International audienceCytotoxic T lymphocytes (CTLs) kill target cells through polarized release of lytic molecules from secretory lysosomes. Loss of munc13-4 function inhibits this process and causes Familial Hemophagocytic Lymphohistiocytosis type 3 (FHL3). Munc13-4 binds rab27a, but the necessity of the complex remains enigmatic, since studies in knock out models suggest separate functions. We here describe a non-canonical rab27a binding motif in the N-terminus of munc13-4. Point mutants in this sequence have severely impaired rab27a binding, allowing dissection of rab27a requirements in munc13-4 function. The munc13-4-rab27a complex is not needed for secretory lysosome maturation, as shown by complementation in CTLs from FHL3 patients, or in a mast cell line silenced for munc13-4. In contrast, fusion of secretory lysosomes with, and content release at the plasma membrane during degranulation, strictly required munc13-4-rab27a complex. Total Internal Reflection Fluorescence Microscopy Imaging reveals that the complex corrals motile secretory lysosomes beneath the plasma membrane during degranulation and controls their docking. The propensity to stall motility of secretory lysosomes is lost in cells expressing munc13-4 point mutants that don't bind rab27. In summary, these results uncovered a mechanism for tethering secretory lysosomes to the plasma membrane, that is essential for degranulation in immune cells
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