High-resolution structure and biochemical properties of the LH1–RC photocomplex from the model purple sulfur bacterium, Allochromatium vinosum

• Published in: Communications biology 2024-02, Vol.7 (1), p.176-10, Article 176
• Main Authors: Tani, Kazutoshi, Kanno, Ryo, Harada, Ayaka, Kobayashi, Yuki, Minamino, Akane, Takenaka, Shinji, Nakamura, Natsuki, Ji, Xuan-Cheng, Purba, Endang R., Hall, Malgorzata, Yu, Long-Jiang, Madigan, Michael T., Mizoguchi, Akira, Iwasaki, Kenji, Humbel, Bruno M., Kimura, Yukihiro, Wang-Otomo, Zheng-Yu
• Format: Article
• Language: English
• Subjects: 101/28; 631/535/1258/1259; 631/57/1464; Biochemical analysis; Biomedical and Life Sciences; Calcium; Chromatiaceae; Chromatiaceae - chemistry; Chromatiaceae - metabolism; Life Sciences; Light-Harvesting Protein Complexes - metabolism; Peptides - metabolism; Photosynthesis; Polypeptides; Sulfur; Thermal stability
• ISSN: 2399-3642; 2399-3642
• DOI: 10.1038/s42003-024-05863-w

The mesophilic purple sulfur phototrophic bacterium Allochromatium ( Alc .) vinosum (bacterial family Chromatiaceae ) has been a favored model for studies of bacterial photosynthesis and sulfur metabolism, and its core light-harvesting (LH1) complex has been a focus of numerous studies of photosynthetic light reactions. However, despite intense efforts, no high-resolution structure and thorough biochemical analysis of the Alc. vinosum LH1 complex have been reported. Here we present cryo-EM structures of the Alc. vinosum LH1 complex associated with reaction center (RC) at 2.24 Å resolution. The overall structure of the Alc. vinosum LH1 resembles that of its moderately thermophilic relative Alc. tepidum in that it contains multiple pigment-binding α- and β-polypeptides. Unexpectedly, however, six Ca ions were identified in the Alc. vinosum LH1 bound to certain α1/β1- or α1/β3-polypeptides through a different Ca 2+ -binding motif from that seen in Alc. tepidum and other Chromatiaceae that contain Ca 2+ -bound LH1 complexes. Two water molecules were identified as additional Ca 2+ -coordinating ligands. Based on these results, we reexamined biochemical and spectroscopic properties of the Alc. vinosum LH1–RC. While modest but distinct effects of Ca 2+ were detected in the absorption spectrum of the Alc. vinosum LH1 complex, a marked decrease in thermostability of its LH1–RC complex was observed upon removal of Ca 2+ . The presence of Ca 2+ in the photocomplex of Alc. vinosum suggests that Ca 2+ -binding to LH1 complexes may be a common adaptation in species of Chromatiaceae for conferring spectral and thermal flexibility on this key component of their photosynthetic machinery. The cryo-EM structure of the partially Ca 2 +  -bound light-harvesting 1–reaction center (LH1–RC) complex from Alc. vinosum , the best-studied model purple sulfur bacterium, is presented, characterized and compared to other photosynthetic bacteria.