Extensive syncytium formation mediated by the reovirus FAST proteins triggers apoptosis-induced membrane instability.
The fusion-associated small transmembrane (FAST) proteins of the fusogenic reoviruses are the only known examples of membrane fusion proteins encoded by non-enveloped viruses. While the involvement of the FAST proteins in mediating extensive syncytium formation in virus-infected and -transfected cells is well established, the nature of the fusion reaction and the role of cell-cell fusion in the virus replication cycle remain unclear. To address these issues, we analyzed the syncytial phenotype induced by four different FAST proteins: the avian and Nelson Bay reovirus p10, reptilian reovirus p14, and baboon reovirus p15 FAST proteins. Results indicate that FAST protein-mediated cell-cell fusion is a relatively non-leaky process, as demonstrated by the absence of significant [3H]uridine release from cells undergoing fusion and by the resistance of these cells to treatment with hygromycin B, a membrane-impermeable translation inhibitor. However, diminished membrane integrity occurred subsequent to extensive syncytium formation and was associated with DNA fragmentation and chromatin condensation, indicating that extensive cell-cell fusion activates apoptotic signaling cascades. Inhibiting effector caspase activation or ablating the extent of syncytium formation, either by partial deletion of the avian reovirus p10 ecto-domain or by antibody inhibition of p14-mediated cell-cell fusion, all resulted in reduced membrane permeability changes. These observations suggest that the FAST proteins do not possess intrinsic membrane-lytic activity. Rather, extensive FAST protein-induced syncytium formation triggers an apoptotic response that contributes to altered membrane integrity. We propose that the FAST proteins have evolved to serve a dual role in the replication cycle of these fusogenic non-enveloped viruses, with non-leaky cell-cell fusion initially promoting localized cell-cell transmission of the infection followed by enhanced progeny virus release from apoptotic syncytia and systemic dissemination of the infection.
Salsman
Jayme
J
Department of Microbiology and Immunology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada.
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Deniz
D
Boutilier
Julie
J
Duncan
Roy
R
D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D002462:Cell Membrane / Q000821:virology; D002463:Cell Membrane Permeability; D002522:Chlorocebus aethiops; D015726:Giant Cells / Q000166:cytology* / Q000502:physiology*; D006921:Hygromycin B / Q000494:pharmacology; D012087:Reoviridae / Q000502:physiology*; D014162:Transfection; D014529:Uridine / Q000378:metabolism; D014709:Vero Cells; D014760:Viral Fusion Proteins / Q000502:physiology*
D014760:Viral Fusion Proteins; D006921:Hygromycin B; D014529:Uridine