Local PI(4,5)P2 generation controls fusion pore expansion during exocytosis

Phosphatidylinositol(4,5)bisphosphate (PI(4,5)P2) is an important signaling phospholipid that is required for regulated exocytosis and some forms of endocytosis. The two processes share a topologically similar pore structure that connects the vesicle lumen with the outside. Widening of the fusion pore during exocytosis leads to cargo release, while its closure initiates kiss&run or cavicapture endocytosis. We show here, using live cell TIRF microscopy of insulin granule exocytosis, that transient accumulation of PI(4,5)P2 at the release site recruits components of the endocytic fission machinery, and stalls the late fusion pore expansion that is required for peptide release. The absence of clathrin differentiates this mechanism from clathrin-mediated endocytosis. The PI(4,5)P2 transients result from local phosphatidylinositol-phosphate-5-kinase-1c (PIP5K1c) activity, and knockdown of PIP5K1c, or optogenetic ablation of PI(4,5)P2 promotes fusion pore expansion. Thus, local phospholipid signaling controls fusion pore expansion peptide release through an unconventional endocytic mechanism.