Reovirus polymerase lambda 3 localized by cryo-electron microscopy of virions at a resolution of 7.6 angstrom

Reovirus is an icosahedral, double-stranded (ds) RNA virus that uses viral polymerases packaged within the viral core to transcribe its ten distinct plus-strand RNAs. To localize these polymerases, the structure of the reovirion was refined to a resolution of 7.6 Angstrom by cryo-electron microscopy (cryo-EM) and three-dimensional (3D) image reconstruction. X-ray crystal models of reovirus proteins, including polymerase lambda3, were then fitted into the density map. Each copy of lambda3 was found anchored to the inner surface of the icosahedral core shell, making major contacts with three molecules of shell protein lambda1 and overlapping, but not centering on, a five-fold axis. The overlap explains why only one copy of lambda3 is bound per vertex. lambda3 is furthermore oriented with its transcript exit channel facing a small channel through the lambda1 shell, suggesting how the nascent RNA is passed into the large external cavity of the pentameric capping enzyme complex formed by protein lambda2.