Structural study of the Cdc25 domain from Ral-specific guanine-nucleotide exchange factor RalGPS1a

The guanine-nucleotide exchange factor (GEF) RalGPS1a activates small GTPase Ral proteins such as RalA and RalB by stimulating the exchange of Ral bound GDP to GTP, thus regulating various downstream cellular processes. RalGPS1a is composed of an Nterminal Cdc25-like catalytic domain, followed by a PXXP motif and a C-terminal pleckstrin homology (PH) domain. The Cdc25 domain of RalGPS1a, which shares about 30% sequence identity with other Cdc25-domain proteins, is thought to be directly engaged in binding and activating the substrate Ral protein. Here we report the crystal structure of the Cdc25 domain of RalGPS1a. The bowl shaped structure is homologous to the Cdc25 domains of SOS and RasGRF1. The most remarkable difference between these three Cdc25 domains lies in their active sites, referred to as the helical hairpin region. Consistent with previous enzymological studies, the helical hairpin of RalGPS1a adopts a conformation favorable for substrate binding. A modeled RalGPS1a-RalA complex structure reveals an extensive binding surface similar to that of the SOS-Ras complex. However, analysis of the electrostatic surface potential suggests an interaction mode between the RalGPS1a active site helical hairpin and the switch 1 region of substrate RalA distinct from that of the SOS-Ras complex.

Opening up the group II chaperonins

A new study in this issue of Structure (Huo et al., 2010) reports the crystal structure of a group II chaperonin in the open state, providing unprecedented levels of detail for the domain movements that result from ATP hydrolysis and a clearer picture of the protein folding mechanism mediated by chaperonins.

Structure-functional analyses of CRHSP-24 plasticity and dynamics in oxidative stress response

The cold shock domain (CSD) is an evolutionarily conserved nucleic acid binding domain that exhibits binding activity to RNA, ssDNA, and dsDNA. Mammalian CRHSP-24 contains CSD, but its structure-functional relationship has remained elusive. Here we report the crystal structure of human CRHSP-24 and characterization of the molecular trafficking of CRHSP-24 between stress granules and processing bodies in response to oxidative stress. The structure of CRHSP-24 determined by single-wavelength anomalous dispersion exhibits an α-helix and a compact β-barrel formed by five curved anti-parallel β strands. Ligand binding activity of the CSD is orchestrated by residues Ser(41) to Leu(43). Interestingly, a phosphomimetic S41D mutant abolishes the ssDNA binding in vitro and causes CRHSP-24 liberated from stress granules in vivo without apparent alternation of its localization to the processing bodies. This new class of phosphorylation-regulated interaction between the CSD and nucleic acids is unique in stress granule plasticity. Importantly, the association of CRHSP-24 with stress granules is blocked by PP4/PP2A inhibitor calyculin A as PP2A catalyzes the dephosphorylation of Ser(41) of CRHSP-24. Therefore, we speculate that CRHSP-24 participates in oxidative stress response via a dynamic and temporal association between stress granules and processing bodies.

Crystal structure and mutagenesis analysis of chitinase CrChi1 from the nematophagous fungus Clonostachys rosea in complex with the inhibitor caffeine

Chitinases are a group of enzymes capable of hydrolysing the β-(1,4)-glycosidic bonds of chitin, an essential component of the fungal cell wall, the shells of nematode eggs, and arthropod exoskeletons. Chitinases from pathogenic fungi have been shown to be putative virulence factors, and can play important roles in infecting hosts. However, very limited information is available on the structure of chitinases from nematophagous fungi. Here, we present the 1.8 Å resolution of the first structure of a Family 18 chitinase from this group of fungi, that of Clonostachys rosea CrChi1, and the 1.6 Å resolution of CrChi1 in complex with a potent inhibitor, caffeine. Like other Family 18 chitinases, CrChi1 has the DXDXE motif at the end of strand β5, with Glu174 as the catalytic residue in the middle of the open end of the (β/α)(8) barrel. Two caffeine molecules were shown to bind to CrChi1 in subsites -1 to +1 in the substrate-binding domain. Moreover, site-directed mutagenesis of the amino acid residues forming hydrogen bonds with caffeine molecules suggests that these residues are important for substrate binding and the hydrolytic process. Our results provide a foundation for elucidating the catalytic mechanism of chitinases from nematophagous fungi and for improving the pathogenicity of nematophagous fungi against agricultural pest hosts.

