Marie-Pierre Egloff

Crystal Structure of the Dengue Virus RNA- Dependent RNA Polymerase Catalytic Domain at 1.85 Angstrom Resolution.

ABSTRACT Dengue fever, a neglected emerging disease for which no vaccine or antiviral agents exist at present, is caused by dengue virus, a member of the Flavivirus genus, which includes several important human pathogens, such as yellow fever and West Nile viruses. The NS5 protein from dengue virus is bifunctional and contains 900 amino acids. The S-adenosyl methionine transferase activity resides within its N-terminal domain, and residues 270 to 900 form the RNA-dependent RNA polymerase (RdRp) catalytic domain. Viral replication begins with the synthesis of minus-strand RNA from the dengue virus positive-strand RNA genome, which is subsequently used as a template for synthesizing additional plus-strand RNA genomes. This essential function for the production of new viral particles is catalyzed by the NS5 RdRp. Here we present a high-throughput in vitro assay partly recapitulating this activity and the crystallographic structure of an enzymatically active fragment of the dengue virus RdRp refined at 1.85-Å resolution. The NS5 nuclear localization sequences, previously thought to fold into a separate domain, form an integral part of the polymerase subdomains. The structure also reveals the presence of two zinc ion binding motifs. In the absence of a template strand, a chain-terminating nucleoside analogue binds to the priming loop site. These results should inform and accelerate the structure-based design of antiviral compounds against dengue virus.

Crystal structure of the RNA polymerase domain of the West Nile virus non-structural protein 5

Viruses of the family Flaviviridae are important human and animal pathogens. Among them, the Flaviviruses dengue (DENV) and West Nile (WNV) cause regular outbreaks with fatal outcomes. The RNA-dependent RNA polymerase (RdRp) activity of the non-structural protein 5 (NS5) is a key activity for viral RNA replication. In this study, crystal structures of enzymatically active and inactive WNV RdRp domains were determined at 3.0- and 2.35-Å resolution, respectively. The determined structures were shown to be mostly similar to the RdRps of the Flaviviridae members hepatitis C and bovine viral diarrhea virus, although with unique elements characteristic for the WNV RdRp. Using a reverse genetic system, residues involved in putative interactions between the RNA-cap methyltransferase (MTase) and the RdRp domain of Flavivirus NS5 were identified. This allowed us to propose a model for the structure of the full-length WNV NS5 by in silico docking of the WNV MTase domain (modeled from our previously determined structure of the DENV MTase domain) onto the RdRp domain. The Flavivirus RdRp domain structure determined here should facilitate both the design of anti-Flavivirus drugs and structure-function studies of the Flavivirus replication complex in which the multifunctional NS5 protein plays a central role.

Crystal structure of human gastric lipase and model of lysosomal acid lipase, two lipolytic enzymes of medical interest.

Fat digestion in humans requires not only the classical pancreatic lipase but also gastric lipase, which is stable and active despite the highly acidic stomach environment. We report here the structure of recombinant human gastric lipase at 3.0-Å resolution, the first structure to be described within the mammalian acid lipase family. This globular enzyme (379 residues) consists of a core domain belonging to the α/β hydrolase-fold family and a “cap” domain, which is analogous to that present in serine carboxypeptidases. It possesses a classical catalytic triad (Ser-153, His-353, Asp-324) and an oxyanion hole (NH groups of Gln-154 and Leu-67). Four N-glycosylation sites were identified on the electron density maps. The catalytic serine is deeply buried under a segment consisting of 30 residues, which can be defined as a lid and belonging to the cap domain. The displacement of the lid is necessary for the substrates to have access to Ser-153. A phosphonate inhibitor was positioned in the active site that clearly suggests the location of the hydrophobic substrate binding site. The lysosomal acid lipase was modeled by homology, and possible explanations for some previously reported mutations leading to the cholesterol ester storage disease are given based on the present model.

