Cytochromeb6fcomplex inhibition by antimycin-A requires Stt7 kinase activation but not PGR5

Abstract

AbstractFerredoxin-plastoquinone reductase (FQR) activity during cyclic electron flow (CEF) was first ascribed to the cytochromeb6fcomplex (b6f). However, this was later dismissed sinceb6finhibition by antimycin-A (AA) could not be reproduced. AA presumably fails to ligate with haembh, at variance with cytochromebc1complex, owing to a specific Qi-site occupation inb6f. Currently, PROTON GRADIENT REGULATION5 (PGR5) and the associated PGR5-Like1 are considered as FQR in the AA-sensitive CEF pathway. Here, we show that theb6fis conditionally inhibited by AA in a PGR5-independent manner when CEF is promoted. AA inhibition, demonstrated by singleb6fturnover and electron transfer measurements, coincided with an altered Qi-site function which required Stt7 kinase activation by a strongly reduced plastoquinone pool. Thus, PGR5 and Stt7 were necessary forb6factivity and AA-sensitive electron transfer in CEF-favouring conditions. Extending previous findings, a new FQR activity model of theb6fis discussed.