspired by recent kinetic data [Vipond, I. B., & Halford, S. E. (1994) Biochemistry (second paper of three in this issue)], we have constructed a transition state model with two metals bound to the scissile phosphorane group.

Preparation of yeast proteinsEndogenous S. cerevisiae ten-subunit Pol II core enzyme was prepared as described 1 . An E. coli expression vector was derived from pET21b (Novagen) for the coexpression of the translational fusion of S. cerevisiae Rpb4:20 glycine linker:TFIIB and Rpb7:His 6 under the control of separate T7 promoters. Details of the vector design are available on request. Following expression in E. coli, cells were lysed by sonication in buffer A (50 mM Tris, 150 mM NaCl; pH 7.5, 0.3 mg/L leupeptin, 1.4 mg/L pepstatin A, 0.17 g/L PMSF, 0.33 g/L benzamidine and 10 mM β -mercaptoethanol). The lysate was cleared by centrifugation and applied to a Ni-NTA agarose column (Qiagen). The column was washed with buffer A containing 2 M NaCl, and the protein was eluted with a gradient of 10 mM to 200 mM imidazole in buffer A containing 150 mM NaCl. Peak fractions were diluted twofold and loaded onto a Mono-S cation exchange column (Amersham) equilibrated with buffer A containing 100 mM NaCl. The fusion protein was eluted over a total of 15 column volumes with a gradient of 0.1-1 M NaCl in buffer A. Peak fractions were concentrated and applied to a Superose 6 gel filtration column (Amersham) equilibrated with buffer B (5 mM HEPES pH 7.25, 40 mM ammonium sulfate, 10 μM ZnCl 2 , 10 mM DTT). Peak fractions were concentrated, shock-frozen in liquid nitrogen, and stored at −80°C. The TBP core domain (S. cerevisiae residues 61-240) expression vector was a generous gift from Dr. Sean Juo. Expression and purification of the yeast TBP core domain was as described 2 except that Superose 12 size exclusion chromatography was performed with buffer B. Peak fractions were concentrated, shock-frozen in liquid nitrogen, and stored at −80°C. 10-subunit Pol II was incubated with two molar equivalents of nucleic acid scaffold (Template, 5'-cgacacagcatcaaatgcacgatgtaacttttataggcgcccaacc;Nontemplate, 5'-ggttgggcgcctataaaagttacatcgtgcaaaatcgttatgagaa; RNA, 5'-gctgtgtcg) as described 3 and 2.5 molar equivalents of TBP. After incubation for 20 minutes at 20°C 3-5 molar equivalents of TFIIB-Rpb4/7 fusion protein were added. After incubation for 20 min. at 20°C, the complex was purified on a Superose 6 size exclusion column (Amersham). Fractions corresponding to the complex were pooled and concentrated to 4 mg/ml.Crystallization, data collection, and structure determination Crystals were grown at 20 °C using the hanging drop vapor diffusion method by mixing 1.5 µl of sample solution with 1.5 µl of reservoir solution (800 mM sodium ammonium tartrate, 100 mM HEPES pH 7.5, 5 mM DTT). Crystals were transferred stepwise to mother solution containing additionally 0-22% glycerol over 8 h, slowly cooled down to 8 °C, incubated for another 24 h, and plunged into liquid nitrogen. Diffraction data were collected in 0.75° increments at the protein crystallography beamline ID 29 at ESRF. Diffraction data were processed with XDS and scaled with XSCALE 4 . The structure was solved by molecular replacement with PHASER 5 using the first 12-subunit Pol II ...

Cytosolic DNA arising from intracellular pathogens triggers a powerful innate immune response. It is sensed by cyclic GMP-AMP synthase (cGAS), which elicits the production of type I interferons by generating the second messenger 2'3'-cyclic-GMP-AMP (cGAMP). Endogenous nuclear or mitochondrial DNA can also be sensed by cGAS under certain conditions, resulting in sterile inflammation. The cGAS dimer binds two DNA ligands shorter than 20 base pairs side-by-side, but 20-base-pair DNA fails to activate cGAS in vivo and is a poor activator in vitro. Here we show that cGAS is activated in a strongly DNA length-dependent manner both in vitro and in human cells. We also show that cGAS dimers form ladder-like networks with DNA, leading to cooperative sensing of DNA length: assembly of the pioneering cGAS dimer between two DNA molecules is ineffective; but, once formed, it prearranges the flanking DNA to promote binding of subsequent cGAS dimers. Remarkably, bacterial and mitochondrial nucleoid proteins HU and mitochondrial transcription factor A (TFAM), as well as high-mobility group box 1 protein (HMGB1), can strongly stimulate long DNA sensing by cGAS. U-turns and bends in DNA induced by these proteins pre-structure DNA to nucleate cGAS dimers. Our results suggest a nucleation-cooperativity-based mechanism for sensitive detection of mitochondrial DNA and pathogen genomes, and identify HMGB/TFAM proteins as DNA-structuring host factors. They provide an explanation for the peculiar cGAS dimer structure and suggest that cGAS preferentially binds incomplete nucleoid-like structures or bent DNA.

