Department of Biomedical Sciences, University of Padova, Via Ugo Bassi 58 B, 35121 Padova, Italy.
Applied Science and Technology Department-BioSolar Lab, Politecnico di Torino, Environment Park, Via Livorno 60, 10144 Torino, Italy.
Int J Mol Sci. 2020 Nov 16;21(22):8643. doi: 10.3390/ijms21228643.
In plant thylakoid membranes Photosystem II (PSII) associates with a variable number of antenna proteins (LHCII) to form different types of supercomplexes (PSII-LHCII), whose organization is dynamically adjusted in response to light cues, with the CS more abundant in high-light and the CSM in low-light. Paired PSII-LHCII supercomplexes interacting at their stromal surface from adjacent thylakoid membranes were previously suggested to mediate stacking. Here, we present the cryo-electron microscopy maps of paired CS and CSM supercomplexes isolated from pea plants grown in high-light and low-light, respectively. These maps show a different rotational offset between the two supercomplexes in the pair, responsible for modifying their reciprocal interaction and energetic connectivity. This evidence reveals a different way by which paired PSII-LHCII supercomplexes can mediate stacking at diverse irradiances. Electrostatic stromal interactions between LHCII trimers almost completely overlapping in the paired CS can be the main determinant by which PSII-LHCII supercomplexes mediate stacking in plants grown in high-light, whereas the mutual interaction of stromal N-terminal loops of two facing Lhcb4 subunits in the paired CSM can fulfil this task in plants grown in low-light. The high-light induced accumulation of the Lhcb4.3 protein in PSII-LHCII supercomplexes has been previously reported. Our cryo-electron microscopy map at 3.8 Å resolution of the CS supercomplex isolated from plants grown in high-light suggests the presence of the Lhcb4.3 protein revealing peculiar structural features of this high-light-specific antenna important for photoprotection.
在植物类囊体膜中，光系统 II（PSII）与数量不定的天线蛋白（LHCII）结合形成不同类型的超复合体（PSII-LHCII），其组织在光信号的刺激下动态调整，CS 在高光下更为丰富，CSM 在低光下更为丰富。先前有人提出，相邻类囊体膜上基质面相互作用的 PSII-LHCII 超复合体可能介导堆叠。在这里，我们展示了分别从高光和低光下生长的豌豆植物中分离出的配对 CS 和 CSM 超复合体的冷冻电镜图谱。这些图谱显示出配对中超复合体之间的不同旋转偏移，这负责改变它们的相互作用和能量连接。这一证据揭示了配对 PSII-LHCII 超复合体在不同辐照度下介导堆叠的不同方式。配对 CS 中几乎完全重叠的 LHCII 三聚体之间的静电基质相互作用可能是 PSII-LHCII 超复合体在高光下介导堆叠的主要决定因素，而配对 CSM 中两个面对的 Lhcb4 亚基的基质 N 端环的相互作用可以在低光下完成这项任务。先前已经报道了高光下 Lhcb4.3 蛋白在 PSII-LHCII 超复合体中的积累。我们在 3.8 Å 分辨率下对高光下生长的植物分离出的 CS 超复合体的冷冻电镜图谱表明存在 Lhcb4.3 蛋白，揭示了这种高光特异天线对光保护很重要的特殊结构特征。
