From Laboratory of Photosynthesis - Photosynthetic conversion of solar energy in cyanobacteria, algae and plants is carried out by two complexes of membrane proteins, chlorophyll and other cofactors. These complexes are referred to as photosystems I and II and their efficient assembly depends on the presence of a number of auxiliary protein factors. In the case of oxygen-evolving photosystem II (PSII), such a factor is a protein called Ycf48, which plays a key role in the initial phase of PSII formation, when the central proteins D1 and D2 together form the first intermediate of PSII. However, the exact mechanism of action of Ycf48 and its localization within the d1/D2 complex remained unknown. In this study, we were able to determine the detailed structure of the isolated complex of D1, D2 and Ycf48 proteins by cryoelectron microscopy. It was found that Ycf48 primarily interacts with the D1 protein at the sites where the cluster of calcium and manganese ions binds and performs the conversion of water to molecular oxygen in functional PSII. Thus, the complex prevents the premature binding of these ions, which could lead to oxidative damage to the D1 protein. Read more