Photoreceptor Specificity in the Light-Induced and COP1-Mediated Rapid Degradation of the Repressor of Photomorphogenesis SPA2 in Arabidopsis.
Photoreceptor Specificity in the Light-Induced and COP1-Mediated Rapid Degradation of the Repressor of Photomorphogenesis SPA2 in Arabidopsis.
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DOI：
10.1371/journal.pgen.1005516
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发表时间：
2015-09
期刊：
影响因子：
4.5
通讯作者：
Hoecker U
中科院分区：
文献类型：
--
作者：
Chen S;Lory N;Stauber J;Hoecker U
拟南芥COP1/SPA E3泛素连接酶是一个关键的负调控因子，通过靶向参与光响应的转录因子降解来抑制黑暗中的光信号。COP1/SPA复合体由COP1和四成员SPA蛋白家族成员（SPA1-SPA4）组成。遗传分析表明，与携带其他三种spa的复合物相比，COP1/SPA2的功能受到光的强烈抑制，从而促进植物在极弱光照下的光响应。在这里，我们发现SPA2蛋白在黑暗生长的幼苗暴露于光脉冲后5-15分钟内降解。光敏色素光感受器是在红光、远红光和蓝光下快速降解SPA2所必需的，而隐色素则不参与蓝光诱导的SPA2蛋白水平的快速降低。这些结果揭示了光诱导抑制COP1/SPA2功能的光感受器特异性机制。仅表达SPA2的spa三突变体强烈的蓝光响应需要光敏色素A (Phytochrome A, phyA)，从而证实了phyA在下调SPA2在蓝光中的功能中的重要作用。在蓝光下，SPA2与隐色素1 （cry1）形成复合物，但在体内不与隐色素2 （cry2）形成复合物，这表明SPA2蛋白缺乏快速的蓝光反应只是部分原因是无法与隐色素相互作用。由于SPA1与cry1和cry2相互作用，这些结果提供了第一个分子证据，表明不同SPA蛋白的光调控在进化过程中存在分歧。光下SPA2的降解需要COP1和与COP1相互作用的SPA2的coil -coil结构域，支持SPA2被COP1泛素化。我们提出光敏色素感知的光导致COP1/SPA2泛素化活性从泛素化下游底物到泛素化SPA2的转换，这随后导致COP1/SPA2功能的抑制。植物已经进化出光感受器，它启动信号级联来调节生长和发育以适应周围的光环境。cul4依赖性COP1/SPA E3泛素连接酶是光信号的关键负调控因子，其功能被光抑制。最近的研究已经确定了光敏色素和隐色素光感受器的共同机制。在这里，我们已经确定了光诱导COP1/SPA抑制的机制，这是光敏色素光感受器特有的。我们发现SPA2蛋白在红光、远红光和蓝光下以光敏色素依赖的方式非常迅速地降解。我们进一步表明，SPA2在光下的降解取决于COP1和SPA2与COP1的相互作用。因此，我们的研究结果表明，光诱导降解SPA2，而不是COP1，通过COP1/SPA2泛素连接酶。COP1的人类同源物在没有植物特异性SPA蛋白的情况下发挥作用，已知受DNA损伤后的自降解调节。因此，这种E3连接酶的组分的自降解是人类和植物中使用的一种调节机制。
The Arabidopsis COP1/SPA E3 ubiquitin ligase is a key negative regulator that represses light signaling in darkness by targeting transcription factors involved in the light response for degradation. The COP1/SPA complex consists of COP1 and members of the four-member SPA protein family (SPA1-SPA4). Genetic analysis indicated that COP1/SPA2 function is particularly strongly repressed by light when compared to complexes carrying the other three SPAs, thereby promoting a light response after exposure of plants to extremely low light. Here, we show that the SPA2 protein is degraded within 5–15 min after exposure of dark-grown seedlings to a pulse of light. Phytochrome photoreceptors are required for the rapid degradation of SPA2 in red, far-red and also in blue light, whereas cryptochromes are not involved in the rapid, blue light-induced reduction in SPA2 protein levels. These results uncover a photoreceptor-specific mechanism of light-induced inhibition of COP1/SPA2 function. Phytochrome A (phyA) is required for the severe blue light responsiveness of spa triple mutants expressing only SPA2, thus confirming the important role of phyA in downregulating SPA2 function in blue light. In blue light, SPA2 forms a complex with cryptochrome 1 (cry1), but not with cryptochrome 2 (cry2) in vivo, indicating that the lack of a rapid blue light response of the SPA2 protein is only in part caused by a failure to interact with cryptochromes. Since SPA1 interacts with both cry1 and cry2, these results provide first molecular evidence that the light-regulation of different SPA proteins diverged during evolution. SPA2 degradation in the light requires COP1 and the COP1-interacting coiled-coil domain of SPA2, supporting that SPA2 is ubiquitinated by COP1. We propose that light perceived by phytochromes causes a switch in the ubiquitination activity of COP1/SPA2 from ubiquitinating downstream substrates to ubiquitinating SPA2, which subsequently causes a repression of COP1/SPA2 function. Plants have evolved photoreceptors that initiate a signaling cascade to adjust growth and development to the ambient light environment. The CUL4-dependent COP1/SPA E3 ubiquitin ligase is a key negative regulator of light signaling whose function is repressed by light. Recent research has identified mechanisms that are common to both phytochrome and cryptochrome photoreceptors. Here, we have identified a mechanism of light-induced COP1/SPA repression that is specific to phytochrome photoreceptors. We show that the SPA2 protein is very rapidly degraded in red, far-red and blue light in a phytochrome-dependent fashion. We further show that SPA2 degradation in the light depends on COP1 and on the interaction of SPA2 with COP1. Hence, our results suggest a light-induced degradation of SPA2, but not of COP1, by the COP1/SPA2 ubiquitin ligase. The human ortholog of COP1, which functions without the plant-specific SPA proteins, is known to be regulated by autodegradation following DNA damage. Hence, autodegradation of components of this E3 ligase is a regulatory mechanism used in both humans and plants.