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<p>Solanesyl diphosphate synthase (SPS) is crucial for photosynthesis, as it supplies prenyl precursors for the biosynthesis of the photosynthetic electron carrier, plastoquinone-9 (PQ-9). Fibrillin 5 (FBN5) stimulates SPS catalytic activity through direct binding, which is essential for normal plant growth. However, the molecular mechanism of FBN5-mediated SPS catalytic regulation remains unclear. In <i>Oryza sativa</i> (rice), <i>Os</i>SPS3 is an important plastid-localized SPS isoform involved in PQ-9 formation. The Os<i>fbn5</i> mutant plants display photodamage with exacerbated PQ-9 deficiency when exposed to high light. Here rice serves as a model organism to study SPS and FBN5. We report the crystal structures of the apo and inhibitor-bound forms of <i>Os</i>SPS3, revealing the alternating catalytic mechanism of the asymmetric <i>Os</i>SPS3 dimer. In addition, we report the cryo-electron microscopy structures of the apo and ligand-bound forms of the <i>Os</i>SPS3–FBN5 complex, showing that <i>Os</i>FBN5 binding triggers an open-to-closed conformational transition of a lid-like capping loop within the inactive monomer of <i>Os</i>SPS3, allowing both monomers of dimeric <i>Os</i>SPS3 to be catalytically active. A comparison of the enzymatic activities of the wild-type <i>Os</i>SPS3 homodimer and a recombinant <i>Os</i>SPS3 heterodimer containing one inactive mutant subunit revealed that <i>Os</i>FBN5 enhances the activity of <i>Os</i>SPS3 by inducing a synchronous catalytic mechanism. This work reveals the dynamic catalytic mechanism of <i>Os</i>SPS3 and provides a structural basis for understanding its function and the FBN5-mediated regulation of the PQ-9 biosynthesis pathway.</p>

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Structural insights into the molecular mechanisms of OsFBN5-induced OsSPS3 catalysis

  • 期刊论文

  • 发表日期：

    2026年1月5日

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  • Han Xiao,
  • Xing-Xing Shi,
  • Min Li,
  • You-Wang Wang,
  • Da-Wei Wang,
  • Long-Can Mei,
  • Hong-Yan Lin,
  • Ping Zhu,
  • Guang-Fu Yang

摘要

Solanesyl diphosphate synthase (SPS) is crucial for photosynthesis, as it supplies prenyl precursors for the biosynthesis of the photosynthetic electron carrier, plastoquinone-9 (PQ-9). Fibrillin 5 (FBN5) stimulates SPS catalytic activity through direct binding, which is essential for normal plant growth. However, the molecular mechanism of FBN5-mediated SPS catalytic regulation remains unclear. In Oryza sativa (rice), OsSPS3 is an important plastid-localized SPS isoform involved in PQ-9 formation. The Osfbn5 mutant plants display photodamage with exacerbated PQ-9 deficiency when exposed to high light. Here rice serves as a model organism to study SPS and FBN5. We report the crystal structures of the apo and inhibitor-bound forms of OsSPS3, revealing the alternating catalytic mechanism of the asymmetric OsSPS3 dimer. In addition, we report the cryo-electron microscopy structures of the apo and ligand-bound forms of the OsSPS3–FBN5 complex, showing that OsFBN5 binding triggers an open-to-closed conformational transition of a lid-like capping loop within the inactive monomer of OsSPS3, allowing both monomers of dimeric OsSPS3 to be catalytically active. A comparison of the enzymatic activities of the wild-type OsSPS3 homodimer and a recombinant OsSPS3 heterodimer containing one inactive mutant subunit revealed that OsFBN5 enhances the activity of OsSPS3 by inducing a synchronous catalytic mechanism. This work reveals the dynamic catalytic mechanism of OsSPS3 and provides a structural basis for understanding its function and the FBN5-mediated regulation of the PQ-9 biosynthesis pathway.