Institute of Low Temperature Science, Hokkaido University, N19 W8 Kita-Ku, Sapporo, 060-0819, Japan.
Department of Natural Sciences and Environmental Health, University College of Southeast Norway, Gullbringvegen 36, 3880, Bø, Norway.
Photosynth Res. 2018 Apr;136(1):49-61. doi: 10.1007/s11120-017-0438-z. Epub 2017 Aug 30.
Diverse light-harvesting complexes (LHCs) have been found in photosynthetic microalgae that originated from secondary endosymbiosis involving primary red algae. However, the associations between LHCs and photosystem I (PSI) and photosystem II (PSII) in these microalgae are not fully understood. Eustigmatophyta is a red algal lineage that appears to have a unique organization in its photosynthetic machinery, consisting of only chlorophyll a and carotenoids that are atypical compared with other closely related groups. In this study, the supramolecular organization of pigment-protein complexes in the eustigmatophyte alga, Nannochloropsis granulata was investigated using Clear Native (CN) PAGE coupled with two-dimensional (2D) SDS-PAGE. Our results showed two slowly migrating green bands that corresponded to PSII supercomplexes, which consisted of reaction centers and LHCs. These green bands were also characterized as PSII complexes by their low temperature fluorescence emission spectra. The protein subunits of the PSII-LHC resolved by 2D CN/SDS-PAGE were analyzed by mass spectrometry, and four different LHC proteins were identified. Phylogenetic analysis of the identified LHC protein sequences revealed that they belonged to four different Lhc groups; (1) stress-related Lhcx proteins, (2) fucoxanthin chlorophyll a/c-binding Lhcf proteins, (3) red-shifted Chromera light-harvesting proteins (Red-CLH), and (4) Lhcr proteins, which are commonly found in organisms possessing red algal plastids. This is the first report showing evidence of a pigment-protein supercomplex consisting of PSII and LHCs, and to identify PSII-associated LHC proteins in Nannochloropsis.
不同的光捕获复合物（LHCs）已在源自涉及初级红藻的二次内共生的光合微藻中被发现。然而，这些微藻中的 LHCs 与光系统 I（PSI）和光系统 II（PSII）之间的关联尚未完全了解。Eustigmatophyta 是红藻谱系，其光合作用机制似乎具有独特的组织，仅由叶绿素 a 和类胡萝卜素组成，与其他密切相关的群体相比是非典型的。在这项研究中，使用 Clear Native（CN）PAGE 与二维（2D）SDS-PAGE 结合，研究了真眼点藻，Nannochloropsis granulata 中色素-蛋白复合物的超分子组织。我们的结果显示了两个缓慢迁移的绿色条带，对应于 PSII 超复合物，由反应中心和 LHCs 组成。这些绿色条带也被证明是 PSII 复合物，因为它们具有低温荧光发射光谱。通过 2D CN/SDS-PAGE 解析的 PSII-LHC 的蛋白质亚基通过质谱进行分析，并鉴定了四种不同的 LHC 蛋白。鉴定的 LHC 蛋白序列的系统发育分析表明，它们属于四个不同的 LHC 组；（1）与应激相关的 Lhcx 蛋白，（2）岩藻黄素叶绿素 a/c 结合的 Lhcf 蛋白，（3）红移 Chromera 光捕获蛋白（Red-CLH），和（4）常见于具有红藻质体的生物中的 Lhcr 蛋白。这是首次报道显示由 PSII 和 LHCs 组成的色素-蛋白超复合物的证据，并在 Nannochloropsis 中鉴定与 PSII 相关的 LHC 蛋白。
