Ahnert-Hilger G, Schäfer T, Spicher K, Grund C, Schultz G, Wiedenmann B
Abteilung Gastroenterologie, Universitätsklinikum Steglitz, Freie Universität, Berlin/Germany.
Eur J Cell Biol. 1994 Oct;65(1):26-38.
Heterotrimeric G proteins, initially believed to be exclusively present in the plasma membrane, have also been found to be associated with intracellular membrane compartments. There they are involved in various membrane trafficking processes including regulated secretion (reviewed in Bomsel, M., K. Mostov, Mol. Biol. Cell 3, 1317-1328 (1992)). Vesicles of two distinct types enter the regulated secretory pathway, i.e. large dense core vesicles and small synaptic vesicles, which differ in their membrane composition and content. Little is known about an association of heterotrimeric G proteins with regulated secretory vesicles, that would explain some aspects of the role heterotrimeric G proteins have during secretion. By immunofluorescence microscopy and immunoreplica analysis, we provide the first demonstration of the presence of complete sets of heterotrimeric G proteins, consisting of alpha-, beta-, and gamma-subunits, on large dense core vesicles from bovine adrenal medulla (chromaffin granules) and small synaptic vesicles from rodent and bovine brain. Each of the two types of secretory vesicles contains beta-subunits (at least beta 1 and beta 2), as well as gamma-subunits (at least gamma 2 or gamma 3). Interestingly, they differ in their composition of alpha-subunits. On small synaptic vesicles, we found two G(o) alpha-subunits (alpha o1 and alpha o2) and two Gi alpha-subunits (alpha i1 and alpha i2). In contrast, on chromaffin granules so far only one alpha o-subunit but no alpha i-subunits could be detected. Functional properties such as transmitter storage and/or exocytotic membrane fusion may be modulated by the various G-protein subunits associated with chromaffin granules and small synaptic vesicles.
异源三聚体G蛋白最初被认为仅存在于质膜中，现在也发现它们与细胞内膜区室有关。在那里，它们参与各种膜运输过程，包括调节性分泌（见Bomsel, M., K. Mostov, Mol. Biol. Cell 3, 1317 - 1328 (1992)的综述）。两种不同类型的囊泡进入调节性分泌途径，即大的致密核心囊泡和小的突触囊泡，它们的膜组成和内容物不同。关于异源三聚体G蛋白与调节性分泌囊泡的关联了解甚少，而这可能解释异源三聚体G蛋白在分泌过程中所起作用的某些方面。通过免疫荧光显微镜和免疫复型分析，我们首次证明了由α-、β-和γ-亚基组成的完整异源三聚体G蛋白存在于牛肾上腺髓质的大致密核心囊泡（嗜铬颗粒）以及啮齿动物和牛脑的小突触囊泡上。这两种类型的分泌囊泡都含有β-亚基（至少β1和β2）以及γ-亚基（至少γ2或γ3）。有趣的是，它们的α-亚基组成不同。在小突触囊泡上，我们发现了两种G(o)α-亚基（αo1和αo2）和两种Giα-亚基（αi1和αi2）。相比之下，在嗜铬颗粒上，到目前为止仅检测到一种αo-亚基，未检测到αi-亚基。与嗜铬颗粒和小突触囊泡相关的各种G蛋白亚基可能会调节诸如递质储存和/或胞吐性膜融合等功能特性。
