Kondo Kumiko, Ochiai Yuriko, Katayama Mitsunori, Ikeuchi Masahiko
Department of Biological Sciences , University of Tokyo, Tokyo 153-8902, Japan.
Plant Physiol. 2007 Jun;144(2):1200-10. doi: 10.1104/pp.107.099267. Epub 2007 Apr 27.
The phycobilisome (PBS) is a supramolecular antenna complex required for photosynthesis in cyanobacteria and bilin-containing red algae. While the basic architecture of PBS is widely conserved, the phycobiliproteins, core structure and linker polypeptides, show significant diversity across different species. By contrast, we recently reported that the unicellular cyanobacterium Synechocystis sp. PCC 6803 possesses two types of PBSs that differ in their interconnecting "rod-core linker" proteins (CpcG1 and CpcG2). CpcG1-PBS was found to be equivalent to conventional PBS, whereas CpcG2-PBS retains phycocyanin rods but is devoid of the central core. This study describes the functional analysis of CpcG1-PBS and CpcG2-PBS. Specific energy transfer from PBS to photosystems that was estimated for cells and thylakoid membranes based on low-temperature fluorescence showed that CpcG2-PBS transfers light energy preferentially to photosystem I (PSI) compared to CpcG1-PBS, although they are able to transfer to both photosystems. The preferential energy transfer was also supported by the increased photosystem stoichiometry (PSI/PSII) in the cpcG2 disruptant. The cpcG2 disruptant consistently showed retarded growth under weak PSII light, in which excitation of PSI is limited. Isolation of thylakoid membranes with high salt showed that CpcG2-PBS is tightly associated with the membrane, while CpcG1-PBS is partly released. CpcG2 is characterized by its C-terminal hydrophobic segment, which may anchor CpcG2-PBS to the thylakoid membrane or PSI complex. Further sequence analysis revealed that CpcG2-like proteins containing a C-terminal hydrophobic segment are widely distributed in many cyanobacteria.
藻胆体（PBS）是蓝细菌和含藻胆素的红藻光合作用所需的超分子天线复合体。虽然PBS的基本结构广泛保守，但藻胆蛋白、核心结构和连接多肽在不同物种间表现出显著差异。相比之下，我们最近报道单细胞蓝细菌集胞藻6803拥有两种类型的PBS，它们的“杆-核心连接”蛋白（CpcG1和CpcG2）不同。发现CpcG1-PBS等同于传统的PBS，而CpcG2-PBS保留了藻蓝蛋白杆但没有中央核心。本研究描述了CpcG1-PBS和CpcG2-PBS的功能分析。基于低温荧光对细胞和类囊体膜估计的从PBS到光系统的特定能量转移表明，与CpcG1-PBS相比，CpcG2-PBS将光能优先转移到光系统I（PSI），尽管它们能够转移到两个光系统。cpcG2缺失突变体中光系统化学计量比（PSI/PSII）增加也支持了这种优先能量转移。cpcG2缺失突变体在弱PSII光下持续表现出生长迟缓，其中PSI的激发受到限制。用高盐分离类囊体膜表明CpcG2-PBS与膜紧密结合，而CpcG1-PBS部分释放。CpcG2的特征在于其C末端疏水片段，它可能将CpcG2-PBS锚定到类囊体膜或PSI复合体上。进一步的序列分析表明，含有C末端疏水片段的CpcG2样蛋白广泛分布于许多蓝细菌中。
