Porphyrins promote the association of GENOMES UNCOUPLED 4 and a Mg-chelatase subunit with chloroplast membranes.
Porphyrins promote the association of GENOMES UNCOUPLED 4 and a Mg-chelatase subunit with chloroplast membranes.
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DOI：
10.1074/jbc.m109.025205
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发表时间：
2009-09-11
期刊：
影响因子：
--
通讯作者：
Larkin RM
中科院分区：
文献类型：
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作者：
Adhikari ND;Orler R;Chory J;Froehlich JE;Larkin RM
在植物中，叶绿素和其他四吡咯是从位于叶绿体内的分支途径合成的。GUN 4（基因组不偶合4）通过激活镁螯合酶（将卟啉转化为叶绿素分支的酶）刺激叶绿素生物合成。GUN 4通过涉及结合Mg-螯合酶的ChlH亚基以及Mg-螯合酶的底物（原卟啉IX）和产物（Mg-原卟啉IX）的机制刺激Mg-螯合酶。我们选择测试GUN 4是否也可能影响镁螯合酶和叶绿体膜（叶绿素生物合成的位点）之间的相互作用。为了测试这一想法，我们诱导的叶绿素前体水平在纯化的豌豆叶绿体喂养这些叶绿体与5-氨基乙酰丙酸，确定GUN 4和镁螯合酶亚基的相对水平的可溶性和含膜的馏分来自这些叶绿体，并定量镁螯合酶活性的膜分离这些叶绿体。我们还监测了GUN 4水平的可溶性和含膜的馏分来自叶绿体喂养各种卟啉。我们的研究结果表明，5-aminolevulinic酸喂养刺激镁螯合酶的活性在叶绿体膜和卟啉结合形式的GUN 4和可能的ChlH协会最稳定的叶绿体膜。这些发现与GUN 4不仅通过激活Mg-螯合酶而且通过促进ChlH和叶绿体膜之间的相互作用来刺激叶绿素生物合成是一致的。
In plants, chlorophylls and other tetrapyrroles are synthesized from a branched pathway that is located within chloroplasts. GUN4 (GENOMES UNCOUPLED 4) stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits porphyrins to the chlorophyll branch. GUN4 stimulates Mg-chelatase by a mechanism that involves binding the ChlH subunit of Mg-chelatase, as well as a substrate (protoporphyrin IX) and product (Mg-protoporphyrin IX) of Mg-chelatase. We chose to test whether GUN4 might also affect interactions between Mg-chelatase and chloroplast membranes, the site of chlorophyll biosynthesis. To test this idea, we induced chlorophyll precursor levels in purified pea chloroplasts by feeding these chloroplasts with 5-aminolevulinic acid, determined the relative levels of GUN4 and Mg-chelatase subunits in soluble and membrane-containing fractions derived from these chloroplasts, and quantitated Mg-chelatase activity in membranes isolated from these chloroplasts. We also monitored GUN4 levels in the soluble and membrane-containing fractions derived from chloroplasts fed with various porphyrins. Our results indicate that 5-aminolevulinic acid feeding stimulates Mg-chelatase activity in chloroplast membranes and that the porphyrin-bound forms of GUN4 and possibly ChlH associate most stably with chloroplast membranes. These findings are consistent with GUN4 stimulating chlorophyll biosynthesis not only by activating Mg-chelatase but also by promoting interactions between ChlH and chloroplast membranes.