Complexities and protein complexes in the antimycin A-sensitive pathway of cyclic electron flow in plants

Antimycin A-sensitive cyclic electron flow (AA-sensitive CEF) was discovered by Arnon and co-workers more than 50 years ago and serves to recycle electrons from ferredoxin (Fd) to plastoquinone (PQ). A role in AA-sensitive CEF has been attributed to the two thylakoid proteins PGR5 and PGRL1 ever since their identification, but this assignment remains controversial. While current technical limitations have prevented unequivocal clarification of their precise function in CEF in vivo, recent biochemical experiments have implied that PGRL1/PGR5 complexes possess Fd-PQ reductase (FQR) activity in vitro. Consequently, PGRL1-PGR5 complexes in flowering plants appear to shuttle between photosystem I (PSI) and the cytochrome (Cyt) b6f complex, whereas in the green alga Chlamydomonas PGRL1 (but not PGR5) has been detected in a PSI-Cyt b6f supercomplex that has intrinsic CEF activity