Crystal structure of a secreted lipase from Gibberella zeae reveals a novel "double-lock" mechanism

Fusarium graminearum (sexual stage: Gibberella zeae) is the causative agent of Fusarium Head Blight (FHB), which is one of the most destructive plant disease of cereals, accounting for high grain yield losses, especially for wheat and maize. Like other fungal pathogens, several extracellular enzymes secreted by G. zeae are known to be involved in host infection. Among these secreted lipases, G. zeae lipase (GZEL), which is encoded by the FGL1 gene, was demonstrated to be crucial to G. zeae pathogenicity. However, the precise mechanism of GZEL remains unclear due to a lack of detailed structural information. In this study, we report the crystal structure of GZEL at the atomic level. The structure of GZEL displays distinct structural differences compared to reported homologues and indicates a unique "double lock" enzymatic mechanism. To gain insight into substrate/inhibitor recognition, we proposed a model of GZEL in complex with substrate and the lipase inhibitor ebelactone B (based on the reported structures of GZEL homologues), which defines possible substrate binding sites within the catalytic cleft and suggests an "anti sn-l" binding mode. These results pave the way to elucidating the mechanism of GZEL and thus provide clues for the design of anti-FHB inhibitors.

Structures of the N- and C-terminal domains of MHV-A59 nucleocapsid protein corroborate a conserved RNA-protein binding mechanism in coronavirus

Coronaviruses are the causative agent of respiratory and enteric diseases in animals and humans. One example is SARS, which caused a worldwide health threat in 2003. In coronaviruses, the structural protein N (nucleocapsid protein) associates with the viral RNA to form the filamentous nucleocapsid and plays a crucial role in genome replication and transcription. The structure of Nterminal domain of MHV N protein also implicated its specific affinity with transcriptional regulatory sequence (TRS) RNA. Here we report the crystal structures of the two proteolytically resistant N- (NTD) and C-terminal (CTD) domains of the N protein from murine hepatitis virus (MHV). The structure of NTD in two different crystal forms was solved to 1.5 Å. The higher resolution provides more detailed structural information than previous reports, showing that the NTD structure from MHV shares a similar overall and topology structure with that of SARS-CoV and IBV, but varies in its potential surface, which indicates a possible difference in RNA-binding module. The structure of CTD was solved to 2.0-Å resolution and revealed a tightly intertwined dimer. This is consistent with analytical ultracentrifugation experiments, suggesting a dimeric assembly of the N protein. The similarity between the structures of these two domains from SARS-CoV, IBV and MHV corroborates a conserved mechanism of nucleocapsid formation for coronaviruses.

Molecular characterization of a class I P450 electron transfer system from Novosphingobium aromaticivorans DSM12444

Cytochrome P450 (CYP) enzymes of the CYP101 and CYP111 families from the oligotrophic bacterium Novosphingobium aromaticivorans DSM12444 are heme monooxygenases that receive electrons from NADH via Arx, a [2Fe-2S] ferredoxin, and ArR, a ferredoxin reductase. These systems show fast NADH turnovers (k(cat) = 39-91 s(-1)) that are efficiently coupled to product formation. The three-dimensional structures of ArR, Arx, and CYP101D1, which form a physiological class I P450 electron transfer chain, have been resolved by x-ray crystallography. The general structural features of these proteins are similar to their counterparts in other class I systems such as putidaredoxin reductase (PdR), putidaredoxin (Pdx), and CYP101A1 of the camphor hydroxylase system from Pseudomonas putida, and adrenodoxin (Adx) of the mitochondrial steroidogenic CYP11 and CYP24A1 systems. However, significant differences in the proposed protein-protein interaction surfaces of the ferredoxin reductase, ferredoxin, and P450 enzyme are found. There are regions of positive charge on the likely interaction face of ArR and CYP101D1 and a corresponding negatively charged area on the surface of Arx. The [2Fe-2S] cluster binding loop in Arx also has a neutral, hydrophobic patch on the surface. These surface characteristics are more in common with those of Adx than Pdx. The observed structural features are consistent with the ionic strength dependence of the activity.

