光合作用需要光，但是过量的光能会导致光合作用装置的氧化性破坏，而植物需要也具备将过量光能耗散掉的光保护机制。叶黄素循环，即强弱光下紫黄质和玉米黄质之间的相互转换，是植物重要的光保护机制之一。光合膜上的主要捕光天线复合体（LHCII）可以发生叶黄素循环，但是光系统I的捕光天线复合体（LHCI）是否能够发生叶黄素循环还存在很大争议。我们PSI-LHCI的晶体结构首次确认了LHCI结合4个紫黄质，具备叶黄素发生的色素基础。本项目拟以豌豆为研究材料，以PSI-LHCI为研究对象，研究过剩光强下LHCI是否发生叶黄素循环；由于强光诱导质子化、质子化诱导LHCI结构调整，结构调整可能诱导叶黄素循环，拟采用结晶学技术解析DCCD-PSI-LHCI的结构，阐明LHCI是否发生叶黄素循环的结构原因。本项目将阐明LHCI是否存在叶黄素循环的光保护机制及其结构基础，也有助于阐明PSI-LHCI的光保护机理。

Photosynthetic organisms, which transfer sunlight into chemical energy, are crucial for life on Earth as they provide food and oxygen and are at the basis of most energy resources. The amount of sunlight reaching the surface of Earth varies substantially, and photosynthetic organisms have strategies that allow them to adapt to different light quantity. Under high light, plants have photoprotection mechanisms to dissipate a large part of the harvested light energy as heat to avoid overexcitation of the photosynthetic machinery. The xanthophyll cycle, that is the interconversion between violaxanthin and zeaxanthin under high and low light, is one of important photoprotection mechanisms. Although it has been proved that the xanthophyll cycle can be happened in light harvesting complex II, there is still disagreement on light harvesting complex I. Our PSI-LHCI crystal structure revealed that LHCI binds 4 violaxanthin molecules, which is the prerequisite for the xanthophyll cycle. In this project, isolated PSI-LHCI from high light treated pea will be investigated by HPLC to elucidate whether the xanthophyll violaxanthin converts into zeaxanthin in LHCI. Since the xanthophyll cycle may be induced by the conformational changes of the proteins, which relies on high light-induced low lumenal pH, we plan to elucidate the relationship between low pH and the conformational changes of proteins by analyzing the crystal structure of DCCD-PSI-LHCI. This project will reveal whether the xanthophyll cycle happens in LHCI to protect from high light and its structural basis, and will elucidate the photoprotection mechanism within LHCI, and even PSI-LHCI.