Noncatalytic cGMP-binding sites of amphibian rod cGMP phosphodiesterase control interaction with its inhibitory gamma-subunits. A putative regulatory mechanism of the rod photoresponse.
Noncatalytic cGMP-binding sites of amphibian rod cGMP phosphodiesterase control interaction with its inhibitory gamma-subunits. A putative regulatory mechanism of the rod photoresponse.
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两栖动物杆 cGMP 磷酸二酯酶的非催化 cGMP 结合位点控制与其抑制性 γ 亚基的相互作用。
DOI：
10.1016/s0021-9258(18)35793-4
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发表时间：
1992
期刊：
影响因子：
--
通讯作者：
M. Bownds
中科院分区：
文献类型：
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作者：
Vadim Y. Arshavsky;C. L. Dumke;M. Bownds
视网膜视杆细胞的cGMP磷酸二酯酶（PDE）在光转导中起着重要作用。光照导致其被杆G蛋白（Gt，transducin）激活，从而导致细胞内cGMP浓度降低，cGMP门控的质膜阳离子通道关闭，并产生光反应。PDE全酶是α β γ 2四聚体。α-和β-亚基各自含有一个催化和一个或可能两个非催化cGMP结合位点。两个相同的γ-亚基作为酶的蛋白质抑制剂。当它们在PDE活化期间与Gt-GTP结合时，其抑制作用被去除。在这里，我们报告说，非催化cGMP结合位点调节PDE α β与PDE γ的结合，并作为结果确定的机制，PDE激活GT。如果非催化位点是空的，GT-GTP物理去除PDE γ从PDE α β激活后。或者，如果非催化位点被cGMP占据，则Gt-GTP释放PDE γ抑制作用，但仍与PDE异源四聚体结合在复合物中。在这两种情况下，活化PDE的动力学参数是不可区分的。这一机制似乎对感光细胞的生理学有两个影响。首先，当非催化位点被cGMP占据时，PDE γ与PDE α β的紧密结合可能是在暗适应细胞中观察到的基础PDE活性水平低的原因。第二，非催化位点的占据最终控制PDE失活的速率（参见图1）。阿尔沙夫斯基，V. Yu.，和Bownds，M. D.（1992）Nature 357，416-417），因为当这些位点被占据并且Gt停留在与PDE全酶的复合物中时，终止PDE活性的GTdR活性较慢。相反，当非催化位点为空且Gt-PDE γ与PDE α β解离时，Gt-PDE γ加速最大。因为cGMP水平是已知的，以减少照明超过相应的非催化位点的结合常数的浓度范围内，结合可能参与确定激活PDE的寿命后，一个单一的闪光灯和/或在黑暗中适应。
The cGMP phosphodiesterase (PDE) of retinal rods plays a central role in phototransduction. Illumination leads to its activation by a rod G-protein (Gt, transducin), thus causing a decrease in intracellular cGMP concentration, closure of plasma membrane cationic channels gated by cGMP, and development of the photoresponse. The PDE holoenzyme is an alpha beta gamma 2 tetramer. The alpha- and beta-subunits each contain one catalytic and one, or possibly two, noncatalytic cGMP-binding sites. Two identical gamma-subunits serve as protein inhibitors of the enzyme. Their inhibition is removed when they bind to Gt-GTP during PDE activation. Here we report that the noncatalytic cGMP-binding sites regulate the binding of PDE alpha beta with PDE gamma and as a result determine the mechanism of PDE activation by Gt. If the noncatalytic sites are empty, Gt-GTP physically removes PDE gamma from PDE alpha beta upon activation. Alternatively, if the noncatalytic sites are occupied by cGMP, Gt-GTP releases PDE gamma inhibitory action but remains bound in a complex with the PDE heterotetramer. The kinetic parameters of activated PDE in these two cases are indistinguishable. This mechanism appears to have two implications for the physiology of photoreceptor cells. First, the tight binding of PDE gamma with PDE alpha beta when the noncatalytic sites are occupied by cGMP may be responsible for the low level of basal PDE activity observed in dark-adapted cells. Second, occupancy of the noncatalytic sites ultimately controls the rate of PDE inactivation (cf. Arshavsky, V. Yu., and Bownds, M. D. (1992) Nature 357, 416-417), for the GTPase activity that terminates PDE activity is slower when these sites are occupied and Gt stays in a complex with PDE holoenzyme. In contrast GTPase acceleration is maximal when the noncatalytic sites are empty and Gt-PDE gamma dissociates from PDE alpha beta. Because cGMP levels are known to decrease upon illumination over a concentration range corresponding to the binding constants of the noncatalytic sites, the binding might be involved in determining the lifetime of activated PDE, after a single flash and/or during dark adaptation.