Höhne-Zell B, Gratzl M
Anatomisches Institut der Technischen Universität München, Munich, Germany.
FEBS Lett. 1996 Sep 30;394(2):109-16. doi: 10.1016/0014-5793(96)00931-3.
Syntaxin and SNAP-25 (synaptosome-associated protein of 25 kDa), associated with the neuronal plasmalemma, and synaptobrevin, a membrane protein of synaptic vesicles, are essential components of the exocytotic apparatus of synaptic vesicles. All three can be proteolytically cleaved by tetanus and/or botulinum neurotoxins. As a consequence of their cleavage, exocytosis of neurotransmitters is blocked. In adrenal chromaffin cells botulinum neurotoxin A only incompletely inhibits exocytosis. This incomplete inhibition of exocytosis is associated with only partial cleavage of SNAP-25 by the toxin, indicating that distinct pools of SNAP-25 may exist in chromaffin cells which differ in their sensitivities to botulinum neurotoxin A. In line with this result we localized SNAP-25 by immunogold electron microscopy not only to the plasmalemma but also to the chromaffin vesicle membrane. Moreover, in addition to SNAP-25 monomers, stable SNAP-25/syntaxin heterodimers were found in chromaffin cells. Subfractionation studies revealed the presence of SNAP-25/syntaxin heterodimers in an enriched fraction of chromaffin vesicles. This complex proved to be stable in SDS, and SNAP-25 within heterodimers was resistant to proteolytic attack by botulinum neurotoxin A. We suggest that these preexisting heterodimers may serve as receptors of soluble NSF attachment proteins (SNAP receptors) during chromaffin vesicle exocytosis.
与神经元质膜相关的 syntaxin 和 SNAP-25（25 kDa 的突触体相关蛋白）以及突触小泡的膜蛋白突触小泡蛋白，是突触小泡胞吐装置的重要组成部分。这三种蛋白都可被破伤风和/或肉毒杆菌神经毒素进行蛋白水解切割。由于它们被切割，神经递质的胞吐作用被阻断。在肾上腺嗜铬细胞中，肉毒杆菌神经毒素 A 仅不完全抑制胞吐作用。这种对胞吐作用的不完全抑制与毒素对 SNAP-25 的部分切割有关，表明嗜铬细胞中可能存在对肉毒杆菌神经毒素 A 敏感性不同的不同 SNAP-25 池。与这一结果一致，我们通过免疫金电子显微镜不仅将 SNAP-25 定位到质膜，还定位到嗜铬小泡膜。此外，除了 SNAP-25 单体外，在嗜铬细胞中还发现了稳定的 SNAP-25/syntaxin 异二聚体。亚分级研究揭示在富含嗜铬小泡的部分中存在 SNAP-25/syntaxin 异二聚体。这种复合物在 SDS 中被证明是稳定的，并且异二聚体内的 SNAP-25 对肉毒杆菌神经毒素 A 的蛋白水解攻击具有抗性。我们认为这些预先存在的异二聚体可能在嗜铬小泡胞吐过程中作为可溶性 NSF 附着蛋白（SNAP 受体）的受体。
