title: Crystal structure and mechanistic determinants of SARS coronavirus nonstructural protein 15 define an endoribonuclease family.
abstract: The approximately 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 Mn(2+)-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-A 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.
authors.affiliation: Case 925, Ecole d'Ingénieurs de Luminy, Architecture et Fonction des Macromolécules Biologiques, Unité Mixte de Recherche 6098, Centre National de la Recherche Scientifique and Universités d'Aix-Marseille I et II, 13288 Marseille Cedex 9, France.
authors.forename: Stefano
authors.initials: S
authors.lastname: Ricagno
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Egloff
authors.forename: Rachel
authors.initials: R
authors.lastname: Ulferts
authors.forename: Bruno
authors.initials: B
authors.lastname: Coutard
authors.forename: Didier
authors.initials: D
authors.lastname: Nurizzo
authors.forename: Valérie
authors.initials: V
authors.lastname: Campanacci
authors.forename: Christian
authors.initials: C
authors.lastname: Cambillau
authors.forename: John
authors.initials: J
authors.lastname: Ziebuhr
authors.forename: Bruno
authors.initials: B
authors.lastname: Canard
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004720:Endonucleases / Q000378:metabolism; D004722:Endoribonucleases / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012324:RNA-Dependent RNA Polymerase / Q000737:chemistry*; D045473:Severe acute respiratory syndrome-related coronavirus / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D017361:Viral Nonstructural Proteins / Q000737:chemistry*; D014779:Virus Replication
chemical_list: D017361:Viral Nonstructural Proteins; D012324:RNA-Dependent RNA Polymerase; D004720:Endonucleases; D004722:Endoribonucleases; C000657925:nidoviral uridylate-specific endoribonuclease