Vesicular pseudopodia define the fusion site on large secretory vesicles of the Drosophila salivary glands

Large secretory vesicles (LSVs) pose a scaling problem for regulated exocytosis. Their micron-scale dimensions greatly increase the vesicular membrane surface area, making productive engagement between the vesicular and target membrane fusion machinery unlikely. Here, we show that vesicular pseudopodia define the fusion sites of LSVs in Drosophila larval salivary glands. Focused ion beam scanning electron microscopy revealed that most LSVs project polarized pseudopodia that interconnect neighboring vesicles and orient toward the apical membrane. Exposed pseudopodia were frequently observed at the apical surface and associated with narrow fusion pores, indicating that fusion occurs at these structures. Three-dimensional correlative light and electron microscopy showed that the I-BAR protein Missing in Metastasis (MIM) selectively localizes to exposed pseudopodia. Proteomic analysis based on a MIM pull-down assay identified exocyst components, including Sec15, which localizes to pseudopodia and persists at fusion sites throughout secretion. Finally, the tetraspanin Tsp42Ee marked complementary apical fusion domains and was required for efficient exocytosis. Our findings support a model in which prepatterned vesicular and apical membrane domains coordinate efficient exocytosis.