The recent emergence of highly pathogenic avian influenza A virus strains with subtype H5N1 pose a global threat to human health. Elucidation of the underlying mechanisms of viral replication is critical for development of anti-influenza virus drugs. The influenza RNA-dependent RNA polymerase (RdRp) heterotrimer has crucial roles in viral RNA replication and transcription. It contains three proteins: PA, PB1 and PB2. PB1 harbours polymerase and endonuclease activities and PB2 is responsible for cap binding; PA is implicated in RNA replication and proteolytic activity, although its function is less clearly defined. Here we report the 2.9 ångström structure of avian H5N1 influenza A virus PA (PA(C), residues 257-716) in complex with the PA-binding region of PB1 (PB1(N), residues 1-25). PA(C) has a fold resembling a dragon's head with PB1(N) clamped into its open 'jaws'. PB1(N) is a known inhibitor that blocks assembly of the polymerase heterotrimer and abolishes viral replication. Our structure provides details for the binding of PB1(N) to PA(C) at the atomic level, demonstrating a potential target for novel anti-influenza therapeutics. We also discuss a potential nucleotide binding site and the roles of some known residues involved in polymerase activity. Furthermore, to explore the role of PA in viral replication and transcription, we propose a model for the influenza RdRp heterotrimer by comparing PA(C) with the lambda3 reovirus polymerase structure, and docking the PA(C) structure into an available low resolution electron microscopy map.

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Raman scattering, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used to study the mechanism of the catalytic crystallization of carbon and metal dusting corrosion. A mechanism is proposed for both metal dusting and the growth of carbon fibers. Carbon cannot crystallize well by deposition from carburizing gases at low temperature without catalytic activation because of its strong C-C bonds and high melting temperature. To form good crystalline carbon, the carbon atoms must dissolve, diffuse through metal particles, and crystallize on an appropriate facet that can act as a template to help the epitaxial growth of carbon crystals. In this process, metal particles are liberated from the pure metal and alloys. This liberation leads to the metal dusting phenomenon. The catalytic growth of carbon filaments is due to the transportation of carbon from one facet of a metal or carbide particle that favors carbon deposition to another facet that favors carbon precipitation. The free energy of poor crystalline carbon is higher than that of good crystalline carbon. The decrease in free energy from highly disordered carbon to well-ordered carbon is the driving force for metal dusting and for growth of carbon filaments through metal particles. IntroductionRecently, the growth of carbon nanotubes has attracted much attention because these nanotubes have many potential applications in electronic devices and as a hydrogen storage medium. 1-5 Meanwhile, metal dusting corrosion has been a long-standing problem in the petrochemical, syngas, and other industries. [6][7][8][9][10][11][12][13][14][15][16][17] This corrosion, which occurs in a strong carburizing gas at high temperature, is accompanied by the formation of carbon filaments (including carbon nanotubes and nanofibers), fine metal carbide, and/or pure metal. Although both nanofilament growth and metal dusting research involve the growth of carbon nanotubes, the two

Thioredoxin, DsbA, the N-terminal active-site domain a and the non-active-site domain b of protein-disulfide isomerase are all monomeric with a thioredoxin fold, and each exhibits low or no isomerase and chaperone activity. We have linked the N terminus of the above four monomers, individually, to the C terminus of the N-terminal domain of DsbC via the flexible linker helix of the latter to produce four domain hybrids, DsbCnTrx, DsbCn-DsbA, DsbCn-PDIa, and DsbCn-PDIb. These four hybrid proteins form homodimers, and except for DsbCn-PDIb they exhibit new or greatly elevated isomerase as well as chaperone activity. Three-dimensional structure prediction indicates that all the four domain hybrids adopt DsbC-like V-shaped structure with a broad uncharged cleft between the two arms for binding of non-native protein folding intermediates. The results provide strong evidence that dimerization creates chaperone and isomerase activity for monomeric thiol-protein oxidases or reductases, and suggesting a pathway for proteins to acquire new functions and/or higher biological efficiency during evolution.Many proteins, such as secretory proteins (antibodies, some peptide hormones) and membrane proteins (receptors, channel proteins), contain disulfide bonds, which play an essential role in stabilizing the tertiary and quaternary structures of these molecules. The formation of native disulfide bonds (including disulfide isomerization) is a key step in protein folding and is usually catalyzed by thiol-protein oxidoreductases, protein-disulfide isomerase (PDI) 1 in eukaryotes, and Dsb proteins in prokaryotes. So far at least six members of the Dsb family, DsbA, DsbB, DsbC, DsbD, DsbE, and DsbG, have been identified. In recent years PDI (1-4), DsbC (5), and DsbG (6) have been characterized to exhibit both disulfide isomerase and chaperone activity. The thiol-protein oxidoreductases contain thioredoxin (Trx) fold with one or more motif(s) of -CXYC-as active site(s). PDI is a homodimeric molecule mainly located within the endoplasmic reticulum, and each subunit is composed of four successive Trx-fold domains (a-b-bЈ-aЈ-) and a C-terminal tail (7). It is known that a and aЈ are homologous, and each has a -CGHC-motif as an active site. However, b and bЈ, without such an active site motif, are homologous with each other but not with the a domain. DsbC, located in the periplasm, is a prokaryotic counterpart of PDI and has been shown by crystal structure analysis (8) to be a V-shaped homodimer with each subunit forming an arm of the V. The N-terminal domain (1-61) of the subunit is linked via an ␣-helix-hinged linker (62-77) to the C-terminal Trx-domain (DsbCc, 78 -216) with a -CGYC-motif as an active site. The N-terminal domain from each monomer forms the dimer interface at the base of the V through ␤-sheet hydrogen bonds. The broad uncharged cleft with a large hydrophobic surface within the V has been suggested to be the site for peptide binding and therefore it plays a role in both the chaperone and the foldase activity of DsbC (8). B...