Structures of EV71 RNA-dependent RNA polymerase in complex with substrate and analogue provide a drug target against the hand-foot-and-mouth disease pandemic in China

Enterovirus 71 (EV71), one of the major causative agents for hand-foot-and-mouth disease (HFMD), has caused more than 100 deaths among Chinese children since March 2008. The EV71 genome encodes an RNAdependent RNA polymerase (RdRp), denoted 3D(pol), which is central for viral genome replication and is a key target for the discovery of specific antiviral therapeutics. Here we report the crystal structures of EV71 RdRp (3D(pol)) and in complex with substrate guanosine-5'-triphosphate and analog 5-bromouridine-5'-triphosphate best to 2.4 Å resolution. The structure of EV71 RdRp (3D(pol)) has a wider open thumb domain compared with the most closely related crystal structure of poliovirus RdRp. And the EV71 RdRp (3D(pol)) complex with GTP or Br-UTP bounded shows two distinct movements of the polymerase by substrate or analogue binding. The model of the complex with the template:primer derived by superimposition with foot-and-mouth disease virus (FMDV) 3D/RNA complex reveals the likely recognition and binding of template:primer RNA by the polymerase. These results together provide a molecular basis for EV71 RNA replication and reveal a potential target for anti-EV71 drug discovery.

Crystal structures of NAC domains of human nascent polypeptide-associated complex (NAC) and its αNAC subunit

Nascent polypeptide associated complex (NAC) and its two isolated subunits, αNAC and βNAC, play important roles in nascent peptide targeting. We determined a 1.9 Å resolution crystal structure of the interaction core of NAC heterodimer and a 2.4 Å resolution crystal structure of αNAC NAC domain homodimer. These structures provide detailed information of NAC heterodimerization and αNAC homodimerization. We found that the NAC domains of αNAC and βNAC share very similar folding despite of their relative low identity of amino acid sequences. Furthermore, different electric charge distributions of the two subunits at the NAC interface provide an explanation to the observation that the heterodimer of NAC complex is more stable than the single subunit homodimer. In addition, we successfully built a βNAC NAC domain homodimer model based on homologous modeling, suggesting that NAC domain dimerization is a general property of the NAC family. These 3D structures allow further studies on structure-function relationship of NAC.

Functional study on GTP hydrolysis by the GTP-binding protein from Sulfolobus solfataricus, a member of the HflX family

GTPase domains from members of the HflX protein family have their catalytic glutamine residue of the DxxGQ motif substituted by phenylalanine, while they are still able to hydrolyse GTP. This appears to challenge the traditional view of GTP hydrolysis mechanism of Ras-like GTPases. SsGBP from the hyperthermophilic archaeon Sulfolobus solfataricus provided the first crystal structure of the HflX family. Here, we report structure-based mutagenesis analyses on SsGBP. Six-point mutations were individually introduced in the Ras-like GTPase domain including regions of P-loop, switches I and II. Intrinsic GTPase activities and thermal stabilities of these variants together with the wild-type full-length SsGBP and its isolated GTPase domain were analysed. Both functional and structural analyses of G235P and G235S mutants, which showed total and partial loss of the GTP hydrolyzing activity, respectively, support our hypothesis that the role of aligning a nucleophilic water molecule by the Ras Gln60 residue is replaced by the backbone amide group of Gly235 in SsGBP. Together with functional studies of other mutants, we conclude that the classical view of GTP hydrolysis mechanism likely remains the same in the HflX family with a twist in the entity of the nucleophilic alignment.

