Roy Duncan, Dalhousie University

Duncan MacKenzie, Nichole McMullen, Gerard Gaspard, Nandini Margam, Roy Duncan Department of Microbiology and Immunology, Dalhousie University

Cell Biology of Viral Infection

Fusogenic reoviruses are the first nonenveloped viruses shown to induce syncytium formation as a novel means for cell-cell transmission. Syncytium formation is mediated by the fusion-associated small transmembrane (FAST) proteins, a diverse family of autonomous, small (~100-150 amino acids) nonstructural viral fusogens that evolved to mediate cell-cell, not virus-cell, membrane fusion. These diminutive protein fusogens have assembled diverse membrane remodeling motifs into minimalist membrane fusion machines. FAST proteins can function autonomously to mediate liposome-cell fusion, which provides the basis for development of the FusogenixTM proteolipid vesicle (PLV) intracellular delivery platform. However, cell-cell fusion requires surrogate adhesins to mediate cell-cell attachment and actin remodelling to generate close membrane apposition and membrane tension at a fusion synapse. We now show that FAST proteins activate a PI3K(p110𝛼)-mTORC2-AKT signaling hub that induces formation of PIP3+/FAST+ macropinosomes in the immediate vicinity of the fusion synapse and concurrent with the onset of pore formation. Inhibiting activation of the PI3K-mTORC2-AKT pathway blocks macropinosome formation and FAST protein-mediated membrane fusion and syncytium formation. Interestingly, this pathway is also required for vertebrate muscle cell fusion but not for syncytium formation induced by enveloped virus spike fusion proteins such as SARS-Cov2 spike or VSV G. This is the first time that macropinocytosis has been linked to formation of a fusion synapse by generating the membrane tension and close membrane apposition needed to drive cell-cell fusion.