乙烯的生物合成与信号及其对种子萌发和休眠的调控
Biosynthesis and signaling of ethylene and their regulation on seed germination and dormancy
乙烯由受体蛋白ETR1、ERS1、ETR2、ERS2和EIN4 (绿色表示)感受, 受体是乙烯信号的负调控因子。受体通过它们的GAF结构域(在受体的细胞质区域用五边形表示)与其他的受体相互作用, 并在ER膜中形成更高层次的复合物。铜(一种乙烯结合的辅因子, 红色圆圈)由铜转运体RAN1 (橙色表示)传递给受体。RTE1 (粉红色)与ETR1相联系, 介导受体信号输出。(A)在乙烯缺乏时, 受体激活CTR1 (黄色)。CTR1通过直接磷酸化EIN2的C-末端(蓝色圆圈)使其失活(紫色)。EIN2能够直接与受体的激酶结构域(在受体的细胞质区域在五边形下较大的椭圆)相互作用。EIN2的水平通过26S蛋白酶体(灰色)被F-box蛋白ETP1和ETP2(绿色星状物)负调控。在细胞核中, 转录因子EIN3/EIL1 (红色)通过蛋白酶体被另外2个F-box蛋白EBF1/2(蓝色星状物)降解。在EIN3/EIL1缺乏时, 乙烯反应基因的转录被关闭。(B)在乙烯存在时, 受体与激素结合并失去活性, 依次关闭CTR1。这种失活阻止正调控因子EIN2的磷酸化。EIN2的C-末端被一种未知的机制所剪切, 并移动到细胞核, 在细胞核中使EIN3/EIL1稳定和诱导EBF1/2的降解。转录因子EIN3/EIL1形成二聚体, 激活乙烯靶基因的表达, 包括F-box基因EBF2 (深蓝色卷曲线, 它产生抑制乙烯途径活性的负反馈环)或者转录因子基因ERF1(淡蓝色线, 它依次始启一个转录级联, 导致数百个乙烯调控基因的活化和抑制)。在乙烯反应基因中有受体基因ETR2 (绿色线), 它的mRNA被乙烯上调, 以及被翻译成为一批新的没有与乙烯结合的受体分子; 这些受体分子然后活化负调控因子CTR1, 从而提供了在不添加乙烯的情况下向下调节乙烯信号的手段。途径中的其他调控节点是核糖核酸外切酶EIN5 (淡橙色, 它控制EBF2 的mRNA水平)以及F-box蛋白ETP1和ETP2 (绿色星状物, 在乙烯存在时, 它们被降解, 导致EIN2的稳定)。正箭头和负箭头分别表示激活和下调这个过程。颜色变浅表示的分子(在‘没有乙烯’中的EIN3/EIL1, 或者在‘乙烯’中的ETP1/2和EBF1/2)相应于蛋白酶体介导降解的被标记的不稳定蛋白。卷曲线表示特定的mRNA, 它们的颜色与相应的蛋白质颜色相一致。引自Merchante等[
Ethylene is perceived by the receptor proteins ETR1, ERS1, ETR2, ERS2, and EIN4 (represented in green), the receptors are negative regulators of ethylene signaling. The receptors interact with other receptors and form higher order complexes in the ER membrane through their GAF domains (represented as pentagons in the receptors’ cytosolic domain). Copper (a cofactor for ethylene binding, red circles) is delivered to the receptors by the copper transporter RAN1 (represented in orange). RTE1 (in pink) is associated with ETR1 and mediates the receptor signal output. (A) In the absence of ethylene, the receptors activate CTR1 (in yellow). CTR1 inactivates EIN2 (in purple) by directly phosphorylating (blue circles) its C-terminal end. EIN2 can directly interact with the kinase domain of the receptors (represented as the larger ovals under the pentagons in the cytosolic domain of the receptors). The levels of EIN2 are negatively regulated by the F-box proteins ETP1 and ETP2 (green star) via the 26S proteasome (gray). In the nucleus, the transcription factors EIN3/EIL1 (in red) are being degraded by two other F-box proteins, EBF1/2 (blue star), through the proteasome. In the absence of EIN3/EIL1, transcription of the ethylene response genes is shut off. (B) In the presence of ethylene, the receptors bind the hormone and become inactivated, which in turn, switches off CTR1. This inactivation prevents the phosphorylation of the positive regulator EIN2. The C-terminal end of EIN2 is cleaved off by an unknown mechanism and moves to the nucleus where it stabilizes EIN3/EIL1 and induces degradation of EBF1/2. The transcription factors EIN3/EIL1 dimerize and activate the expression of ethylene target genes, including the F-box gene EBF2 (dark blue curly line) [which generates a negative feedback loop dampening the activity of the ethylene pathway] or the transcription factor gene ERF1 (light blue line) [which, in turn, initiates a transcriptional cascade resulting in the activation and repression of hundreds of ethylene-regulated genes]. Among the ethylene responsive genes the receptor gene is ETR2 (green line), whose mRNA is up-regulated by ethylene and translated into the new batch of ethylene-free receptor molecules which then activate the negative regulator CTR1, thus providing the means of tuning down ethylene signaling in the absence of additional ethylene. Other regulatory nodes in the pathway are the exoribonuclease EIN5 (light orange), which controls the levels of EBF2 mRNA, and the F-box proteins ETP1 and ETP2 (green star) that are degraded in the presence of ethylene leading to the stabilization of EIN2. Positive and negative arrows represent activation and down-regulation processes, respectively. Molecules shown in fading colors (EIN3/EIL1 in ‘no ethylene’, or ETP1/2 and EBF1/2 in ‘ethylene’) correspond to unstable proteins targeted to proteasome-mediated degradation. Curly lines indicate specific mRNAs, with their colors matching that of the corresponding proteins. From Merchante et al.[
