Mol. Cell, published online 3 January 2013; doi:10.1016/j.molcel.2012.11.030

Plant photosynthesis occurs in the thylakoid membranes of chloroplasts, where light energy captured by photosystems I and II (PSI and PSII) is converted to the chemical energy of ATP and NADPH through regulated electron transport. The cyclic electron flow (CEF) pathway centers on PSI and generates proton gradients for ATP synthesis: PSI photoreduces ferredoxin (Fd) and then recovers its electrons from plastocyanin (Pc), which similarly inherits them in an ordered electron transport cascade from the cytochrome b6f complex (Cyt b6f) and plastoquinone (PQ). Current models of CEF propose that a ferredoxin-plastoquinone reductase (FQR) must exist to complete the redox cycle between Fd and PQ. Hertle et al. now show that PGRL1, a thylakoid membrane-associated protein in Arabidopsis thaliana that was previously linked to CEF, is the missing FQR. Biochemical analyses of thylakoid membranes revealed that PGRL1 exists both as a monomer and in two dimeric forms, including a homodimer and a heterodimer with PGR5, another protein previously associated with CEF. Systematic mutagenesis of six conserved cysteine residues in PGRL1 identified three functional redox-regulated domains: a homodimerization motif that is regulated by thioredoxins, an iron-containing cofactor site and a PGR5-heterodimerization domain. In vitro assays showed that the PGRL1–PGR5 complex can accept electrons from Fd, whereas PGRL1 alone is sufficient to reduce quinones. In parallel, PGRL1 redox kinetics in planta validate that PRGL1 receives electrons from PSI in a PGR5-dependent manner. Taken together, the results support a model that PGRL1 acts as the plant FQR with the help of PGR5.