THE CHANGING ROLE OF THE SMALL PSII SUBUNITS PSB27 AND PSB28 DURING EVOLUTION

The 3D structure of the protein from Arabidopsis thaliana (B) was obtained with the SWISS-MODEL workspace (Arnold et al. 2006) using the known structure of the Synechocystis sp. PCC 6803 (PDB: 2KVO) (A). Polar residues are shown in blue and hydrophobic residues in green.
During assembly the PSII core (green) is not phosphorylated. By this means Psb27-H2 (blue-grey) can bind and Psb27-H1 binding is blocked (black arrow). After photodamage the PSII core gets phosphorylated (red-green) which enables Psb27-H1 (blue) to bind. Phosphorylation prevents binding of Psb27-H2.

Photosystem II (PSII) catalyzes the unique reaction of light-dependent water oxidation and subsequent reduction of plastoquinone at the beginning of the photosynthetic electron transport chain. The mature complex consists of at least 20 protein-subunits and over 80 cofactors. Further proteins are required for biogenesis and repair of PSII. Most of these proteins interact specifically with assembly intermediates during defined steps in PSII assembly. This opinion article emphasizes the function of the two factors Psb27 and Psb28 during the biogenesis and repair of PSII in cyanobacteria and give an impression of their potential biochemical, structural and physiological properties in plants considering the fact that they both have homologues in all oxygenic photosynthetic organisms.

The authors, Falko Flügge and Lars Dietzel, hypothesize that Psb28 may have retained its function in higher plants while the two Psb27 forms bind differently to PSII intermediates depending on PSII core phosphorylation state.