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Rotavirus RNA-dependent RNA polymerase VP1 catalyzes RNA synthesis within a subviral particle. This activity depends on core shell protein VP2. A conserved sequence at the 3' end of plus-strand RNA templates is important for polymerase association and genome replication. We have determined the structure of VP1 at 2.9 A resolution, as apoenzyme and in complex with RNA. The cage-like enzyme is similar to reovirus lambda3, with four tunnels leading to or from a central, catalytic cavity. A distinguishing characteristic of VP1 is specific recognition, by conserved features of the template-entry channel, of four bases, UGUG, in the conserved 3' sequence. Well-defined interactions with these bases position the RNA so that its 3' end overshoots the initiating register, producing a stable but catalytically inactive complex. We propose that specific 3' end recognition selects rotavirus RNA for packaging and that VP2 activates the autoinhibited VP1/RNA complex to coordinate packaging and genome replication.

Two distinct partitiviruses, Penicillium stoloniferum viruses S and F, can be isolated from the fungus Penicillium stoloniferum. The bisegmented dsRNA genomes of these viruses are separately packaged in icosahedral capsids containing 120 coat-protein subunits. We used transmission electron cryomicroscopy and three-dimensional image reconstruction to determine the structure of Penicillium stoloniferum virus S at 7.3 A resolution. The capsid, approximately 350 A in outer diameter, contains 12 pentons, each of which is topped by five arched protrusions. Each of these protrusions is, in turn, formed by a quasisymmetric dimer of coat protein, for a total of 60 such dimers per particle. The density map shows numerous tubular features, characteristic of alpha helices and consistent with secondary structure predictions for the coat protein. This three-dimensional structure of a virus from the family Partitiviridae exhibits both similarities to and differences from the so-called "T = 2" capsids of other dsRNA viruses.

Reovirus is a useful model for addressing the molecular basis of membrane penetration by one of the larger nonenveloped animal viruses. We now report the structure of the reovirus virion at approximately 7.0 A resolution as obtained by electron cryomicroscopy and three-dimensional image reconstruction. Several features of the myristoylated outer capsid protein mu1, not seen in a previous X-ray crystal structure of the mu1-sigma3 heterohexamer, are evident in the virion. These features appear to be important for stabilizing the outer capsid, regulating the conformational changes in mu1 that accompany perforation of target membranes, and contributing directly to membrane penetration during cell entry.

Viruses commonly evolve distinct mechanisms to perform some of the same functions as cells. In the January 11 issue of Cell, Deo et al. describe the structure of rotavirus nonstructural protein 3 in complex with RNA, which explains how it acts as a functional homolog of cellular poly(A) binding protein to promote translation of the nonpolyadenylated rotavirus mRNAs.