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bioRxiv · 10.1101/2025.02.21.639470

N-terminal region of PetD is essential for cytochrome b6f function and controls STT7 kinase activity via STT7-dependent feedback loop phosphorylation

Zaeem, A.·Milrad, Y.·Buetfering, S.·Stoffel, C.·Burlacot, A.·Scholz, M.·Buchert, F.·Hippler, M.
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Abstract

The cytochrome b6f complex (b6f) links photosystem I (PSI) and photosystem II (PSII) in the photosynthetic electron transport chain and is distributed across appressed and non-appressed thylakoid membranes. The b6f also activates the state-transition 7 kinase (STT7), which phosphorylates light-harvesting complex proteins, triggering their migration to enable energy redistribution between PSII and PSI. STT7-dependent phosphorylation has also been observed at T4 in the N-terminal domain of the b6f subunit-IV (PetD), though its functional significance has remained unclear. To investigate its role, we generated several chloroplast mutants. The phosphomimic mutation PetD T4E - but not T4A - inhibited STT7 kinase activity, as indicated by the absence of STT7-dependent phosphorylation and a State 1-locked phenotype. This reveals a novel feedback mechanism regulating STT7-dependent phosphorylation. Deletion of the first five N-terminal amino acids similarly inhibited STT7 activity and additionally disrupted electron transfer, underscoring a crucial role of the PetD N-terminus in b6f function.

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Zaeem, A., Milrad, Y., Buetfering, S., Stoffel, C., Burlacot, A., Scholz, M., Buchert, F., Hippler, M.. 2025-02-25. N-terminal region of PetD is essential for cytochrome b6f function and controls STT7 kinase activity via STT7-dependent feedback loop phosphorylation. https://doi.org/10.1101/2025.02.21.639470

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