Structural implications for a phycobilisome complex from the thermophilic cyanobacterium Thermosynechococcus vulcanus

• Published in: Biochimica et biophysica acta. Bioenergetics 2021-09, Vol.1862 (9), p.148458-148458, Article 148458
• Main Authors: Kawakami, Keisuke, Nagao, Ryo, Tahara, Yuhei O., Hamaguchi, Tasuku, Suzuki, Takehiro, Dohmae, Naoshi, Kosumi, Daisuke, Shen, Jian-Ren, Miyata, Makoto, Yonekura, Koji, Kamiya, Nobuo
• Format: Article
• Language: English
• Subjects: Cyanobacteria; Electron microscopy; GraFix; Phycobilisome
• ISSN: 0005-2728; 1879-2650
• DOI: 10.1016/j.bbabio.2021.148458

Phycobilisomes (PBSs) are huge, water-soluble light-harvesting complexes used by oxygenic photosynthetic organisms. The structures of some subunits of the PBSs, including allophycocyanin (APC) and phycocyanin (PC), have been solved by X-ray crystallography previously. However, there are few reports on the overall structures of PBS complexes in photosynthetic organisms. Here, we report the overall structure of the PBS complex isolated from the cyanobacterium Thermosynechococcus vulcanus, determined by negative-staining electron microscopy (EM). Intact PBS complexes were purified by trehalose density gradient centrifugation with a high-concentration phosphate buffer and then subjected to a gradient-fixation preparation using glutaraldehyde. The final map constructed by the single-particle analysis of EM images showed a hemidiscoidal structure of the PBS, consisting of APC cores and peripheral PC rods. The APC cores are composed of five cylinders: A1, A2, B, C1, and C2. Each of the cylinders is composed of three (A1 and A2), four (B), or two (C1 and C2) APC trimers. In addition, there are eight PC rods in the PBS: one bottom pair (Rb and Rb'), one top pair (Rt and Rt'), and two side pairs (Rs1/Rs1′ and Rs2/Rs2′). Comparison with the overall structures of PBSs from other organisms revealed structural characteristics of T. vulcanus PBS. •PBS complexes are purified from a cyanobacterium Thermosynechococcus vulcanus.•A surface structure of the PBS is solved by negative-staining EM analysis.•Characteristic APC and PC structures of the PBS are observed.•Unique assembly model of the T. vulcanus PBS is proposed.