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

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.