Structure of the ribosome associating GTPase HflX

The HflX-family is a widely distributed but poorly characterized family of translation factor-related guanosine triphosphatases (GTPases) that interact with the large ribosomal subunit. This study describes the crystal structure of HflX from Sulfolobus solfataricus solved to 2.0-A resolution in apo- and GDP-bound forms. The enzyme displays a two-domain architecture with a novel "HflX domain" at the N-terminus, and a classical G-domain at the C-terminus. The HflX domain is composed of a four-stranded parallel beta-sheet flanked by two alpha-helices on either side, and an anti-parallel coiled coil of two long alpha-helices that lead to the G-domain. The cleft between the two domains accommodates the nucleotide binding site as well as the switch II region, which mediates interactions between the two domains. Conformational changes of the switch regions are therefore anticipated to reposition the HflX-domain upon GTP-binding. Slow GTPase activity has been confirmed, with an HflX domain deletion mutant exhibiting a 24-fold enhanced turnover rate, suggesting a regulatory role for the HflX domain. The conserved positively charged surface patches of the HflX-domain may mediate interaction with the large ribosomal subunit. The present study provides a structural basis to uncover the functional role of this GTPases family whose function is largely unknown.

Crystal structure of the Mp1p ligand binding domain 2 reveals its function as a fatty acid-binding protein

Penicillium marneffei is a dimorphic, pathogenic fungus in Southeast Asia that mostly afflicts immunocompromised individuals. As the only dimorphic member of the genus, it goes through a phase transition from a mold to yeast form, which is believed to be a requisite for its pathogenicity. Mp1p, a cell wall antigenic mannoprotein existing widely in yeast, hyphae, and conidia of the fungus, plays a vital role in host immune response during infection. To understand the function of Mp1p, we have determined the x-ray crystal structure of its ligand binding domain 2 (LBD2) to 1.3 A. The structure reveals a dimer between the two molecules. The dimer interface forms a ligand binding cavity, in which electron density was observed for a palmitic acid molecule interacting with LBD2 indirectly through hydrogen bonding networks via two structural water molecules. Isothermal titration calorimetry experiments measured the ligand binding affinity (K(d)) of Mp1p at the micromolar level. Mutations of ligand-binding residues, namely S313A and S332A, resulted in a 9-fold suppression of ligand binding affinity. Analytical ultracentrifugation assays demonstrated that both LBD2 and Mp1p are mostly monomeric in vitro, no matter with or without ligand, and our dimeric crystal structure of LBD2 might be the result of crystal packing. Based on the conformation of the ligand-binding pocket in the dimer structure, a model for the closed, monomeric form of LBD2 is proposed. Further structural analysis indicated the biological importance of fatty acid binding of Mp1p for the survival and pathogenicity of the conditional pathogen.

Structural implications of a G170R mutation of alanine:glyoxylate aminotransferase that is associated with peroxisome-to-mitochondrion mistargeting

In a subset of patients with the hereditary kidney-stone disease primary hyperoxaluria type 1 (PH1), the liver-specific enzyme alanine:glyoxylate aminotransferase (AGT) is mistargeted from peroxisomes to mitochondria. This is a consequence of the combined presence of the common P11L polymorphism and a disease-specific G170R mutation. In this paper, the crystal structure of mutant human AGT containing the G170R replacement determined at a resolution of 2.6 A is reported. The crystal structure of AGT consists of an intimate dimer in which an extended N-terminal segment of 21 amino acids from one subunit wraps as an elongated irregular coil around the outside of the crystallographic symmetry-related subunit. In addition to the N-terminal segment, the monomer structure contains a large domain of 261 amino acids and a small C-terminal domain of 110 amino acids. Comparison of the mutant AGT structure and that of wild-type normal AGT shows that the two structures are almost identical, with a backbone-atom r.m.s. deviation of 0.34 A. However, evidence of significant local structural changes in the vicinity of the G170R mutation might be linked to the apparent decrease in protein stability.

Crystal structure of cytotoxin protein suilysin from Streptococcus suis

Cholesterol-dependent cytolysins (CDC) are pore forming toxins. A prototype of the CDC family members is perfringolysin O (PFO), which directly binds to the cell membrane enriched in cholesterol, causing cell lysis. However, an exception of this general observation is intermedilysin (ILY) of Streptococcus intermedius, which requires human CD59 as a receptor in addition to cholesterol for its hemolytic activity. A possible explanation of this functional difference is the conformational variation between the C-terminal domains of the two toxins, particularly in the highly conserved undecapeptide termed tryptophan rich motif. Here, we present the crystal structure of suilysin, a CDC toxin from the infectious swine pathogen Streptococcus suis. Like PFO, suilysin does not require a host receptor for hemolytic activity; yet the crystal structure of suilysin exhibits a similar conformation in the tryptophan rich motif to ILY. This observation suggests that the current view of the structure-function relationship between CDC proteins and membrane association is far from complete.

