Liu W, Clark W A, Sharma P, Northup J K
Laboratory of Cellular Biology, NIDCD, National Institutes of Health, Rockville, Maryland 20850, USA.
J Biol Chem. 1998 Dec 18;273(51):34284-92. doi: 10.1074/jbc.273.51.34284.
The G protein alpha subunit (Galpha) is composed of two distinct folding domains: a GTP-binding Ras-like domain and an alpha helical domain (HD). We have recently reported that the helical domain (HDt) of the vertebrate visual transducin alpha subunit (Galphat) synergizes activation of retinal cyclic GMP phosphodiesterase (PDE) by activated Galphat (Liu, W., and Northup, J. K., (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12878-12883). Here, we examine the molecular basis for this HD-based signaling regulation, and we provide a new model for the activation of the target effector. The HD proteins derived from visual transducin or taste gustducin alpha subunits, but no other Galpha HD proteins, each attenuate the PDE catalytic core (Palphabeta) and synergize Galphat stimulation of the holoPDE (Palphabetagamma2) with similar apparent affinities. The data from studies of both HDt-mediated attenuation and stimulation indicate that the HDt and the PDE inhibitory subunit (Pgamma) interact with PDE at independent sites and that Palphabeta contains the binding sites for HD. The saturation of both processes by HDt displays positive cooperativity with Hill coefficients of 1.5 for the attenuation of Palphabeta activity and 2.1 for synergism of holoPDE activation. Our data suggest the that Galphat-HDt regulates PDE by allosterically decreasing the affinity of Palphabeta for Pgamma and thus simultaneously facilitating the interaction of the activated Galphat-Ras-like domain with Pgamma. Thus, we propose a new model for the high efficiency of PDE activation as well as deactivation, and, overall, a novel mechanism for controlling fidelity, sensitivity, and efficacy of G protein signaling.
G蛋白α亚基（Galpha）由两个不同的折叠结构域组成：一个结合GTP的类Ras结构域和一个α螺旋结构域（HD）。我们最近报道，脊椎动物视觉转导蛋白α亚基（Galphat）的螺旋结构域（HDt）可协同激活的Galphat激活视网膜环鸟苷酸磷酸二酯酶（PDE）（刘，W.，和诺思拉普，J.K.，（1998年）美国国家科学院院刊95，12878 - 12883）。在此，我们研究了这种基于HD的信号调节的分子基础，并为靶效应器的激活提供了一个新模型。源自视觉转导蛋白或味觉转导素α亚基的HD蛋白，但没有其他Galpha HD蛋白，各自减弱PDE催化核心（Palphabeta），并以相似的表观亲和力协同Galphat对全酶PDE（Palphabetagamma2）的刺激。来自HDt介导的减弱和刺激研究的数据表明，HDt和PDE抑制亚基（Pgamma）在独立位点与PDE相互作用，并且Palphabeta包含HD的结合位点。HDt对这两个过程的饱和显示出正协同性，Palphabeta活性减弱的希尔系数为1.5，全酶PDE激活协同作用的希尔系数为2.1。我们的数据表明，Galphat - HDt通过变构降低Palphabeta对Pgamma的亲和力来调节PDE，从而同时促进激活的Galphat - 类Ras结构域与Pgamma的相互作用。因此，我们提出了一个关于PDE高效激活以及失活的新模型，总体而言，是一种控制G蛋白信号保真度、敏感性和功效的新机制。
