Rotaviruses (RVs), members of the Reoviridae family, have been resolved into 8 genetically distinct groups (RVA-RVH). The RV genome consists of eleven segments of double-stranded (ds) RNA and is contained in a non-enveloped icosahedral capsid composed of three concentric protein layers. The outer layer consists of VP7 trimers, organized with T=13 symmetry, and VP4 trimeric spikes. The intermediate layered is formed by VP6 trimers, also with T=13 symmetry. The innermost protein layer is a smooth, thin, pseudo T=1 assembly formed from 12 decamers of the core lattice protein, VP2. Tethered to the underside of the VP2 layer are complexes comprised of the viral RNA-dependent RNA polymerase (RdRP), VP1, and the RNA-capping enzyme, VP3. Together, VP1, VP2, VP3, and the dsRNA genome form the core of the virion. The core proteins function together to transcribe the segmented dsRNA genome, producing eleven capped plus-sense (+)RNAs. The viral RdRP uses the (+)RNAs as templates for the synthesis of the dsRNA genome. Although the RdRP alone can recognize viral (+)RNAs, the polymerase is only active when VP2 is present. The VP2-dependent activity of VP1 provides a means by which genome replication (dsRNA synthesis) can be linked with genome packaging and core assembly. Newly made (+)RNAs pass from the RdRP to VP3, an enzyme which introduces m7G caps to the 5'-end of the transcripts through associated guanylyltransferase and methyltransferase activities. The overriding goal of this project is to characterize the structure and function of RV proteins, focusing on the structural and nonstructural proteins involved in RNA synthesis and capsid assembly. Progress toward this goal in 2014-2015 is summarized below. Humans are primarily infected by RVs that belong to the RVA, RVB or RVC group. RVA accounts for more than 90% of all RV-related gastroenteritis in humans while RVB and RVC are principally responsible for sporadic outbreaks of gastroenteritis. In contrast, RVCs are a major cause of morbidity and mortality due to gastroenteritis in piglets, indicating a need for effective porcine RVC vaccines. Virus strains comprising RV groups are further resolved into G and P types based on the antigenic and sequence properties of their outer capsid proteins VP7 and VP4. Both VP7 and VP4 proteins contain multiple antigenic epitopes that can induce the production of neutralizing antibodies, which makes them a main target for vaccine development. While the atomic structure of the RVA TLP has been determined, there is little to no structural information available for the individual capsid proteins of RVC TLP. This limits our ability to define antigenic domains on the RVC TLP that could be useful for developing vaccines. To gain information for RVC, we used Phyre2-based homology-modeling to build a predicted structure for the RVC TLP (VP2, VP4, VP6, VP7) of the human Bristol strain, relying on the known atomic structure of the RVA TLP. The model, along with sequence alignments, indicates that the exposed surface of the RVC VP7 outer capsid likely includes 1 or 2 putative antigenic loops not present on the RVA VP7 outer capsid. Thus, the location and number of neutralizing epitopes on the RVC TLP may be distinct from those of the RVA TLP. In addition, RVC VP7 proteins appear to have 1 or 2 more surface-exposed N-linked glycosylation sites than RVA VP7 proteins; additional glycosylation may help RVC mask their more elaborate antigenic virion surface. Because RVC strains grow poorly in cell culture, hindering the generation of RVC TLP structures via cryo-electron microscopy and crystallography, our modeled structure should provide a useful alternative for studies on RVC biology and vaccine development.
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