Law S F, Zaina S, Sweet R, Yasuda K, Bell G I, Stadel J, Reisine T
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104.
Mol Pharmacol. 1994 Apr;45(4):587-90.
A major cellular action of the neuropeptide somatostatin (SRIF) is the inhibition of adenylyl cyclase activity. SRIF induces this effect after its interaction with membrane-bound receptors. Five SRIF receptors (SSTRs), which differ in their functional coupling to adenylyl cyclase, have recently been cloned. The third SSTR cloned, SSTR3, effectively mediates the inhibition of adenylyl cyclase by SRIF. The molecular mechanism by which SRIF modulates intracellular cAMP synthesis via SSTR3 was investigated by initially identifying which G alpha subunits are involved in coupling SSTR3 to adenylyl cyclase. SRIF did not inhibit cAMP formation in Chinese hamster ovary cells stably expressing SSTR3 and Gi alpha 2 or Gi alpha 3 but lacking Gi alpha 1. However, SRIF did inhibit forskolin-stimulated cAMP formation in Chinese hamster ovary cells stably expressing SSTR3 and Gi alpha 1, indicating that Gi alpha 1 selectively couples SSTR3 to adenylyl cyclase. To investigate the functional domains of Gi alpha 1 necessary for interaction with SSTR3, a chimeric alpha subunit (Gi alpha 2/Gi alpha 1) was constructed, consisting of the amino-terminal two thirds of Gi alpha 2 ligated to the carboxyl-terminal third of Gi alpha 1. SRIF inhibited cAMP formation in cells expressing SSTR3 and the Gi alpha 2/Gi alpha 1 chimera. These findings indicate that the carboxy-terminal third of Gi alpha 1 interacts with SSTR3 and is important in transmitting the signal of SSTR3 activation to adenylyl cyclase. In contrast, a similar Gi alpha 2/Gi alpha 3 chimera did not couple SSTR3 to adenylyl cyclase, further indicating that Gi alpha 3 does not contribute to SRIF inhibition of adenylyl cyclase activity. These findings demonstrate that Gi alpha 1 selectively couples SSTR3 to adenylyl cyclase, and they indicate that the carboxyl-terminal region of this alpha subunit is involved in mediating SRIF inhibition of adenylyl cyclase activity.
神经肽生长抑素（SRIF）的一个主要细胞作用是抑制腺苷酸环化酶活性。SRIF与膜结合受体相互作用后会诱导这种效应。最近已克隆出五种SRIF受体（SSTRs），它们在与腺苷酸环化酶的功能偶联方面存在差异。克隆出的第三个SSTR，即SSTR3，可有效介导SRIF对腺苷酸环化酶的抑制作用。通过首先确定哪些Gα亚基参与将SSTR3与腺苷酸环化酶偶联，研究了SRIF通过SSTR3调节细胞内cAMP合成的分子机制。在稳定表达SSTR3和Giα2或Giα3但缺乏Giα1的中国仓鼠卵巢细胞中，SRIF并未抑制cAMP的形成。然而，SRIF确实抑制了稳定表达SSTR3和Giα1的中国仓鼠卵巢细胞中福斯可林刺激的cAMP形成，这表明Giα1选择性地将SSTR3与腺苷酸环化酶偶联。为了研究Giα1与SSTR3相互作用所必需的功能结构域，构建了一个嵌合α亚基（Giα2/Giα1），它由Giα2的氨基末端三分之二连接到Giα1的羧基末端三分之一组成。SRIF抑制了表达SSTR3和Giα2/Giα1嵌合体的细胞中cAMP的形成。这些发现表明，Giα1的羧基末端三分之一与SSTR3相互作用，并且在将SSTR3激活信号传递给腺苷酸环化酶方面很重要。相比之下，类似的Giα2/Giα3嵌合体并未将SSTR3与腺苷酸环化酶偶联，这进一步表明Giα3对SRIF抑制腺苷酸环化酶活性没有作用。这些发现证明Giα1选择性地将SSTR3与腺苷酸环化酶偶联，并且表明该α亚基的羧基末端区域参与介导SRIF对腺苷酸环化酶活性的抑制作用。
