Multiple rotavirus species encode fusion-associated small transmembrane (FAST) proteins with cell type-specific activity

Fusion-associated small transmembrane (FAST) proteins are viral nonstructural proteins that mediate cell-cell fusion to form multinucleated syncytia. We previously reported that human species B rotavirus NSP1-1 is a FAST protein that induces syncytia in primate epithelial cells but not rodent fibroblasts. We hypothesized that NSP1-1 proteins of other rotavirus species could mediate cell-cell fusion but might be limited to cell types derived from homologous hosts. To test this hypothesis, we predicted the structure and domain organization of NSP1-1 of species B rotavirus from a human, goat, and pig, species G rotavirus from a pigeon and turkey, and species I rotavirus from a dog and cat. We transiently expressed the NSP1-1 proteins in avian, canine, hamster, human, porcine, and simian cells. Regardless of the host origin of the virus, each NSP1-1 protein induced syncytia in primate cells, while few induced syncytia in other cell types. In some cell types, a lack of protein expression or RNA stability failed to explain the inability of NSP1-1 to mediate cell-cell fusion. To identify the domains that determine cell-specific fusion activity for human species B rotavirus NSP1-1, we engineered chimeric proteins containing domain exchanges with the p10 FAST protein from Nelson Bay orthoreovirus and found that the N-terminal and transmembrane domains were determinants. Together, these findings suggest that rotavirus species B, G, and I NSP1-1 are functional FAST proteins whose N termini contribute to specifying the cells in which they mediate syncytium formation.IMPORTANCEMechanisms of membrane fusion and determinants of host range for pathogens remain poorly understood. Improved understanding of these concepts could open new areas for therapeutic development and shed light on virus epidemiology. Our analyses of NSP1-1 proteins from species B, G, and I rotaviruses provide insights into the variability tolerated by functional FAST proteins. Further, the observation that all putative FAST proteins tested can induce syncytium formation in at least some cell types provides evidence that rotaviruses that encode NSP1-1 proteins are fusogenic viruses. Finally, although the criteria for their specificity remain unclear, our observations regarding fusion capacities of different NSP1-1 proteins and of chimeric FAST proteins suggest a potential role for rotavirus FAST proteins in determining the efficiency of viral replication within a given host or cell type.