nsp8

Rao et al. have recently determined the crystal structure of a supercomplex consisting of eight subunits each of nsp7 and nsp8. This hollow cylinder-like assembly is purported to allows the passage of RNA, and likely forms the core framework of the replicase complex. The central channel of the cylinder has an internal diameter of ~30 Å, and is positively charged, favoring nucleic acid binding, implying a role in conferring processivity to the RNA-dependent RNA polymerase . A recent paper by Ziebuhr and co-workers have suggested that nsp8 may be a second non-canonical RNA-dependent RNA polymerase. It is likely that the nsp7-nsp8 supercomplex is only a part of a much larger infrastructure for the replicase complex, which may itself be dynamic in composition over the duration of the viral lifecycle. However, this is still speculation, since the structure of the replicase/transcriptase complex is still pending.

Imbert I, Guillemot JC, Bourhis JM, Bussetta C, Coutard B, Egloff MP, Ferron F, Gorbalenya AE, Canard B. A second, non-canonical RNA-dependent RNA polymerase in SARS coronavirus.EMBO J. 2006 Oct 18;25(20):4933-42. Epub 2006 Oct 5.

Zhai Y, Sun F, Li X, Pang H, Xu X, Bartlam M, Rao Z. Insights into SARS-CoV transcription and replication from the structure of the nsp7-nsp8 hexadecamer.Nat Struct Mol Biol. 2005 Nov;12(11):980-6.

Prentice E, McAuliffe J, Lu X, Subbarao K, Denison MR. Identification and characterization of severe acute respiratory syndrome coronavirus replicase proteins.J Virol. 2004 Sep;78(18):9977-86.

Bartlam M, Yang H, Rao Z. Structural insights into SARS coronavirus proteins. Curr Opin Struct Biol. 2005 Dec;15(6):664-72.