The chlorophyll biosynthesis regulator GENOMES UNCOUPLED 4 (GUN4) is conserved in nearly all oxygenic photosynthetic organisms. Recently, GUN4 has been found to be able to bind the linear tetrapyrroles (bilins) and stimulate the magnesium chelatase activity in the unicellular green alga Chlamydomonas reinhardtii. Here, we characterize GUN4 proteins from Arabidopsis thaliana and the cyanobacterium Synechocystis sp. PCC 6803 for their ability to bind bilins, and present the crystal structures of Synechocystis GUN4 in biliverdin-bound, phycocyanobilin-bound, and phytochromobilin-bound forms at the resolutions of 1.05, 1.10, and 1.70 Å, respectively. These linear molecules adopt a cyclic-helical conformation, and bind more tightly than planar porphyrins to the tetrapyrrole-binding pocket of GUN4. Based on structural comparison, we propose a working model of GUN4 in regulation of tetrapyrrole biosynthetic pathway, and address the role of the bilin-bound GUN4 in retrograde signaling.

中文翻译：

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叶绿素生物合成调节剂 GUN4 结合胆素的结构基础

叶绿素生物合成调节因子 GENOMES UNCOUPLED 4 (GUN4) 在几乎所有含氧光合生物中都是保守的。最近，已发现 GUN4 能够结合线性四吡咯 (bilins) 并刺激单细胞绿藻莱茵衣藻中的镁螯合酶活性。在这里，我们描述了来自拟南芥和蓝藻集胞藻属的GUN4 蛋白。PCC 6803 能够结合胆素，并呈现集胞藻的晶体结构GUN4 在胆绿素结合、藻蓝胆素结合和植物色素结合形式中的分辨率分别为 1.05、1.10 和 1.70 Å。这些线性分子采用环状螺旋构象，并且比平面卟啉更紧密地与 GUN4 的四吡咯结合袋结合。基于结构比较，我们提出了 GUN4 在调节四吡咯生物合成途径中的工作模型，并解决了胆素结合的 GUN4 在逆行信号传导中的作用。