STX1A localizes to the lysosome and controls its exocytosis

Lysosome exocytosis is one of the critical functions of lysosomes in maintaining cellular homeostasis and plasma membrane (PM) repair. At the basal level, the SNAREs (soluble-N-ethylmaleimide-sensitive-factor accessory-protein receptors) regulating the lysosome fusion with the cell surface have been poorly defined. Here, we identified a Qa-SNARE STX1A, localized majorly to lysosomes and a cohort to the PM in HeLa cells. Overexpression of GFP-STX1A in HeLa cells causes decreased lysosome number and their peripheral dispersion. However, STX1A knockdown in HeLa cells displayed an accumulation of lysosomes beneath the cell surface with reduced lysosome exocytosis. Consistently, TIRF imaging microscopy demonstrated an enhanced enrichment of LAMP1-positive vesicles at the cell surface in STX1A-depleted compared with control cells. Moreover, STX1A depletion reduces proteolytic activity without affecting the lysosome content or acidity. Additionally, these cells showed enhanced lysosome dispersion and autolysosome accumulation. Functionally, GFP-STX1A also localizes to LLOMe-induced GAL3-positive damaged lysosomes and reduces their number by enhancing exocytosis. Biochemically, STX1A forms a SNARE complex with SNAP23 or SNAP25 (Qbc) and VAMP2 (R), and their knockdown in HeLa cells mimics the STX1A-depletion phenotypes. Overall, these studies demonstrate a unique function of STX1A in regulating lysosomal exocytosis by localizing to these degradative organelles.