A Structural Basis for the Inhibition of the NS5 Dengue Virus mRNA 2′-O-Methyltransferase Domain by Ribavirin 5′-Triphosphate

Ribavirin is one of the few nucleoside analogues currently used in the clinic to treat RNA virus infections, but its mechanism of action remains poorly understood at the molecular level. Here, we show that ribavirin 5′-triphosphate inhibits the activity of the dengue virus 2′-O-methyltransferase NS5 domain (NS5MTaseDV). Along with several other guanosine 5′-triphosphate analogues such as acyclovir, 5-ethynyl-1-β-d-ribofuranosylimidazole-4-carboxamide (EICAR), and a series of ribose-modified ribavirin analogues, ribavirin 5′-triphosphate competes with GTP to bind to NS5MTaseDV. A structural view of the binding of ribavirin 5′-triphosphate to this enzyme was obtained by determining the crystal structure of a ternary complex consisting of NS5MTaseDV, ribavirin 5′-triphosphate, and S-adenosyl-l-homocysteine at a resolution of 2.6 Å. These detailed atomic interactions provide the first structural insights into the inhibition of a viral enzyme by ribavirin 5′-triphosphate, as well as the basis for rational drug design of antiviral agents with improved specificity against the emerging flaviviruses.

A second, non-canonical RNA-dependent RNA polymerase in SARS Coronavirus

In (+) RNA coronaviruses, replication and transcription of the giant ∼30 kb genome to produce genome‐ and subgenome‐size RNAs of both polarities are mediated by a cognate membrane‐bound enzymatic complex. Its RNA‐dependent RNA polymerase (RdRp) activity appears to be supplied by non‐structural protein 12 (nsp12) that includes an RdRp domain conserved in all RNA viruses. Using SARS coronavirus, we now show that coronaviruses uniquely encode a second RdRp residing in nsp8. This protein strongly prefers the internal 5′‐(G/U)CC‐3′ trinucleotides on RNA templates to initiate the synthesis of complementary oligonucleotides of <6 residues in a reaction whose fidelity is relatively low. Distant structural homology between the C‐terminal domain of nsp8 and the catalytic palm subdomain of RdRps of RNA viruses suggests a common origin of the two coronavirus RdRps, which however may have evolved different sets of catalytic residues. A parallel between the nsp8 RdRp and cellular DNA‐dependent RNA primases is drawn to propose that the nsp8 RdRp produces primers utilized by the primer‐dependent nsp12 RdRp.

Crystal Structure of Maltose Phosphorylase from Lactobacillus brevis: Unexpected Evolutionary Relationship with Glucoamylases

BACKGROUND Maltose phosphorylase (MP) is a dimeric enzyme that catalyzes the conversion of maltose and inorganic phosphate into beta-D-glucose-1-phosphate and glucose without requiring any cofactors, such as pyridoxal phosphate. The enzyme is part of operons that are involved in maltose/malto-oligosaccharide metabolism. Maltose phosphorylases have been classified in family 65 of the glycoside hydrolases. No structure is available for any member of this family. RESULTS We report here the 2.15 A resolution crystal structure of the MP from Lactobacillus brevis in complex with the cosubstrate phosphate. This represents the first structure of a disaccharide phosphorylase. The structure consists of an N-terminal complex beta sandwich domain, a helical linker, an (alpha/alpha)6 barrel catalytic domain, and a C-terminal beta sheet domain. The (alpha/alpha)6 barrel has an unexpected strong structural and functional analogy with the catalytic domain of glucoamylase from Aspergillus awamori. The only conserved glutamate of MP (Glu487) superposes onto the catalytic residue Glu179 of glucoamylase and likely represents the general acid catalyst. The phosphate ion is bound in a pocket facing the carboxylate of Glu487 and is ideally positioned for nucleophilic attack of the anomeric carbon atom. This site is occupied by the catalytic base carboxylate in glucoamylase. CONCLUSIONS These observations strongly suggest that maltose phosphorylase has evolved from glucoamylase. MP has probably conserved one carboxylate group for acid catalysis and has exchanged the catalytic base for a phosphate binding pocket. The relative positions of the acid catalytic group and the bound phosphate are compatible with a direct-attack mechanism of a glycosidic bond by phosphate, in accordance with inversion of configuration at the anomeric carbon as observed for this enzyme.

