Thylakoid membranes in chloroplasts contain photosynthetic protein complexes that convert light energy into chemical energy. Photosynthetic protein complexes are considered to undergo structural reorganization to maintain the efficiency of photochemical reactions. A detailed description of the mobility of photosynthetic complexes in real time is necessary to understand how macromolecular organization of the membrane is altered by environmental fluctuations. Here, we used high-speed atomic force microscopy to visualize and characterize the in situ mobility of individual protein complexes in grana thylakoid membranes isolated from Spinacia oleracea. Our observations reveal that these membranes can harbor complexes with at least two distinctive classes of mobility. A large fraction of grana membranes contained proteins with quasistatic mobility exhibiting molecular displacements smaller than 10 nm2. In the remaining fraction, the protein mobility is variable with molecular displacements of up to 100 nm2. This visualization at high spatiotemporal resolution enabled us to estimate an average diffusion coefficient of ∼1 nm2 s−1. Interestingly, both confined and Brownian diffusion models could describe the protein mobility of the second group of membranes. We also provide the first direct evidence, to our knowledge, of rotational diffusion of photosynthetic complexes. The rotational diffusion of photosynthetic complexes could be an adaptive response to the high protein density in the membrane to guarantee the efficiency of electron transfer reactions. This characterization of the mobility of individual photosynthetic complexes in grana membranes establishes a foundation that could be adapted to study the dynamics of the complexes inside intact and photosynthetically functional thylakoid membranes to be able to understand its structural responses to diverse environmental fluctuations.

中文翻译：

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原子力显微镜可视化光合蛋白在颗粒类囊体膜中的流动性

叶绿体中的类囊体膜含有光合蛋白质复合物，可将光能转化为化学能。光合蛋白质复合物被认为进行结构重组以保持光化学反应的效率。实时详细描述光合复合物的流动性对于了解环境波动如何改变膜的大分子组织是必要的。在这里，我们使用高速原子力显微镜来可视化和表征单个蛋白质复合物在从菠菜中分离的格兰纳类囊体膜中的原位迁移率。我们的观察表明，这些膜可以包含具有至少两种不同移动性类别的复合物。大部分颗粒膜包含具有准静态迁移率的蛋白质，其分子位移小于 10 nm2。在剩下的部分中，蛋白质迁移率随着分子位移的变化而变化，最高可达 100 nm2。这种高时空分辨率的可视化使我们能够估计~1 nm2 s-1 的平均扩散系数。有趣的是，受限扩散模型和布朗扩散模型都可以描述第二组膜的蛋白质流动性。据我们所知，我们还提供了光合复合物旋转扩散的第一个直接证据。光合复合物的旋转扩散可能是对膜中高蛋白质密度的适应性反应，以保证电子转移反应的效率。