Model for the fusion focus multi-step formation, architecture, and function. (A) Schematic representation of the cell–cell fusion process in fission yeast. Type V myosins first assume a crescent localization, decorating the shmoo tip. Focalization is observed in the h⁻ cell fist and then in the h⁺ cell. The distance between the two dots then reduces over time, indicating cell wall thinning, until the two structures merge into one and fusion occurs. (B) Illustration of the architecture of the fusion focus. The formin Fus1 nucleates short actin filaments, which are focalized with type V myosins near the plasma membrane. Focalization requires both Fus1 and type V myosins. Longer For3-nucleated cables are also polarized toward the shmoo tip. (C) Model of the function of the fusion focus. For comparison, the wild-type situation is compared with that in fus1Δ cells. In the wild type, glucanases are concentrated at the fusion focus, therefore segregating them from the location of cell wall synthases, which decorate the entire shmoo tip. This geometrical organization permits cell wall thinning and fusion. In absence of Fus1, the localizations of glucanases and cell wall synthases overlap over the shmoo tip, promoting cell growth.