Transcription of ribosomal RNA by RNA polymerase (Pol) I initiates ribosome biogenesis and regulates eukaryotic cell growth. The crystal structure of Pol I fromthe yeast Saccharomyces cerevisiae at 2.8A˚ resolution reveals all 14 subunits of the 590-kilodalton enzyme, and shows differences to Pol II. An ‘expander’ element occupies the DNA template site and stabilizes an expanded active centre cleft with an unwound bridge helix. A ‘connector’ element invades the cleft of an adjacent polymerase and stabilizes an inactive polymerase dimer. The connector and expander must detach during Pol I activation to enable transcription initiation and cleft contraction by convergent movement of the polymerase ‘core’ and ‘shelf’ modules. Conversion between an inactive expanded and an active contracted polymerase state may generally underlie transcription. Regulatory factors can modulate the core–shelf interface that includes a ‘composite’ active site for RNA chain initiation, elongation, proofreading and termination

DNA in the eukaryotic nucleus is packaged in the form of nucleosomes, ~147 base pairs of DNA wrapped around a histone protein octamer. The position and histone composition of nucleosomes is governed by ATP dependent chromatin remodelers1–3 such as the 15 subunit INO80 complex4. INO80 regulates gene expression, DNA repair and replication by sliding nucleosomes, exchanging histone H2A.Z with H2A, and positioning +1 and -1 nucleosomes at promoter DNA5–8. A structure and mechanism for these remodeling reactions is lacking. Here we report the cryo-electron microscopy structure at 4.3Å resolution, with parts at 3.7Å, of an evolutionary conserved core INO80 complex from Chaetomium thermophilum bound to a nucleosome. INO80core cradles one entire gyre of the nucleosome through multivalent DNA and histone contacts. A Rvb1/2 AAA+ ATPase hetero-hexamer is an assembly scaffold for the complex and acts as stator for the motor and nucleosome gripping subunits. The Swi2/Snf2 ATPase motor binds to SHL-6, unwraps ~15 base pairs, disrupts the H2A:DNA contacts and is poised to pump entry DNA into the nucleosome. Arp5-Ies6 grip SHL-2/-3 acting as counter grip for the motor on the other side of the H2A/H2B dimer. The Arp5 insertion domain forms a grappler element that binds the nucleosome dyad, connects the Arp5 core and entry DNA over a distance of ~90Å and packs against histone H2A/H2B near the acidic patch. Our structure together with biochemical data8 suggest a unified mechanism for nucleosome sliding and histone editing by INO80. The motor pumps entry DNA across H2A/H2B against Arp5 and the grappler, sliding nucleosomes as a ratchet. Transient exposure of H2A/H2B by the motor and differential recognition of H2A.Z and H2A may regulate histone exchange during translocation.

Random screening provided no suitable lead structures in a search for novel inhibitors of the bacterial enzyme DNA gyrase. Therefore, an alternative approach had to be developed. Relying on the detailed 3D structural information of the targeted ATP binding site, our approach combines as key techniques (1) an in silico screening for potential low molecular weight inhibitors, (2) a biased high throughput DNA gyrase screen, (3) validation of the screening hits by biophysical methods, and (4) a 3D guided optimization process. When the in silico screening was performed, the initial data set containing 350 000 compounds could be reduced to 3000 molecules. Testing these 3000 selected compounds in the DNA gyrase assay provided 150 hits clustered in 14 classes. Seven classes could be validated as true, novel DNA gyrase inhibitors that act by binding to the ATP binding site located on subunit B: phenols, 2-amino-triazines, 4-amino-pyrimidines, 2-amino-pyrimidines, pyrrolopyrimidines, indazoles, and 2-hydroxymethyl-indoles. The 3D guided optimization provided highly potent DNA gyrase inhibitors, e. g., the 3,4-disubstituted indazole 23 being a 10 times more potent DNA gyrase inhibitor than novobiocin (3).

Protein deposition as amyloid fibrils underlies many debilitating human disorders. The complexity and size of disease-related polypeptides, however, often hinders a detailed rational approach to study effects that contribute to the process of amyloid formation. We report here a simplified peptide sequence successfully designed de novo to fold into a coiled-coil conformation under ambient conditions but to transform into amyloid fibrils at elevated temperatures. We have determined the crystal structure of the coiled-coil form and propose a detailed molecular model for the peptide in its fibrillar state. The relative stabilities of the two structural forms and the kinetics of their interconversion were found to be highly sensitive to small sequence changes. The results reveal the importance of specific packing interactions on the kinetics of amyloid formation and show the potential of this exceptionally favorable system for probing details of the molecular origins of amyloid disease.