Structure of Dunaliella Photosystem II reveals conformational flexibility of stacked and unstacked supercomplexes

Abstract

Photosystem II (PSII) generates an oxidant whose redox potential is high enough to enable water oxidation1,2, a substrate so abundant that it assures a practically unlimited electron source for life on earth3. Our knowledge on the mechanism of water photooxidation was greatly advanced by high-resolution structures of prokaryotic PSII4–6. Here we show high-resolution structures of eukaryotic PSII from the green algae Dunaliella salina at two distinct conformations. The conformers are also present in stacked PSII, exhibiting flexibility that is relevant to the grana formation in chloroplasts of the green lineage. CP29, one of PSII associated light harvesting antennae, plays a major role in distinguishing the two conformations of the supercomplex. We also show that the stacked PSII dimer, a form suggested to support the organization of thylakoid membranes7,8, can appear in many different orientations providing a flexible stacking mechanism for the arrangement of grana stacks in thylakoids. Our findings provide a structural basis for the heterogenous nature of the eukaryotic PSII on multiple levels.

The effect of ultrafine WO3 nanoparticles on the organization of thylakoids enriched in photosystem II and energy transfer in photosystem II complexes

“…The At the same time, monomerization and aggregation of LCHII trimers was further observed. The direction of these changes is consistent with the knowledge of how the disassembly of large PSII supercomplexes (C 2 S 2 M 2 ) and the formation of smaller ones affect the energy transfer capabilities of its outer antenna complexes to the inner ones (Barera et al, 2012;Caffarri et al, 2009;Caspy et al, 2021;Su et al, 2017;Wei et al, 2016). In samples treated with higher concentrations of 0.01 and 0.1 μg NPs/μg Chl, the absorbance unexpectedly increased compared to the control.…”

“…In addition, different conformers may occur within the supercomplexes. For example, it has been shown that in eukaryotic PSII, a monomeric CP29 antenna can switch between two different C 2 S 2 conformations, stretched C 2 S 2 str and compact C 2 S 2 comp, by changing its orientation, each of which may exhibit additional structural heterogeneity (Caspy et al, 2021). This flexibility of the PSII dimer core most likely ensures its optimal functioning under global as well as local variable conditions, including (i) efficient utilization of light for charge separation, (ii) access of water and plastoquinones to the donor and acceptor PSII sides, respectively, and (iii) discharge of O 2 and H + outside the OEC.…”