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

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. Photosystem I drives photosynthesis by harvesting light through antennas. Here, the authors resolve the cryoEM structure of the Euglena gracilis PSI– LHCI–LHCII supercomplex, revealing its antenna organization and pigment arrangement that support efficient energy transfer in a secondary plastid.