Probing the Dual Role of Ca<sup>2+</sup> in the <i>Allochromatium tepidum</i> LH1–RC Complex by Constructing and Analyzing Ca<sup>2+</sup>-Bound and Ca<sup>2+</sup>-Free LH1 Complexes
The genome of the mildly thermophilic hot spring purple sulfur bacterium, <i>Allochromatium</i> (<i>Alc</i>.) <i>tepidum</i>, contains a multigene <i>pufBA</i> family that encodes a series of α- and β-polypeptides, collectively forming a heterogeneous...
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| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-01-01
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| Series: | Biomolecules |
| Subjects: | |
| Online Access: | https://www.mdpi.com/2218-273X/15/1/124 |
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| Summary: | The genome of the mildly thermophilic hot spring purple sulfur bacterium, <i>Allochromatium</i> (<i>Alc</i>.) <i>tepidum</i>, contains a multigene <i>pufBA</i> family that encodes a series of α- and β-polypeptides, collectively forming a heterogeneous light-harvesting 1 (LH1) complex. The <i>Alc. tepidum</i> LH1, therefore, offers a unique model for studying an intermediate phenotype between phototrophic thermophilic and mesophilic bacteria, particularly regarding their LH1 <i>Qy</i> transition and moderately enhanced thermal stability. Of the 16 α-polypeptides in the <i>Alc. tepidum</i> LH1, six α1 bind Ca<sup>2+</sup> to connect with β1- or β3-polypeptides in specific Ca<sup>2+</sup>-binding sites. Here, we use the purple bacterium <i>Rhodospirillum rubrum</i> strain H2 as a host to express Ca<sup>2+</sup>-bound and Ca<sup>2+</sup>-free <i>Alc. tepidum</i> 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<sup>2+</sup> affects their thermostability and spectral features. The cryo-EM structures of both complexes were closed circular rings consisting of 14 αβ-polypeptides. The <i>Q<sub>y</sub></i> absorption maximum of Ca<sup>2+</sup>-bound LH1 (α1/β1 and α1/β3) was at 894 nm, while that of Ca<sup>2+</sup>-free (α2/β1) was at 888 nm, indicating that Ca<sup>2+</sup> imparts a <i>Q<sub>y</sub></i> transition of 6 nm. Crucially for the ecological success of <i>Alc. tepidum</i>, Ca<sup>2+</sup>-bound LH1 complexes were more thermostable than Ca<sup>2+</sup>-free complexes, indicating that calcium plays at least two major roles in photosynthesis by <i>Alc. tepidum</i>—improving photocomplex stability and modifying its spectrum. |
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| ISSN: | 2218-273X |