Fig. 7. Proposed mechanism of the fusion-pore expansion in syncytium formation. (A) Membrane curvature of an expanding fusion pore is similar to that in budding vesicles. (B) Proposed pathway of syncytium formation. Red networks under the contacting membranes in the pre-fusion state depict actin structures. Protein fusogens, such as baculovirus gp64, form nascent fusion pores. The subsequent expansion of these pores to yield syncytium is controlled by cell machinery. A dynamic resistance of the actin network disrupted by a pore slows down pore expansion, which is driven by membrane-bending proteins (banana-like shapes). These proteins, which are involved in the generation of highly curved intracellular membrane compartments, accumulate at the strongly bent rim of the fusion pores, lower the line tension of the pores and, thus, drive their expansion.