Plays a critical role in several secretory processes, including cuticle secretion and neurotransmitter release, and probably assists in neuronal membrane maturation or the final stages of neuronal differentiation (PubMed:7834751). Essential for embryonic viability and development (PubMed:7834751). Required for coordinated peristaltic contractions (PubMed:7834751). Recruited by Unc-13-4B to secretory lysosome-related organelles (SLs) that are essential for tracheal lumen fusion between previously separate tracheal branches (anastomosis). Possibly promotes the intracellular fusion of the extending tracheal stalk cell lumens in tracheal fusion cells (Fcs) by interacting with complementary SNAREs (such as Syb) present in the apical membrane of the FC-FC interface and the membranes of the separate tracheal stalk cells (PubMed:27323327). Prior to embryonic stage 12 expression is ubiquitous, then it becomes concentrated in the neuropil. In third instar larvae, expression is seen at the synaptic boutons of the ventral muscles. In adult brain, expression is seen in the optic lobe neuropils. Expressed throughout development. Belongs to the syntaxin family.