Medically important flaviviruses cause diverse disease pathologies and collectively are responsible for a major global disease burden. A contributing factor to pathogenesis is secreted flavivirus nonstructural protein 1 (NS1). Despite demonstrated protection by NS1-specific antibodies against lethal flavivirus challenge, the structural and mechanistic basis remains unknown. Here, we present three crystal structures of full-length dengue virus NS1 complexed with a flavivirus–cross-reactive, NS1-specific monoclonal antibody, 2B7, at resolutions between 2.89 and 3.96 angstroms. These structures reveal a protective mechanism by which two domains of NS1 are antagonized simultaneously. The NS1 wing domain mediates cell binding, whereas the β-ladder triggers downstream events, both of which are required for dengue, Zika, and West Nile virus NS1–mediated endothelial dysfunction. These observations provide a mechanistic explanation for 2B7 protection against NS1-induced pathology and demonstrate the potential of one antibody to treat infections by multiple flaviviruses.

For the field of virology, perhaps one of the most paradigm-shifting events so far in the 21st century was the identification of the giant double-stranded DNA virus that infects amoebae. Remarkably, this virus, known as Mimivirus, has a genome that encodes for nearly 1,000 proteins, some of which are involved in the biosynthesis of unusual sugars. Indeed, the virus is coated by a layer of glycosylated fibers that contain D-glucose, N-acetyl-D-glucosamine, L-rhamnose, and 4-amino-4,6-dideoxy-D-glucose. Here we describe a combined structural and enzymological investigation of the protein encoded by the open-reading frame L780, which corresponds to an L-rhamnose synthase. The structure of the L780/NADP + /UDP-L-rhamnose ternary complex was determined to 1.45 Å resolution and refined to an overall R-factor of 19.9%. Each subunit of the dimeric protein adopts a bilobal-shaped appearance with the N-terminal domain harboring the dinucleotide-binding site and the C-terminal domain positioning the UDP-sugar into the active site. The overall molecular architecture of L780 places it into the short-chain dehydrogenase/reductase superfamily. Kinetic analyses indicate that the enzyme can function on either UDP-and dTDP-sugars but displays a higher catalytic efficiency with the UDP-linked substrate. Site-directed mutagenesis experiments suggest that both Cys 108 and Lys 175 play key roles in catalysis. This structure represents the first model of a viral UDP-L-rhamnose synthase and provides new details into these fascinating enzymes.

Psychrobacter cryohalolentis K5 T is a Gram-negative bacterium first isolated from Siberian permafrost in 2006. It has a complex O-antigen containing L-rhamnose, D-galactose, two diacetamido-sugars, and one triacetamido-sugar. The biosynthetic pathway for one of the diacetamido-sugars, namely 2,3-diacetamido-2,3-dideoxy-D-glucuronic acid, is presently unknown. Utilizing the published genome sequence of P. cryohalolentis K5 T , we hypothesized that the genes designated Pcryo_0613, Pcryo_0614, Pcryo_0616, and Pcryo_0615 encode for a uridine dinucleotide (UDP)-N-acetyl-D-glucosamine 6-dehydrogenase, an nicotinamide adenine dinucleotide (oxidized) (NAD + )-dependent dehydrogenase, a pyridoxal 5 0 -phosphate (PLP)-dependent aminotransferase, and an N-acetyltransferase, respectively, activities of which would be required for the biosynthesis of this unusual carbohydrate. Here we present the cloning, overexpression, and purification of these hypothetical proteins. Kinetic data on the enzymes encoded by Pcryo_0613, Pcryo_0614, and Pcryo_0615 confirmed their postulated biochemical activities. In addition, the high-resolution X-ray structures of both the internal and external aldimine forms of the aminotransferase were determined to 1.25 and 1.0 Å, respectively. Finally, the three-dimensional architecture of the N-acetyltransferase in complex with its substrate and coenzyme A was solved to 1.8 Å resolution. Strikingly, the N-acetyltransferase was shown to adopt a new motif for UDP-sugar binding. The data presented herein provide additional insight into sugar biosynthesis in Gram-negative bacteria.Abbreviations: CoA, coenzyme A; HEPES, N-2-hydroxyethylpiperazie-N 0 -2-ethanesulfonic acid; HEPPS, N-2-hydroxyethylpiperazine-N 0 -3-propanesulfonic acid; HPLC, high-performance liquid chromatography; MES, 2-(N-morpholino)ethanesulfonic acid; NAD + , nicotinamide adenine dinucleotide (oxidized); NADH, nicotinamide adenine dinucleotide (reduced); NDP, nucleotide diphosphate; Ni-NTA, nickel-nitrilotriacetic acid; PLP, pyridoxal 5 0 -phosphate; rTEV, recombinant tobacco etch virus; Tris, tris-(hydroxymethyl)aminomethane; UDP, uridine dinucleotide; UDP-GlcNAc, uridine diphosphate N-acetylglucosamine.

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