Crystal structure of a ferredoxin reductase for the CYP199A2 system from Rhodopseudomonas palustris

Cytochrome P450-199A2 from Rhodopseudomonas palustris oxidizes para-substituted benzoic acids and may play a role in lignin and aromatic acid degradation pathways in the bacterium. CYP199A2 has an associated [2Fe-2S] ferredoxin, palustrisredoxin (Pux) but not a ferredoxin reductase. A genome search identified the palustrisredoxin reductase (PuR) gene. PuR was produced in Escherichia coli and shown to be a flavin-dependent protein that supports efficient electron transfer from NADH to Pux, thus reconstituting CYP199A2 monooxygenase activity (k(cat) = 37.9 s(-1) with 4-methoxybenzoic acid). The reduction of Pux by PuR shows K(m) = 4.2 microM and k(cat) = 262 s(-1) in 50 mM Tris, pH 7.4. K(m) is increased to 154 microM in the presence of 200 mM KCl, indicating the importance of ionic interactions in PuR/Pux binding. The crystal structure of PuR has been determined at 2.2 A resolution and found to be closely related to that of other oxygenase-coupled NADH-dependent ferredoxin reductases. Residues on the surface that had been proposed to be involved in ferredoxin reductase-ferredoxin binding are conserved in PuR. However, Lys328 in PuR lies over the FAD isoalloxazine ring and, together with His11 and Gln41, render the electrostatic potential of the surface more positive and may account for the greater involvement of electrostatic interactions in ferredoxin binding by PuR. Consistent with these observations the K328G mutation weakened Pux binding and virtually eliminated the dependence of PuR/Pux binding on salt concentration, thus confirming that the FAD si side surface in the vicinity of Lys328 is the ferredoxin binding site.

New nsp8 isoform suggests mechanism for tuning viral RNA synthesis

During severe acute respiratory syndrome coronavirus (SARS-CoV) infection, the activity of the replication/transcription complexes (RTC) quickly peaks at 6 hours post infection (h.p.i) and then diminishes significantly in the late post-infection stages. This "down-up-down" regulation of RNA synthesis distinguishes different viral stages: primary translation, genome replication, and finally viron assembly. Regarding the nsp8 as the primase in RNA synthesis, we confirmed that the proteolysis product of the primase (nsp8) contains the globular domain (nsp8C), and indentified the resectioning site that is notably conserved in all the three groups of coronavirus. We subsequently crystallized the complex of SARS-CoV nsp8C and nsp7, and the 3-D structure of this domain revealed its capability to interfuse into the hexadecamer super-complex. This specific proteolysis may indicate one possible mechanism by which coronaviruses to switch from viral infection to genome replication and viral assembly stages.

Crystal structure of ArgP from Mycobacterium tuberculosis confirms two distinct conformations of full-length LysR transcriptional regulators and reveals its function in DNA binding and transcriptional regulation

Mycobacterium tuberculosis presents a challenging medical problem partly due to its persistent nonreplicative state. The inhibitor of chromosomal replication (iciA) protein encoded by M. tuberculosis has been suggested to inhibit chromosome replication initiation in vitro. However, iciA has also been identified as arginine permease (ArgP), a regulatory transcription factor for arginine outward transport. In order to understand the function of ArgP, we have determined its crystal structure by X-ray crystallography to a resolution of 2.7 A. ArgP is a member of the LysR-type transcriptional regulators (LTTRs) and forms a homodimer with each subunit containing two domains: a DNA binding domain (DBD) and a regulatory domain (RD). Two conformationally distinct subunits were identified: closed subunit and open subunit. This phenomenon was first observed in LTTR CbnR, but not in LTTR CrgA, and might be common in LTTRs. We identified two forms of dimers: DBD-type dimers and RD-type dimers. The former is confirmed in solution, and the latter is considered to form oligomers during function. We provide the first structural insights into the interaction of the extreme C-terminal residues with the DBD, which is confirmed by mutagenesis and analytical ultracentrifugation to be important for stability of the functional dimer. The structure serves as a model to suggest how three critical aspects, namely, DNA binding, homo-oligomerization, and interaction with RNAP, are mediated during regulation processing. A model is proposed for the LysR family of dimeric regulators.

