Exocytosis occurs in all living cells and is essential for many cellular processes including metabolism, signaling, and trafficking. During exocytosis, cargo loaded vesicles dock and fuse with the plasma membrane to release their content. To accommodate different cargos and cellular needs exocytosis must occur across scales; From synaptic vesicles that are only ~50nm in diameter, and neuroendocrine vesicles that are in the ~500nm range to giant secretory vesicles filled with viscous cargo, such as in the acinar cells in the exocrine pancreas, that reach up to a few µm in diameter. Yet, how fusion and content release are adapted to remain function across these scales is not well understood. It is well established that during exocytosis of small vesicles, vesicle fusion can proceed through one of two pathways: The first is complete incorporation, when the vesicular membrane fuses to the target membrane and the fusion pore expand irreversibly, incorporating the vesicular membrane into the target membrane. The second is “kiss-and-run”, when the fusion pore flickers, opening briefly and collapsing rapidly into two separate membranes. I am interested in understanding how exocytosis occurs in giant vesicles witch challenge efficient secretion and membrane homeostasis due to their massive size and viscous content. I am using the salivary gland of D. Melanogaster, as a model system for giant vesicles secretion. The vesicles in the gland measure between 5-8 µm, fuse and secrete viscous content into a preformed lumen. To visualize the secretion process, I adapted a method for super-resolution microscopy to live-gland imaging. I observed that fusion pores of giant vesicles expand to a stable opening of up to 3µm and slowly constricts down to hundreds of nm or less during secretion. Because constricting a membrane pore from “infinity” in molecular terms, back to a very narrow ‘stalk’ demands an investment of energy, I hypothesized that this is mediated by a specialized protein machinery. I am currently screening for the components of the machinery using the enormous power of Drosophila genetics by taking a candidate gene approach. My preliminary results identify the BAR domain containing protein, MIM (missing in metastasis) as a key regulator of pore dynamics, leading to new and exciting insights into the molecular mechanism of cellular secretion and membrane homeostasis in live tissues.
