Probing the Dual Role of Ca 2+ in the Allochromatium tepidum LH1-RC Complex by Constructing and Analyzing Ca 2+ -Bound and Ca 2+ -Free LH1 Complexes.
Zou, M.J., Sun, S., Wang, G.L., Yan, Y.H., Ji, W., Wang-Otomo, Z.Y., Madigan, M.T., Yu, L.J.(2025) Biomolecules 15
- PubMed: 39858518
- DOI: https://doi.org/10.3390/biom15010124
- Primary Citation of Related Structures:
8YPB, 8YPD - PubMed Abstract:
The genome of the mildly thermophilic hot spring purple sulfur bacterium, Allochromatium ( Alc .) tepidum , contains a multigene pufBA family that encodes a series of α- and β-polypeptides, collectively forming a heterogeneous light-harvesting 1 (LH1) complex. The Alc. tepidum LH1, therefore, offers a unique model for studying an intermediate phenotype between phototrophic thermophilic and mesophilic bacteria, particularly regarding their LH1 Qy transition and moderately enhanced thermal stability. Of the 16 α-polypeptides in the Alc. tepidum LH1, six α1 bind Ca 2+ to connect with β1- or β3-polypeptides in specific Ca 2+ -binding sites. Here, we use the purple bacterium Rhodospirillum rubrum strain H2 as a host to express Ca 2+ -bound and Ca 2+ -free Alc. tepidum 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 2+ affects their thermostability and spectral features. The cryo-EM structures of both complexes were closed circular rings consisting of 14 αβ-polypeptides. The Q y absorption maximum of Ca 2+ -bound LH1 (α1/β1 and α1/β3) was at 894 nm, while that of Ca 2+ -free (α2/β1) was at 888 nm, indicating that Ca 2+ imparts a Q y transition of 6 nm. Crucially for the ecological success of Alc. tepidum , Ca 2+ -bound LH1 complexes were more thermostable than Ca 2+ -free complexes, indicating that calcium plays at least two major roles in photosynthesis by Alc. tepidum -improving photocomplex stability and modifying its spectrum.
Organizational Affiliation:
Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.