Protein-protein interactions in the early stages of chlorophyll biosynthesis

光合作用是地球上发生的最重要的过程，其中二氧化碳和水通过利用来自太阳的光能由绿色色素叶绿素转化为碳水化合物。所有的绿色植物，藻类和几种细菌都可以进行光合作用，它最终是地球上所有生命的基础。光合作用是一个多步骤的过程，涉及许多化学反应，由称为酶的专门蛋白质和大量其他蛋白质催化，这些蛋白质与叶绿素一起作为“捕光复合物（LHC）”吸收光线。叶绿素及其化学前体在不与LHC蛋白质结合时可能非常危险，对细胞造成氧化损伤。因此，必须严格控制和调节叶绿素的生物合成，以防止这种积累。叶绿素生物合成的第一个关键步骤是由多亚基酶镁（Mg）螯合酶进行的，该酶由H、I和D亚基组成。这是一个关键酶，因为它位于四吡咯生物合成的分支点。叶绿素和血红素是这一途径的最终产物;血红素是由亚铁螯合酶产生的，在呼吸过程中起着至关重要的作用，呼吸过程产生细胞所需的能量。根据细胞的需要，该途径的两个分支需要平衡;过多的血红素和/或过少的叶绿素和镁螯合酶活性将被刺激，过多的叶绿素和/或过少的血红素和亚铁螯合酶活性将被刺激。镁螯合酶被刺激的一种方式是通过添加一种称为GUN 4的蛋白质。这种蛋白质最初在高等植物拟南芥中被描述，其中它参与从叶绿体到细胞核的通信途径之一。通过该途径的信号传导由镁螯合酶产生的叶绿素生物合成中间体镁原卟啉IX介导。有趣的是，虽然GUN 4在Mg螯合酶上充当“加速剂”，但它在叶绿素生物合成途径中的下一个酶Mg原甲基转移酶（ChIM）上充当“制动器”。相反，镁螯合酶的H亚基刺激后者。这些刺激和抑制必须是GUN 4，螯合酶和甲基转移酶之间的物理相互作用的结果。我们将使用各种技术来研究H，I，D，ChlM和GUN 4之间的相互作用，使用纯化的蛋白质，也直接在模式绿色藻类集胞藻。确定这些蛋白质如何相互作用将使我们对叶绿素生物合成如何控制和调节有一个重要的了解。这项工作至关重要，因为它可以提供一种提高植物光合产量的潜在方法，特别是在胁迫条件下。

Photosynthesis is the most important process occurring on Earth whereby carbon dioxide and water is converted into carbohydrate using light energy harnessed from the sun by the green pigment chlorophyll. All green plants, algae and several species of bacteria can carry out photosynthesis and it is ultimately the basis of all life on the planet. Photosynthesis is a multi-step process involving many chemical reactions catalysed by specialised proteins known as enzymes and a large number of other proteins that act with chlorophyll as a 'light harvesting complex (LHC)' to absorb light. Chlorophyll, and its chemical precursors, can be highly dangerous when not bound to LHC proteins, causing oxidative damage to the cell. Chlorophyll biosynthesis must therefore be tightly controlled and regulated to prevent such a build-up. The first committed step of chlorophyll biosynthesis is carried out by the multi-subunit enzyme magnesium (Mg) chelatase that is made up of the H, I and D subunits. This is a key enzyme as it lies at the branch point of tetrapyrolle biosynthesis. Chlorophyll and haem are the end products of this pathway; haem is produced by the enzyme ferrochelatase and plays a crucial role in the process of respiration which generates energy for the cells needs. Depending on the cell's requirements the two branches of this pathway need to be balanced; too much haem and/or too little chlorophyll and magnesium chelatase activity will be stimulated, too much chlorophyll and/or too little haem and ferrochelatase activity will be stimulated. One way in which Mg chelatase is stimulated is by the addition of a protein known as GUN4. This protein was originally described in the higher plant Arabidopsis where it is involved in one of the communication pathways from the chloroplast to the nucleus. Signalling through this pathway is mediated by the chlorophyll biosynthetic intermediate magnesium protoporphyrin IX produced by magnesium chelatase. Interestingly, whilst GUN4 acts as an 'accelerator' on Mg chelatase it works as a 'brake' on the next enzyme in the chlorophyll biosynthetic pathway, Mg proto methyltransferase (ChlM). In contrast, the H subunit of Mg chelatase stimulates the latter. These stimulations and inhibitions must be the result of physical interactions between GUN4, the chelatase and the methyltransferase. We will use various techniques to investigate interactions between H, I, D, ChlM and GUN4 using the purified proteins and also directly in the model green algae Synechocystis. Determining how these proteins interact with each other will give us an important insight into how chlorophyll biosynthesis is controlled and regulated. This work is of fundamental importance as it could provide a potential way of improving photosynthetic yield in plants, particularly under stress conditions.