The crystal structures of two cuticle-degrading proteases from nematophagous fungi and their contribution to infection against nematodes

Cuticle-degrading proteases are involved in the breakdown of cuticle/eggshells of nematodes or insects, a hard physical barrier against fungal infections. Understanding the 3-dimensional structures of these proteins can provide crucial information for improving the effectiveness of these fungi in biocontrol applications, e.g., by targeted protein engineering. However, the structures of these proteases remain unknown. Here, we report the structures of two cuticle-degrading proteases from two species of nematophagous fungi. The two structures were solved with X-ray crystallography to resolutions of 1.65 A (Ver112) and 2.1 A (PL646), respectively. Crystal structures of PL646 and Ver112 were found to be very similar to each other, and similar to that of proteinase K from another fungus Tritirachium album. Differences between the structures were found among residues of the substrate binding sites (S1 and S4). Experimental studies showed that the enzymes differed in hydrolytic activity to synthetic peptide substrates. Our analyses of the hydrophobic/hydrophilic and electrostatic features of these two proteins suggest that their surfaces likely play important roles during fungal infection against nematodes. The two crystal structures provide a solid basis for investigating the relationship between structure and function of cuticle-degrading proteases.

Engineered Saccharomyces cerevisiae that produces 1,3-propanediol from D-glucose

AIMS

Saccharomyces cerevisiae is a safe micro-organism used in fermentation industry. 1,3-Propanediol is an important chemical widely used in polymer production, but its availability is being restricted owing to its expensively chemical synthesis. The aim of this study is to engineer a S. cerevisiae strain that can produce 1,3-propanediol at low cost.

METHODS AND RESULTS

By using D-glucose as a feedstock, S. cerevisiae could produce glycerol, but not 1,3-propanediol. In this study, we have cloned two genes yqhD and dhaB required for the production of 1,3-propanediol from glycerol, and integrated them into the chromosome of S. cerevisiae W303-1A by Agrobacterium tumefaciens-mediated transformation. Both genes yqhD and dhaB functioned in the engineered S. cerevisiae and led to the production of 1,3-propanediol from D-glucose.

CONCLUSION

Saccharomyces cerevisiae can be engineered to produce 1,3-propanediol from low-cost feedstock D-glucose.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our knowledge, this is the first report on developing S. cerevisiae to produce 1,3-propanediol by using A. tumefaciens-mediated transformation. This study might lead to a safe and cost-efficient method for industrial production of 1,3-propanediol.

Structural basis for a reciprocating mechanism of negative cooperativity in dimeric phosphagen kinase activity

Phosphagen kinase (PK) family members catalyze the reversible phosphoryl transfer between phosphagen and ADP to reserve or release energy in cell energy metabolism. The structures of classic quaternary complexes of dimeric creatine kinase (CK) revealed asymmetric ligand binding states of two protomers, but the significance and mechanism remain unclear. To understand this negative cooperativity further, we determined the first structure of dimeric arginine kinase (dAK), another PK family member, at 1.75 A, as well as the structure of its ternary complex with AMPPNP and arginine. Further structural analysis shows that the ligand-free protomer in a ligand-bound dimer opens more widely than the protomers in a ligand-free dimer, which leads to three different states of a dAK protomer. The unexpected allostery of the ligand-free protomer in a ligand-bound dimer should be relayed from the ligand-binding-induced allostery of its adjacent protomer. Mutations that weaken the interprotomer connections dramatically reduced the catalytic activities of dAK, indicating the importance of the allosteric propagation mediated by the homodimer interface. These results suggest a reciprocating mechanism of dimeric PK, which is shared by other ATP related oligomeric enzymes, e.g., ATP synthase.