# Structural basis of diadinoxanthin-Chl a/b–binding proteins in the photosystem I supercomplex of Euglena gracilis

## TL;DR

This study reveals the high-resolution structure of a photosystem I complex in Euglena gracilis, showing how it evolved through secondary endosymbiosis.

## Contribution

The paper presents the first detailed structure of the PSI-LHCI supercomplex in E. gracilis, highlighting unique pigment-binding features and evolutionary adaptations.

## Key findings

- The PSI-LHCI supercomplex has 8 core subunits and 16 LHCIs arranged in a two-layered configuration.
- The LHCIs include 4 diadinoxanthin-Chl a/b-binding and 12 diadinoxanthin-Chl a-binding antennae.
- The structure provides insights into light-harvesting mechanisms and evolutionary changes in green-lineage organisms.

## Abstract

Euglena gracilis is a phototrophic flagellate that has evolved through secondary endosymbiotic events and belongs to green-lineage organisms. We report a unique structure of photosystem I-light harvesting complex I (PSI-LHCI) supercomplex from E. gracilis at a 2.23-angstrom resolution by cryo–electron microscopy. The supercomplex is composed of 8 core subunits and 16 LHCIs and exhibits distinctive structural features compared to its counterparts in green algae and plants. The LHCI subunits encircle the core complex, forming a two-layered arrangement that comprises six pairs of tightly packed heterodimers. Specifically, the 16 LHCIs consist of 4 diadinoxanthin-chlorophyll (Chl) a/b–binding antennae (Lhcbm) and 12 diadinoxanthin-Chl a–binding antennae (Lhca), and they exhibit characteristic pigment compositions combining features of green- and red-lineage organisms. These findings provide a robust structural foundation for elucidating the mechanisms of light harvesting and energy transfer as well as insights into the evolutionary changes of green-lineage PSI-LHCI.

The high-resolution Euglena PSI structure reveals how diverse two-layered antennas adapt to secondary endosymbiotic evolution.