|
The cytochrome b6f complex (b6f) links photosystem II and I and is distributed between appressed and non-appressed thylakoid membranes. Beyond electron transfer, b6f also activates the STT7 kinase, which phosphorylates light-harvesting complex (LHC) proteins, promoting their redistribution to balance excitation energy between PSII and PSI. STT7-dependent phosphorylation has also been detected at threonine 4 (T4) in the N-terminal region of the b6f subunit PetD; however, its physiological relevance remains poorly understood. To explore this, we generated chloroplast mutants. The phosphomimetic PetD T4E mutation suppressed STT7 kinase activity, evidenced by the absence of downstream phosphorylation and a persistent State 1 conformation. This suggests a previously unrecognized feedback mechanism modulating STT7 activity. Similarly, deletion of
the first five N-terminal residues of PetD impaired both STT7 activation and electron transport, highlighting the N-terminus of PetD as essential for b6f function and its regulatory role (Zaeem et al., 2026). In parallel, C-terminal GFP tagging of PetA at the stromal side reduced electron transfer rates, altered b6f distribution, and abolished state transitions. Cryo-electron tomography confirmed lower b6f abundance in appressed membranes, and phosphoproteomics revealed strong impairment of STT7 function. Together, these findings demonstrate the dual role of b6f in electron transport and regulatory signaling and show that balanced membrane distribution of b6f is essential for efficient photosynthetic energy conversion.
|
