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 repair. The SNAREs regulating the lysosome fusion with the cell surface at the basal level have been poorly defined. Here, we identified STX1A, a Qa-SNARE localized majorly to lysosomes and a cohort to the plasma membrane in HeLa cells. Overexpression of GFP-STX1A in HeLa cells causes decreased lysosome number and their peripheral dispersion. In line, STX1A knockdown resulted in the accumulation of lysosomes beneath the cell surface and showed reduced lysosome exocytosis in HeLa cells. TIRF imaging microscopy demonstrated an enhanced enrichment of LAMP1-positive vesicles at the cell surface in STX1A depleted compared to control cells. Moreover, STX1A depletion reduces the proteolytic activity without affecting the lysosome content or acidity and increases peripheral lysosome dispersion compared to control cells. Consistently, these cells display an enhanced number of autophagosomes and accumulate autolysosomes. 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.