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<p>Photosystem I (PSI) harvests light through light-harvesting complexes (LHCs) to drive electron transfer. <i>Euglena gracilis</i>, a secondary endosymbiotic green alga, yet the architecture of its PSI–LHCs supercomplex has remained unresolved. Here, we report a 2.06 Å cryo-electron microscopy structure of the <i>E. gracilis</i> PSI–LHCI–LHCII supercomplex, revealing a minimal PSI core associated with ten antennas arranged in two layers on the PsaM-facing side. Each inner and outer LHC forms a face-to-face pair, while four LHCI heterodimers, stabilized by conserved Loop_23 motifs, assemble into two tetramers bridged by a central LHCI–LHCII pair. Notably, a ~ 40-residue insertion in PsaD was observed, anchoring LHC-3 and stabilizing LHC belts. Moreover, <i>Euglena</i>-specific chlorophylls and a distinctive arrangement of red chlorophylls establish a distinct pigment network, channeling excitation-energy into the core. These findings uncover a lineage-specific strategy for light-harvesting, highlighting how secondary plastids remodel PSI–LHCs architecture to support photosynthesis.</p>

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The architecture and energy transfer pathways of PSI–LHCI–LHCII in the phototrophic flagellate Euglena gracilis

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年3月5日

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  • Guoqiang Huang,
  • Shishang Dong,
  • Lin Li,
  • Lixia Zhu,
  • Jiayi Bai,
  • Xing Han,
  • Jiasheng Jiang,
  • Jinxin Ju,
  • Sen-Fang Sui,
  • Xiaochun Qin

摘要

Photosystem I (PSI) harvests light through light-harvesting complexes (LHCs) to drive electron transfer. Euglena gracilis, a secondary endosymbiotic green alga, yet the architecture of its PSI–LHCs supercomplex has remained unresolved. Here, we report a 2.06 Å cryo-electron microscopy structure of the E. gracilis PSI–LHCI–LHCII supercomplex, revealing a minimal PSI core associated with ten antennas arranged in two layers on the PsaM-facing side. Each inner and outer LHC forms a face-to-face pair, while four LHCI heterodimers, stabilized by conserved Loop_23 motifs, assemble into two tetramers bridged by a central LHCI–LHCII pair. Notably, a ~ 40-residue insertion in PsaD was observed, anchoring LHC-3 and stabilizing LHC belts. Moreover, Euglena-specific chlorophylls and a distinctive arrangement of red chlorophylls establish a distinct pigment network, channeling excitation-energy into the core. These findings uncover a lineage-specific strategy for light-harvesting, highlighting how secondary plastids remodel PSI–LHCs architecture to support photosynthesis.