Expression and localization of synaptotagmin I in rat parotid gland

Synaptotagmins are a gene family of membrane proteins with distinct expression patterns. Synaptotagmin I is an abundant protein of the synaptic vesicle membrane and was implicated as the Ca²⁺ sensor in fast responding synapses. Yet, its precise role along the synaptic vesicle life cycle is not fully understood. In this report we show that synaptotagmin I is not exclusively confined to neuronal and neuroendocrine systems, rather, it is also expressed in the exocrine system of the parotid gland. The gene for synaptotagmin I was isolated and sequenced from rat parotid cDNA. The identity of synaptotagmin I protein was further confirmed by several independent antibodies. The protein is exclusively found in the membranous fraction of purified granules, similarly to VAMP-2, another major integral membrane protein of synaptic vesicles. Synaptotagmin I represents 0.4% of the total membrane protein mass of the granule. Using immunoelectron microscopy the two proteins were also localized primarily to the granules' membranes. These findings suggest that synaptotagmin I which regulates Ca²⁺-dependent neurotransmitter release also plays a role which is common to all secretory organelles - neuronal, endocrine and exocrine. A role for synaptotagmin I in integrating signals with protein secretion in the parotid gland is suggested.