Supplementary Figure 10: Model of membrane trafficking in fusion cells.

Schematic view of early (top) and late (bottom) stages of lumen fusion. Extension of the stalk cell lumen (left) into the fusion cell (right) is brought about by recycling endosomes (yellow), while, in a second step, fusion of the extending stalk cell lumen with the central lumen is mediated by Stac-positive secretory lysosomes (orange). Top: Early during fusion, Rab11-positive recycling endosomes (a, yellow) contribute to expansion of the two apical domains (magenta), which are possibly being pulled towards each other by the cytoskeletal track (dark green). Arl3-mediated maturation of late endosomes (b, dark red) gives rise to secretory lysosome-like vesicles (orange), which are marked by Rab39, Lamp1 and Stac, and are brought into vicinity of the apical membrane domains through Dynein-mediated anchoring to microtubules. Bottom: Late during fusion, Stac recruits Syntaxin (red) to secretory lysosomes (c), which may engage with other SNARE proteins (blue) on opposite membranes. The local release of Ca²⁺ (d) from ER close to the site of lumen fusion may trigger the formation of mature SNARE complexes between the juxtaposed Stac vesicle and apical plasma membrane to promote fusion (e). The stage after completion of fusion, where the intracellular FC lumen has expanded, is depicted as a small cartoon at the bottom.