Metallochaperones and metal-sensors in metal-allocation

“首先，你现在在这里，数万亿个漂浮的原子必须以一种复杂而奇怪的方式聚集在一起才能创造出你”，“为什么原子会这么麻烦是个谜”[比尔·布莱森（2003），摘自2004年安万特科学书籍奖得主《几乎所有事物的简史》导言的开头几行]。在组装的原子中有必需的金属，包括铜、锌、钴和铁。据估计，大约三分之一的基因产物需要一种或另一种金属才能发挥作用，我们不知道每种蛋白质是如何获得正确的金属的。一种天真的期望是，蛋白质会紧紧地与正确的金属结合，而只与其他所有金属弱结合，或者根本不结合。然而，从我们最近的工作中，我们报道了两种蛋白质，一种是在细菌细胞内感知多余的锌原子，另一种是将多余的锌原子运输出细胞，它们与错误的金属铜的结合比锌紧密得多。r·j·p·威廉姆斯（现为牛津大学化学名誉教授）指出，电池面临一个问题，因为金属的亲和顺序遵循一个一般的系列，这四个原子是铜>锌>钴>铁。一个细胞怎么可能包含需要最具竞争性的金属（如铜）的蛋白质，同时包含需要较不具竞争性的原子（如钴或铁）的蛋白质？一个简单的预测是，铜将与它们结合，使非铜蛋白质失去功能。据推测，除了正确的蛋白质外，其他金属（如铜）不知何故远离了所有蛋白质的金属结合位点。我们已经开发了一个理想的实验系统来测试这一点。比尔·布莱森的谜题的部分答案是，组装这些原子的大部分工作是由植物和一些微生物为你完成的，它们利用阳光作为能量将无机元素构建成初级有机分子。在植物叶绿体和与蓝藻有亲缘关系的细菌中，这种重要的能量转换涉及电子通过与质体青素结合的铜原子流动。因此可以认为，质体青素是地球上铜最重要的目的地。铜原子必须被运送到内部隔室，即类囊体，才能到达质体青素，但如果上述假设是正确的，那么就暴露了根本问题。如何防止铜在通往质体青素的途中与所有其他蛋白质错误结合？我们发现了一种途径，其中包括一种将铜带入细胞的进口蛋白，第二种蛋白质将铜运送到类囊体，第三种蛋白质，金属伴侣蛋白，在另外两种蛋白质之间运送铜。通过将铜从一种蛋白质传递到另一种蛋白质，假设这种金属可以远离所有其他蛋白质，但这意味着蛋白质之间的相互作用现在决定了金属蛋白质获得哪些金属，而不是它们固有的金属结合偏好。我们最近在分子水平上可视化了金属伴侣-转运体相互作用的细节，现在可以测试是什么选择了相互作用的伙伴。我们将测试这种相互作用的特异性是否会使铜远离锌转运体和检测其他金属的传感器。金属传感器有特殊用途，因为当它们在活细胞内获得金属时，它们可以报告。如果正确的话，这个假设意味着像铜这样的金属有可能与错误的蛋白质紧密结合。这对整个生物学都有影响，因为它会产生畸变的风险。这可能是细胞对金属营养过剩、限制和其他条件的反应的一个关键特征，这些条件可以促进金属从真正的位点释放，包括衰老和氧化应激。

'To begin with, for you to be here now trillions of drifting atoms had somehow to assemble in an intricate and curiously obliging manner to create you', 'why atoms take this trouble is a bit of a puzzle' [Bill Bryson (2003), taken from the opening lines of the Introduction to 'A short history of nearly everything', winner of the 2004 Aventis prize for science books]. Among the assembled atoms are essential metals including copper, zinc, cobalt and iron. It is estimated that approximately one third of all gene products need one or other metal to function and we do not understand how each protein acquires the correct metal. A naive expectation was that proteins would tightly bind the correct metal and only bind all others weakly, or not at all. However, from our recent work we have reported two proteins, one that senses surplus zinc atoms within a bacterial cell and the other that transports surplus zinc atoms out of the cell, that bind the wrong metal, copper, much more tightly than zinc. R.J.P. Williams (now Emeritus Professor of Chemistry in Oxford) notes that the cell faces a problem because the order of affinities for metals follows a general series which for these four atoms is copper>zinc>cobalt>iron. How can a cell contain proteins that require the most competitive metals, such as copper, while simultaneously containing proteins that require less competitive atoms, such as cobalt or iron? A simplistic prediction is that copper will bind to them all rendering the non-copper proteins non-functional. It is hypothesised that somehow or other metals such as copper are kept away from the metal-binding sites of all but the correct proteins. We have developed an ideal experimental system in which to test this. Part of the answer to Bill Bryson's puzzle is that much of the work to assemble these atoms is done for you by plants and some microbes, which build inorganic elements into primary organic molecules using sunlight as energy. In plant chloroplasts, and ancestrally related cyanobacteria, this vital energy conversion involves electrons flowing through copper atoms bound to plastocyanin. It could therefore be argued that plastocyanin is the most important destination for copper on Earth. Copper atoms must be delivered to internal compartments, thylakoids, to reach plastocyanin but if the above hypothesis is correct then this exposes the fundamental problem. How can copper be prevented from erroneously binding to all other proteins while en route to plastocyanin? We discovered a pathway involving an importer that brings copper into the cell, a second protein that transports it into the thylakoids and a third protein, a metallochaperone, which shuttles copper between the other two. By handing copper from one protein to the next in the pathway it is hypothesised that this metal can be kept away from all other proteins, but this implies that the protein-interactions now dictate which metals are acquired by metalloproteins rather than their inherent metal-binding preferences. We have recently visualised the metallochaperone-transporter interaction at a molecular level of detail and can now test what selects the interacting partners. We will test whether the specificity of this interaction keeps copper away from the zinc transporter and away from sensors that detect other metals. Metal-sensors are of special use because they can report when they acquire a metal inside a living cell. If correct, the hypothesis implies that metals such as copper have the potential to associate tightly with the wrong proteins. This has implications across biology because it creates a risk for aberrations. It is likely to be a key feature of cellular responses to metal nutrient-excess, -limitation and other conditions which could promote metal-release from bona fide sites, including senescence and oxidative stress.