Insights into plastocyanin–cytochrome <i>b6f</i> complex formation: The role of plastocyanin phosphorylation

Plastocyanin (PC) is a copper-containing protein that acts as a mobile electron carrier in plants during photosynthesis. In this work, we investigated the role of PC phosphorylation in photosynthetic electron transfer, focusing on interactions with both cytochrome b6f (Cytb6f) and photosystem I (PSI) in Chlamydomonas reinhardtii. While the binding and electron transfer between PC and PSI are well characterized, the interaction between PC and Cytf remains less clear. Using chemical cross-linking combined with mass-spectrometry, we identified 2 potential binding models for PC and Cytf: "side-on" and "head-on." To evaluate electron transfer, we developed an in vitro system that allowed oxidized PC, formed via light-driven electron transfer at PSI, to reoxidize Cytf. Our data show that a phosphomimetic variant of PC, where phosphorylated PC S49 residue interacts with PetA-K188, displays faster Cytf oxidation, likely optimizing binding and electron transfer between PC and Cytf. Additionally, PC phosphomimetic variants exhibited slower transfer rates than the wild type, suggesting that phosphorylation also modulates PC's interaction with PSI. This regulation likely optimizes Cytf oxidation and electron transfer under conditions of low PC availability, such as during high light stress. Overall, PC phosphorylation appears to play a role in fine-tuning electron transfer between PSI, Cytb6f, and PC, thereby ensuring efficient photosynthesis in dynamic environmental conditions.