STRUCTURAL STUDIES OF GUN4 AND THE GUN4/GUN5 COMPLEX

该子项目是利用NIH/NCRR资助的中心赠款提供的资源的许多研究子项目之一。子项目和研究者（PI）可能从另一个NIH来源获得了主要资金，因此可以在其他CRISP条目中表示。所列机构为中心，不一定是研究者所在机构。质体正常发育所需的许多因子实际上都编码在植物的核基因组中。此外，越来越多的证据表明，这些基因的协调表达部分取决于叶绿体的功能状态。例如，导致发育停滞的叶绿体的核突变也导致核定位的光合基因的表达减少。这种质体衍生的信号级联对于协调编码光合作用组分的核和叶绿体定位基因是必不可少的。拟南芥GUN（genomes uncoupled）基因座被认为是质体到细胞核信号传导途径的组成部分。目前，在GUN基因座内有五个已知的基因（GUN 1、GUN 2、GUN 3、GUN 4和GUN 5），其中，直到最近，GUN 5是编码已知功能的蛋白质（Mg-螯合酶的ChlH亚基）的唯一基因。GUN 4最近被证明编码一种功能未知的新蛋白，该蛋白似乎直接与GUN 5相互作用，以增强其Mg-螯合酶活性，并赋予其ATP酶活性（未发表的数据RL）。有趣的是，在gun 4和gun 5植物中叶绿素积累减少，并且gun 4gun 5双突变体产生白化病植物。这些发现表明，四吡咯生物合成途径控制核基因编码质体定位蛋白质的转录。为了更清楚地了解这种信号传导途径的机制，我们建议从结构上表征新型蛋白质gun 4，并确定gun 4/gun 5介导的Mg-螯合酶活性背后的生物物理机制。这项工作将提供一个模板，为进一步研究这一独特的信号通路在体外和体内的模式真核生物，拟南芥。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Many of the factors necessary for proper plastid development are actually encoded in the plant's nuclear genome. Furthermore a growing the body of evidence suggests that the coordinated expression of these genes depends in part on the functional state of the chloroplast. For example, nuclear mutations that result in developmentally arrested chloroplasts also result in the reduced expression of nuclear-localized photosynthetic genes. This plastid-derived signaling cascade is essential for coordination of both nuclear and chloroplast localized genes that encode components of photosynthesis. Arabidopsis GUN (genomes uncoupled) loci have been identified as components of the plastid to nucleus signaling pathway. There are currently, five known genes within the GUN loci (GUN1, GUN2, GUN3, GUN4 and GUN5), of which, until recently, GUN5 was the only gene to encode a protein of known function, the ChlH subunit of Mg-chelatase. GUN4 was recently shown to encode a novel protein of unknown function that appears to directly interact with GUN5 to both enhance its Mg-chelatase activity, as well as confer on it, ATPase activity (unpublished data RL). Interestingly, there is a reduction in chlorophyll accumulation in both gun4 and gun5 plants and gun4gun5 double mutants produce albino plants. These findings suggest that the tetrapyrrole biosynthetic pathway controls transcription of nuclear genes encoding plastid-localized proteins. In an effort to understanding the mechanism of this signaling pathway more clearly, we propose to structurally characterize the novel protein gun4 and also determine the biophysical mechanism behind gun4/gun5 mediated Mg-chelatase activity. This work will provide a template for further investigation of this unique signaling pathway both in vitro and in vivo in a model eukaryote, Arabidopsis thaliana.