Abstract
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 from Thermosynechococcus elongatus at 2.94 Å resolution. It contains three assembly factors (Psb27, Psb28 and 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 (QB) and replace the bicarbonate ligand of non-haem iron with glutamate, a structural motif found in reaction centres of non-oxygenic photosynthetic bacteria. These results reveal 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 Mn4CaO5 cluster, which performs the unique water-splitting reaction.
Highlights
Photosystem II (PSII) is the only enzyme that catalyzes the light-driven oxidation of water, a thermodynamically demanding reaction that drives photosynthesis, sustaining life on our planet[1,2,3]
Our study provides mechanistic answers to three long-standing questions: i) How do assembly factors modulate the structures of photosystem II (PSII) subunits to assist biogenesis? ii) How is PSII protected from photodamage during assembly? iii) How is the PSII
Stable PSII intermediates were purified from the T. elongatus DpsbJ mutant[43] by affinity chromatography using a twin-strep-tag fused to the C-terminus of the CP43 subunit and subsequent ion exchange chromatography (Fig. S1a)
Summary
Photosystem II (PSII) is the only enzyme that catalyzes the light-driven oxidation of water, a thermodynamically demanding reaction that drives photosynthesis, sustaining life on our planet[1,2,3]. This multi-subunit membrane protein complex is located in the thylakoid membranes of cyanobacteria, algae and plants. PSII strips electrons from water and injects them into the photosynthetic electron transport chain (PET). It forms a homodimer with a molecular mass of.
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