Fusion pore dynamics of large secretory vesicles define a distinct mechanism of exocytosis.

Exocrine cells utilize large secretory vesicles (LSVs) up to 10 µm in diameter. LSVs fuse with the apical surface, often recruiting actomyosin to extrude their content through dynamic fusion pores. The molecular mechanism regulating pore dynamics remains largely uncharacterized. We observe that the fusion pores of LSVs in the Drosophila larval salivary glands expand, stabilize, and constrict. Arp2/3 is essential for pore expansion and stabilization, while myosin II is essential for pore constriction. We identify several Bin-Amphiphysin-Rvs (BAR) homology domain proteins that regulate fusion pore expansion and stabilization. We show that the I-BAR protein Missing-in-Metastasis (MIM) localizes to the fusion site and is essential for pore expansion and stabilization. The MIM I-BAR domain is essential but not sufficient for localization and function. We conclude that MIM acts in concert with actin, myosin II, and additional BAR-domain proteins to control fusion pore dynamics, mediating a distinct mode of exocytosis, which facilitates actomyosin-dependent content release that maintains apical membrane homeostasis during secretion.

The yeast oligopeptide transporter Opt2 is localized to peroxisomes and affects glutathione redox homeostasis.

Glutathione, the most abundant small-molecule thiol in eukaryotic cells, is synthesized de novo solely in the cytosol and must subsequently be transported to other cellular compartments. The mechanisms of glutathione transport into and out of organelles remain largely unclear. We show that budding yeast Opt2, a close homolog of the plasma membrane glutathione transporter Opt1, localizes to peroxisomes. We demonstrate that deletion of OPT2 leads to major defects in maintaining peroxisomal, mitochondrial, and cytosolic glutathione redox homeostasis. Furthermore, ∆opt2 strains display synthetic lethality with deletions of genes central to iron homeostasis that require mitochondrial glutathione redox homeostasis. Our results shed new light on the importance of peroxisomes in cellular glutathione homeostasis.