Journal: Biomolecules / Year: 2025 Title: Probing the Dual Role of Ca in the LH1-RC Complex by Constructing and Analyzing Ca-Bound and Ca-Free LH1 Complexes. Authors: Mei-Juan Zou / Shuai Sun / Guang-Lei Wang / Yi-Hao Yan / Wei Ji / Zheng-Yu Wang-Otomo / Michael T Madigan / Long-Jiang Yu / Abstract: The genome of the mildly thermophilic hot spring purple sulfur bacterium, (.) , contains a multigene family that encodes a series of α- and β-polypeptides, collectively forming a heterogeneous ...The genome of the mildly thermophilic hot spring purple sulfur bacterium, (.) , contains a multigene family that encodes a series of α- and β-polypeptides, collectively forming a heterogeneous light-harvesting 1 (LH1) complex. The LH1, therefore, offers a unique model for studying an intermediate phenotype between phototrophic thermophilic and mesophilic bacteria, particularly regarding their LH1 transition and moderately enhanced thermal stability. Of the 16 α-polypeptides in the LH1, six α1 bind Ca to connect with β1- or β3-polypeptides in specific Ca-binding sites. Here, we use the purple bacterium strain H2 as a host to express Ca-bound and Ca-free LH1-only complexes composed of α- and β-polypeptides that either contain or lack the calcium-binding motif WxxDxI; purified preparations of each complex were then used to test how Ca affects their thermostability and spectral features. The cryo-EM structures of both complexes were closed circular rings consisting of 14 αβ-polypeptides. The absorption maximum of Ca-bound LH1 (α1/β1 and α1/β3) was at 894 nm, while that of Ca-free (α2/β1) was at 888 nm, indicating that Ca imparts a transition of 6 nm. Crucially for the ecological success of , Ca-bound LH1 complexes were more thermostable than Ca-free complexes, indicating that calcium plays at least two major roles in photosynthesis by -improving photocomplex stability and modifying its spectrum.
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