EXPRESSION AND LOCALIZATION OF SYNAPTOTAGMI-I IN RAT PAROTID-GLAND

Synaptotagmins are a gene family of membrane proteins with distinct ex pression patterns, Synaptotagmin I is an abundant protein of the synap tic vesicle membrane and was implicated as the Ca2+ sensor in fast res ponding synapses. Yet, its precise role along the synaptic vesicle lif e cycle is not fully understood. In this report we show that synaptota gmin I is not exclusively confined to neuronal and neuroendocrine syst ems, rather, it is also expressed in the exocrine system of the paroti d 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 exclusivel y 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 als o localized primarily to the granules' membranes, These findings sugge st that synaptotagmin I which regulates Ca2+-dependent neurotransmitte r release also plays a role which is common to all secretory organelle s - neuronal, endocrine and exocrine, A role for synaptotagmin I in in tegrating signals with protein secretion in the parotid gland is sugge sted.