Soluble NSF attachment protein receptors (SNAREs) in RBL-2H3 mast cells: Functional role of syntaxin 4 in exocytosis and identification of a vesicle-associated membrane protein 8-containing secretory compartment

Mast cells upon stimulation through high affinity IgE receptors massively release inflammatory mediators by the fusion of specialized secretory granules (related to lysosomes) with the plasma membrane. Using the RBL-2H3 rat mast cell line, we investigated whether granule secretion involves components of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) machinery. Several isoforms of each family of SNARE proteins were expressed. Among those, synaptosome-associated protein of 23 kDa (SNAP23) was central in SNARE complex formation. Within the syntaxin family, syntaxin J interacted with SNAP23 and all vesicle-associated membrane proteins (VAMPs) examined, except tetanus neurotoxin insensitive VAMP (TI-VAMP), Overexpression of syntaxin 4, but not of syntaxin 2 nor syntaxin 3, caused inhibition of Fc epsilon RI-dependent exocytosis. Four VAMP proteins, i.e., VAMP2, cellubrevin, TI-VAMP, and VAMP8, were present on intracellular membrane structures, with VAMPS residing mainly on mediator-containing secretory granules. We suggest that syntaxin 4, SNAP23, and VAMPS may be involved in regulation of mast cell exocytosis, Furthermore, these results are the first demonstration that the nonneuronal VAMPS isoform, originally localized on early endosomes, is present in a regulated secretory compartment.