Abstract
In higher plant thylakoids, the heterogeneous distribution of photosynthetic protein complexes is a determinant for the formation of grana, stacks of membrane discs that are densely populated with Photosystem II (PSII) and its light harvesting complex (LHCII). PSII associates with LHCII to form the PSII-LHCII supercomplex, a crucial component for solar energy conversion. Here, we report a biochemical, structural and functional characterization of pairs of PSII-LHCII supercomplexes, which were isolated under physiologically-relevant cation concentrations. Using single-particle cryo-electron microscopy, we determined the three-dimensional structure of paired C2S2M PSII-LHCII supercomplexes at 14 Å resolution. The two supercomplexes interact on their stromal sides through a specific overlap between apposing LHCII trimers and via physical connections that span the stromal gap, one of which is likely formed by interactions between the N-terminal loops of two Lhcb4 monomeric LHCII subunits. Fast chlorophyll fluorescence induction analysis showed that paired PSII-LHCII supercomplexes are energetically coupled. Molecular dynamics simulations revealed that additional flexible physical connections may form between the apposing LHCII trimers of paired PSII-LHCII supercomplexes in appressed thylakoid membranes. Our findings provide new insights into how interactions between pairs of PSII-LHCII supercomplexes can link adjacent thylakoids to mediate the stacking of grana membranes.
Highlights
In higher plant thylakoids, the heterogeneous distribution of photosynthetic protein complexes is a determinant for the formation of grana, stacks of membrane discs that are densely populated with Photosystem II (PSII) and its light harvesting complex (LHCII)
The structure reported here for paired C2S2M PSII-LHCII supercomplexes is representative of the predominant form of PSII in pea plants grown at moderate light intensity
There is a specific overlap between facing LHCII trimers that may be due to the formation of salt bridges between highly conserved negatively-charged residues of the Lhcb subunits in facing LHCII trimers
Summary
The heterogeneous distribution of photosynthetic protein complexes is a determinant for the formation of grana, stacks of membrane discs that are densely populated with Photosystem II (PSII) and its light harvesting complex (LHCII). Photosynthetic membrane proteins are segregated into different thylakoid domains; grana regions contain mainly PSII and LHCII, complexes with flat stromal surfaces that do not project into the narrow stromal gap between thylakoids. Cytochrome b6/f (Cyt b6/f) is evenly distributed between stacked and unstacked thylakoid regions[13, 14] This lateral heterogeneity between PSI and PSII is thought to be intimately linked to membrane appression, allowing higher plants to optimize photosynthesis in ever-changing light conditions[15]
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