# Cryo-EM structure of Chlamydomonas reinhardtii Photosystem I complexed with cytochrome c6

## TL;DR

The paper reveals the cryo-EM structure of the Cyt c6:PSI complex in green algae, showing how electrons are transferred to Photosystem I.

## Contribution

The study provides the first cryo-EM structure of Cyt c6 bound to PSI in Chlamydomonas reinhardtii, revealing novel and ancestral interaction features.

## Key findings

- The heme group of Cyt c6 is positioned ~11 Å from P700, stabilized by a helix-loop-helix motif of PSAF.
- Cyt c6 retains an arginine residue (R66) crucial for donor:PSI interactions, forming electrostatic and π(cation)-π interactions.
- The structure reveals evolutionary insights into the replacement of Cyt c6 by plastocyanin in plants.

## Abstract

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome b₆f complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, the heme protein cytochrome c₆ (Cyt c₆) is also employed in algae and cyanobacteria. Here, we present a cryo–electron microscopy structure of a Cyt c₆:PSI complex from Chlamydomonas reinhardtii. We observe that the heme group of Cyt c₆ is positioned ~11 Å away from P700, stabilized by extensive contacts involving a N-terminal helix-loop-helix motif of PSAF, characteristic of eukaryotic PSI. Notably, the algal Cyt c₆ also retains an arginine residue (R66) which is crucial for cyanobacterial donor:PSI reactions. Our structure reveals the previously uncharacterized interactions involving this residue; it can form a putative electrostatic contact with PsaB-D623 while also contributing to a tri-planar π(cation)-π interactions with adjacent residues. Our findings provide a structural framework for understanding the mechanism and evolution of donor:PSI interactions.

In photosynthesis, cytochrome c6 (Cyt c6) has been evolutionarily replaced by plastocyanin as the electron donor to Photosystem I (PSI). Here, the authors present the cryo-EM structure Cyt c6: PSI complex from green algae, revealing both novel and ancestral features of donor:PSI interactions.