Levius O, Feinstein N, Linial M
Department of Biological Chemistry, University of Jerusalem, Israel.
Eur J Cell Biol. 1997 May;73(1):81-92.
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 Ca(2+)-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.
突触结合蛋白是一类具有独特表达模式的膜蛋白基因家族。突触结合蛋白I是突触小泡膜中的一种丰富蛋白质，被认为是快速反应突触中的钙离子传感器。然而，其在突触小泡生命周期中的精确作用尚未完全明确。在本报告中，我们表明突触结合蛋白I并非仅局限于神经元和神经内分泌系统，相反，它也在腮腺的外分泌系统中表达。从大鼠腮腺cDNA中分离并测序了突触结合蛋白I的基因。几种独立的抗体进一步证实了突触结合蛋白I的身份。与突触小泡的另一种主要整合膜蛋白VAMP - 2类似，该蛋白仅存在于纯化颗粒的膜部分中。突触结合蛋白I占颗粒总膜蛋白质量的0.4%。利用免疫电子显微镜，这两种蛋白也主要定位于颗粒膜上。这些发现表明，调节钙离子依赖性神经递质释放的突触结合蛋白I在所有分泌细胞器（神经元、内分泌和外分泌）中都发挥着共同作用。提示突触结合蛋白I在腮腺中整合信号与蛋白质分泌方面发挥作用。
