Molecular Mechanism ofTtrapyrrole Distribution and Transportation in Higher Plants

在植物中，四氢吡咯被分为四大类：叶绿素、胆红素、血红素和四氢铁血红素。这些四吡咯对植物细胞的各种生命活动是不可或缺的。这些四吡咯在质体中合成，随后分配到各种细胞器。叶绿素只存在于质体中，而血红素是存在于细胞中各种细胞器中的大量脱氢血红素蛋白所必需的。此外，最近对叶绿体逆行信号的研究表明，一些叶绿素中间体，如镁原卟啉IX和镁原卟啉IX单甲酯，作为信号从叶绿体转移到细胞核，控制核编码的光合作用基因。然而，这些四吡咯在植物细胞中的分布和运输机制还不是很清楚。由于四吡咯在生理条件下在水溶液中很难溶解，因此推测植物细胞中一定存在四吡咯的运输系统。在本研究项目中，为了阐明四吡咯在高等植物中分布和运输的分子机制，我们首先通过辣根过氧化物酶的重组建立了极其灵敏的血红素分析方法，该方法可以检测从质体中流出的血红素。然后，我们对与p22HBP/SOLE家族同源的拟南芥胞质血红素结合蛋白(CHBPs)进行了生化特征分析。此外，我们通过叶绿体硫氧还蛋白阐明了氧化还原依赖的镁螯合酶的调控，它负责产生逆行信号，即镁原卟啉IX。这些贡献极大地加深了我们对植物细胞中四吡咯的调控和功能的理解。

In plants, tetrapyrroles are classified into four major groups, chlorophylls, bilins, hemes, and siroheme. These tetrapyrroles are indispensable for various vital activities of plant cells. These tetrapyrroles are synthesized in plastids, subsequently distributed to various organelles. Chlorophylls exist in only plastids, while hemes are required for numerous apo-heme proteins existing in various organelles in the cell. Furthermore, recent studies on retrograde signaling from the chloroplast suggest that some chlorophyll intermediates, such as Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester, act as signals transferred from plastids to the nucleus to control nuclear-encoded photosynthetic genes. However, the mechanisms of distribution and transportation of these tetrapyrroles in plant cells are not well understood. Since tetrapyrroles are poorly soluble in aqueous solutions under physiological conditions, it is assumed that there must be tetrapyrrole transportation system that are responsible for tetrapyrrole trafficking in the plant cell.In this research project, to clarify the molecular mechanism of tetrapyrrole distribution and transportation in higher plants, we first developed extremely sensitive heme assay by reconstitution with horseradish peroxidase apo-enzyme, which allows to detect effused heme from plastids. We then performed biochemical characterization of Arabidopsis cytosolic heme-binding proteins (cHBPs) homologous to the p22HBP/SOUL family. Furthermore, we clarified the redox-dependent regulation of Mg-chelatase via chloroplast thioredoxins, whish is responsible for production of retrograde signals, Mg-protoporphyrin IX.These contributions significantly enhanced our understandings of regulations and functions of tetrapyrroles in plant cells.