SNAP-24, a Drosophila SNAP-25 homologue on granule membranes, is a putative mediator of secretion and granule-granule fusion in salivary glands

Fusion of vesicles with target membranes is dependent on the interaction of target (t) and vesicle (v) SNARE (soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor) proteins located on opposing m embranes. For fusion at the plasma membrane, the t-SNARE SNAP-25 is essenti al. In Drosophila, the only known SNAP-25 isoform is specific to neuronal a xons and synapses and additional t-SNAREs must exist that mediate both non- synaptic fusion in neurons and constitutive and regulated fusion in other c ells. Here we report the identification and characterization of SNAP-24, a closely related Drosophila SNAP-25 homologue, that is expressed throughout development. The spatial distribution of SNAP-24 in the nervous system is p unctate and, unlike SNAP-25, is not concentrated in synaptic regions. In vi tro studies, however, show that SNAP-24 can form core complexes with syntax in and both synaptic and non-synaptic v-SNAREs, High levels of SNAP-24 are found in larval salivary glands, where SNAP-24 localizes mainly to granule membranes rather than the plasma membrane. During glue secretion, the massi ve exocytotic event of these glands, SNAP-24 containing granules fuse with one another and the apical membrane, suggesting that glue secretion utilize s compound exocytosis and that SNAP-24 mediates secretion.