How to build a water-splitting machine: structural insights into photosystem II assembly

Biogenesis of photosystem II (PSII), nature’s water splitting catalyst, is assisted by auxiliary proteins that form transient complexes with PSII components to facilitate stepwise assembly events. Using cryo-electron microscopy, we solved the structure of such a PSII assembly intermediate with 2.94 Å resolution. It contains three assembly factors (Psb27, Psb28, Psb34) and provides detailed insights into their molecular function. Binding of Psb28 induces large conformational changes at the PSII acceptor side, which distort the binding pocket of the mobile quinone (Q B ) and replace bicarbonate with glutamate as a ligand of the non-heme iron, a structural motif found in reaction centers of non-oxygenic photosynthetic bacteria. These results reveal novel mechanisms that protect PSII from damage during biogenesis until water splitting is activated. Our structure further demonstrates how the PSII active site is prepared for the incorporation of the Mn 4 CaO 5 cluster, which performs the unique water splitting reaction. One Sentence Highlight The high-resolution Cryo-EM structure of the photosystem II assembly intermediate PSII-I reveals how nature’s water splitting catalyst is assembled, protected and prepared for photoactivation by help of the three assembly factors Psb27, Psb28 and Psb34.