Photosystem II complex in vivo is a monomer.

This paper is important because it attempts to address the long-standing question of the in vivo oligomeric state of membrane complexes that are present as dimers in vitro. Specifically, it challenges the notion that the photosystem II complex (PSII) that was solved by X-ray crystallography as a dimer exhibits the same state in situ.

PSII is a unique molecular machine, which converts solar energy into physiologically available redox energy by oxidizing water into molecular oxygen and protons. It is generally accepted that PSII acts as a homo-dimer as it was obtained by crystallographic studies. Moreover, the existence of functional multimeric complexes was suggested. The authors of this paper attempt to challenge the well-known models suggesting that PSII functions mainly in a monomeric form in vivo. Analyzing the photochemical reaction and blue-native gel electrophoresis of different PSII particles, they suggest that the conversion of natural PSII monomers to dimeric form is caused by deprivation of lipids through the course of biochemical purification procedures. However, while this speculation is intriguing, it is far from being proven. Thus, before the known models will be re-evaluated, some testable questions raised by this novel paper should be answered. For example, in which stage of the purification of PSII is the dimer formed and what are the biochemical driving forces that are pertinent for efficient dimer formation?