Multiscale structural basis of photoprotection in plant light-harvesting proteins

植物生产力是由有效和安全地收集阳光的能力定义的。高效的光捕获是由“天线”确保的，这是一个充满叶绿素的蛋白质的大集合，它捕获太阳能并将其传递到少数几个“反应中心”蛋白质，将太阳能转化为化学能。然而，光水平可以在几分钟内从中等亮度上升到危险亮度，导致能量的过度吸收和对微妙反应中心的损害。这可以通过能量依赖性猝灭（或“qE”）来缓解，这是一种保护机制，其中天线感应高强度的光并切换到保护状态。在这种状态下，多余的能量在损坏发生之前被无害地耗散（或“淬火”）。最近，研究表明，在分子水平上控制qE是提高粮食产量的一条非常有前途的途径。与其他重要的生物学机制不同，量化宽松的确切运作机制尚不清楚，这在一定程度上阻碍了这一点。我们知道主要的天线蛋白LHCII起着核心作用。高光强导致单个LHCII蛋白在能量收集和能量淬灭状态之间切换。它们还集体重组自己，在叶绿体膜上形成大的簇状网络。因此，qE是一种“多尺度”机制，涉及天线蛋白内部和整个天线组装中的结构变化。不幸的是，我们没有这些结构变化的详细描述，这使得实验数据难以解释，并导致了许多关于量化宽松如何运作的相互矛盾的想法。我们将建立比以往任何时候都更精确的量化宽松机制。我们将使用一种“自下而上”的方法，首先研究单个的、孤立的LHCII如何运作，并以此来建立它们如何作为一个完整的“天线”集体运作。首先，我们将预测LHCII的捕光结构。由于实验技术无法做到这一点，我们将使用严格的计算模拟。这使我们能够模拟现实条件，在这种情况下，LHCII在弱光下的膜中，并看到结构的运动和灵活性。使用理论生物物理学的技术，我们将解释这种结构如何促进能量的有效收集，而不是故意的保护性耗散。然后，我们将预测潜在的保护，LHCII的能量耗散结构。目前的3D结构LHCII经常被用作保护状态的原型，但它可能是一个很差的近似。我们将使用我们的生物模拟方法来考虑高光条件，预测这些结构，并充分表征它们如何发挥保护作用。在最后一部分，我们将通过实验和理论来考虑天然膜中出现的LHCII的大型网络。使用超高分辨率显微镜，我们将表征这些网络的结构。与此同时，使用高分辨率荧光成像，我们将直接测量能量在其中被淬灭的速率。当结合我们的LHCII模拟时，我们开发了qE机制的全貌：它如何在分子水平上运作，它如何受蛋白质结构和外部条件的控制，以及这如何影响植物天线的整体功能。这将最终回答植物科学中一个长期存在的争议问题。更重要的是，它将为我们理解植物如何捕获、管理和转化太阳能提供分子基础。

Plant productivity is defined by the ability to efficiently and safely harvest sunlight. Efficient light-harvesting is ensured by the 'antenna', a large assembly of chlorophyll-filled proteins that capture solar energy and deliver it to the few 'reaction centre' proteins that convert solar energy into chemical energy. However, light levels can to go from moderate to dangerously bright within minutes, leading to excess absorption of energy and damage to the delicate reaction centres. This is mitigated by Energy-dependent quenching (or 'qE'), a protective mechanism in which the antenna senses the high intensity of light and switches to a protective state. In this state excess energy is harmlessly dissipated (or 'quenched') before damage occurs. Recently, it was shown that controlling qE at the molecular level is a very promising route to enhancing food production. This is somewhat hindered by the fact that, unlike other important biological mechanisms, the precise workings of qE are unclear. We do know that the major antenna protein, LHCII, plays a central role. High light intensity causes individual LHCII proteins to switch between energy-harvesting and energy-quenching states. They also collectively reorganize themselves to form large, clustered networks in the chloroplast membrane. qE is therefore a 'multi-scale' mechanism involving structural changes inside antenna proteins and in the whole antenna assembly. Unfortunately, we don't have a detailed picture of what these structural changes are which makes experimental data difficult to interpret and has led to many contradictory ideas of how qE works. We will establish the mechanism of qE with greater accuracy than ever possible. We will use a 'bottom-up' approach, first studying how individual, isolated LHCII operate and using this to establish how they collectively operate as an entire 'antenna'. First we will predict the light-harvesting structure of LHCII. Since experimental techniques have failed to do this, we will instead use rigorous computational simulation. This allows us to mimic realistic conditions, in this case LHCII in a membrane under low light, and to see the movement and flexibility of the structure. Using the techniques of theoretical biophysics we will explain how this structure promotes efficient harvesting of energy rather than deliberate protective dissipation. We will then predict the potential protective, energy-dissipating structures of LHCII. The current 3D structure LHCII is often used as a prototype for the protective state but it is likely a poor approximation. We will use our bio-simulation approach to consider high light conditions, predict these structures, and fully characterize how they function protectively. In the final part, we will consider the large networks of LHCII that occur in natural membranes by experimentation as well as theory. Using ultra-high resolution microscopy we will characterize the structures of these networks. At the same time, using high resolution fluorescence imaging, we will directly measure the rate at which energy is being quenched within them. When combined with our LHCII simulations we develop a complete picture of the qE mechanism: how it operates at the molecular level, how it is controlled by protein structure and external conditions, and how this effects the function of the plant antenna as a whole. This will finally answer a long-standing and controversial question in plant science. More importantly, it will provide a molecular foundation to our efforts to understand how plants capture, manage and transform the sun's energy.