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How paired PSII-LHCII supercomplexes mediate the stacking of plant thylakoid membranes unveiled by structural mass-spectrometry.

ABSTRACT: Grana are a characteristic feature of higher plants' thylakoid membranes, consisting of stacks of appressed membranes enriched in Photosystem II (PSII) and associated light-harvesting complex II (LHCII) proteins, together forming the PSII-LHCII supercomplex. Grana stacks undergo light-dependent structural changes, mainly by reorganizing the supramolecular structure of PSII-LHCII supercomplexes. LHCII is vital for grana formation, in which also PSII-LHCII supercomplexes are involved. By combining top-down and crosslinking mass spectrometry we uncover the spatial organization of paired PSII-LHCII supercomplexes within thylakoid membranes. The resulting model highlights a basic molecular mechanism whereby plants maintain grana stacking at changing light conditions. This mechanism relies on interactions between stroma-exposed N-terminal loops of LHCII trimers and Lhcb4 subunits facing each other in adjacent membranes. The combination of light-dependent LHCII N-terminal trimming and extensive N-terminal α-acetylation likely affects interactions between pairs of PSII-LHCII supercomplexes across the stromal gap, ultimately mediating membrane folding in grana stacks.

SUBMITTER: Albanese P

PROVIDER: S-EPMC7069969 | biostudies-literature |

REPOSITORIES: biostudies-literature

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How paired PSII-LHCII supercomplexes mediate the stacking of plant thylakoid membranes unveiled by integrative structural mass-spectrometry

Project description:Photosynthesis drives all life on Earth by exploiting solar energy to split water molecules through the Photosystem (PS) II enzyme. In plant thylakoid membranes, PSII binds a modular set of light harvesting complexes (LHCII) to form different types of PSII-LHCII supercomplex (PSII-LHCIIsc). Plant PSII-LHCIIsc are localized in the stacked region of the thylakoid membranes called grana, from which they can be isolated in paired conformations of type (C2S2M2)x2 and (C2S2M)x2, with the two supercomplexes facing each other at their stromal surface. Although the atomic structure of PSII-LHCIIsc has recently been solved, there is still a lack of knowledge on their mutual interactions when facing each other within apposing thylakoid membranes, as well as their structural dynamics in response to light variations. The major challenges in the structural determination of the stromal interactions are posed not only by the dynamic nature of these over 2-megadalton assemblies, but also by the heterogeneity of the LHCII subunits and the high flexibility of their stromally exposed N-terminal loops. Here, we explored the potential of combining top-down mass spectrometry (TD-MS) and crosslinking mass spectrometry (XL-MS) to peek through the keyhole of the tight stromal gap between two facing supercomplexes, unveiling so far hidden structural details. A first goal of experiments was to identify the distinct sequence variants and proteoforms involved in PSII-LHCIIsc structural dynamics in response to light modulation. To investigate their structural interactions, we treated paired PSII-LHCIIsc isolated from the three light conditions with two complementary chemical cross-linkers, targeting different residues and producing partially overlapping distance restraints. Most interactions detected “in-vitro” on the isolated paired supercomplexes were further supported by XL-MS results obtained “in-situ” on the corresponding thylakoid membranes.

2020-03-19 | PXD017382 | Pride

Paired C2S2M PSII-LHCII supercomplexes from thylakoid membranes of Pisum sativum

Project description: Not available

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Pea PSII-LHCII supercomplexes form pairs by making connections across the stromal gap.

Project description: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.

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