CGMP BINDING-SITES ON PHOTORECEPTOR PHOSPHODIESTERASE - ROLE IN FEEDBACK-REGULATION OF VISUAL TRANSDUCTION
CGMP BINDING-SITES ON PHOTORECEPTOR PHOSPHODIESTERASE - ROLE IN FEEDBACK-REGULATION OF VISUAL TRANSDUCTION
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DOI：
10.1073/pnas.91.11.4845
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发表时间：
1994-05-24
影响因子：
11.1
通讯作者：
ARSHAVSKY, VY
中科院分区：
文献类型：
--
作者：
COTE, RH;BOWNDS, MD;ARSHAVSKY, VY
脊椎动物视觉转导的中心步骤是cGMP水平的快速下降，导致光感受器细胞膜上cGMP门控离子通道关闭。长期以来，cGMP磷酸二酯酶（PDE）的活化导致了这种降低，它不仅含有cGMP水解的催化位点，而且还含有非催化的cGMP结合位点，这一直是一个谜。最近的研究表明，这些非催化位点的占用减缓了PDE失活的速度。我们在这里报道，激活的转导蛋白诱导的PDE活化降低了cGMP对PDE上非催化位点的结合亲和力，并加速了cGMP与这些位点的分离。这些位点可以以三种状态存在：对未激活的PDE具有高亲和力（K-d = 60 nM），当酶与转导蛋白在复合物中激活时具有中等亲和力（K-d约为180 nM），当转导蛋白从PDE催化亚基中物理去除PDE的抑制亚基时具有低亲和力（K-d约为1 μ M）。通过转导蛋白激活PDE导致cGMP从两个非催化位点之一解离的速率增加10倍；物理去除PDE催化亚基的抑制亚基进一步加速了cGMP从两个位点的解离速率bb0 50倍。由于缺乏结合cGMP的PDE分子更快地失活，这表明cGMP的长期降低可能作为负反馈调节器，产生光适应光感受器更快、更小的光响应特征。
A central step in vertebrate visual transduction is the rapid drop in cGMP levels that causes cGMP-gated ion channels in the photoreceptor cell membrane to close. It has long been a puzzle that the cGMP phosphodiesterase (PDE) whose activation causes this decrease contains not only catalytic sites for cGMP hydrolysis but also noncatalytic cGMP binding sites. Recent work has shown that occupancy of these noncatalytic sites slows the rate of PDE inactivation. We report here that PDE activation induced by activated transducin lowers the cGMP binding affinity for noncatalytic sites on PDE and accelerates the dissociation of cGMP from these sites. These sites can exist in three states: high affinity (K-d = 60 nM) for the nonactivated PDE, intermediate affinity (K-d approximate to 180 nM) when the enzyme is activated in a complex with transducin, and low affinity (K-d > 1 mu M) when transducin physically removes the inhibitory subunits of PDE from the PDE catalytic subunits. Activation of PDE by transducin causes a 10-fold increase in the rate of cGMP dissociation from one of the two noncatalytic sites; physical removal of the inhibitory subunits from the PDE catalytic subunits further accelerates the cGMP dissociation rate from both sites >50-fold. Because PDE molecules lacking bound cGMP inactivate more rapidly, this suggests that a prolonged cGMP decrease may act as a negative feedback regulator to generate the faster, smaller photoresponses characteristic of light-adapted photoreceptors.