Involved in the targeting and/or fusion of transport vesicles to their target membrane. Part of the SNARE core complex containing Snap25 and syntaxin. Neuronal synaptic vesicles (By similarity). In embryos, expressed in the plasma membrane of tracheal cells (PubMed:27323327). The ratio of isoform Syb-A to isoform Syb-B is highly regulated during development. Not nervous system-specific; abundant in cells of the gut and Malpighian tubules. Ubiquitinated by gzl, regulating endocytic trafficking (PubMed:23353890). In wing imaginal disks, ubiquitination by gzl promotes transcytosis of wingless (wg) to the basolateral surface (PubMed:26974662). Cell lethality. Belongs to the synaptobrevin family.