Insights into SARS-CoV transcription and replication from the structure of the nsp7–nsp8 hexadecamer

Coronavirus replication and transcription machinery involves multiple virus-encoded nonstructural proteins (nsp). We report the crystal structure of the hexadecameric nsp7–nsp8 supercomplex from the severe acute respiratory syndrome coronavirus at 2.4-Å resolution. nsp8 has a novel 'golf-club' fold with two conformations. The supercomplex is a unique hollow, cylinder-like structure assembled from eight copies of nsp8 and held tightly together by eight copies of nsp7. With an internal diameter of ∼30 Å, the central channel has dimensions and positive electrostatic properties favorable for nucleic acid binding, implying that its role is to confer processivity on RNA-dependent RNA polymerase. SUPPLEMENTARY INFORMATION: The online version of this article (doi:10.1038/nsmb999) contains supplementary material, which is available to authorized users.