The Molecular Mechanism of Multidrug Resistance by the Q151M Human Immunodeficiency Virus Type 1 Reverse Transcriptase and Its Suppression Using α-Boranophosphate Nucleotide Analogues

Nucleoside analogues are currently used to treat human immunodeficiency virus infections. The appearance of up to five substitutions (A62V, V75I, F77L, F116Y, and Q151M) in the viral reverse transcriptase promotes resistance to these drugs, and reduces efficiency of the antiretroviral chemotherapy. Using pre-steady state kinetics, we show that Q151M and A62V/V75I/F77L/F116Y/Q151M substitutions confer to reverse transcriptase (RT) the ability to discriminate an analogue relative to its natural counterpart, and have no effect on repair of the analogue-terminated DNA primer. Discrimination results from a selective decrease of the catalytic rate constant k pol: 18-fold (from 7 to 0.3 s−1), 13-fold (from 1.9 to 0.14 s−1), and 12-fold (from 13 to 1 s−1) in the case of ddATP, ddCTP, and 3′-azido-3′-deoxythymidine 5′-triphosphate (AZTTP), respectively. The binding affinities of the triphosphate analogues for RT remain unchanged. Molecular modeling explains drug resistance by a selective loss of electrostatic interactions between the analogue and RT. Resistance was overcome using α-boranophosphate nucleotide analogues. Using A62V/V75I/F77L/F116Y/Q151M RT, k polincreases up to 70- and 13-fold using α-boranophosphate-ddATP and α-boranophosphate AZTTP, respectively. These results highlight the general capacity of such analogues to circumvent multidrug resistance when RT-mediated nucleotide resistance originates from the selective decrease of the catalytic rate constantk pol.

Crystal structure and mechanistic determinants of SARS coronavirus nonstructural protein 15 define an endoribonuclease family

The ≈30-kb coronavirus (+)RNA genome is replicated and transcribed by a membrane-bound replicase complex made up of 16 viral nonstructural proteins (nsp) with multiple enzymatic activities. The complex includes an RNA endonuclease, NendoU, that is conserved among nidoviruses but no other RNA virus, making it a genetic marker of this virus order. NendoU (nsp15) is a Mn2+-dependent, uridylate-specific enzyme, which leaves 2′–3′-cyclic phosphates 5′ to the cleaved bond. Neither biochemical nor sequence homology criteria allow a classification of nsp15 into existing endonuclease families. Here, we report the crystal structure of the severe acute respiratory syndrome coronavirus nsp15 at 2.6-Å resolution. Nsp15 exhibits a unique fold and assembles into a toric hexamer with six potentially active, peripheric catalytic sites. The structure and the spatial arrangement of the catalytic residues into an RNase A-like active site define a separate endonuclease family, endoU, and represent another spectacular example of convergent evolution toward an enzymatic function that is critically involved in the coronavirus replication cycle.

Structural genomics of the SARS coronavirus: cloning, expression, crystallization and preliminary crystallographic study of the Nsp9 protein

The aetiologic agent of the recent epidemics of Severe Acute Respiratory Syndrome (SARS) is a positive‐stranded RNA virus (SARS‐CoV) belonging to the Coronaviridae family and its genome differs substantially from those of other known coronaviruses. SARS‐CoV is transmissible mainly by the respiratory route and to date there is no vaccine and no prophylactic or therapeutic treatments against this agent. A SARS‐CoV whole‐genome approach has been developed aimed at determining the crystal structure of all of its proteins or domains. These studies are expected to greatly facilitate drug design. The genomes of coronaviruses are between 27 and 31.5 kbp in length, the largest of the known RNA viruses, and encode 20–30 mature proteins. The functions of many of these polypeptides, including the Nsp9–Nsp10 replicase‐cleavage products, are still unknown. Here, the cloning, Escherichia coli expression, purification and crystallization of the SARS‐CoV Nsp9 protein, the first SARS‐CoV protein to be crystallized, are reported. Nsp9 crystals diffract to 2.8 Å resolution and belong to space group P61/522, with unit‐cell parameters a = b = 89.7, c = 136.7 Å. With two molecules in the asymmetric unit, the solvent content is 60% (V M = 3.1 Å3 Da−1).

The Kinetics, Specificities and Structural Features of Lipases

The four main classes of biological substances are carbohydrates, proteins, nucleic acids and lipids. The first three of these substances have been clearly defined on the basis of their structural features, whereas the property which is common to all lipids is a physicochemical one. Lipids are in fact a group of structurally heterogeneous molecules which are all insoluble in water but soluble in apolar and slightly polar solvents such as ether, chloroform and benzene.