How paired PSII\u2013LHCII supercomplexes mediate the stacking of plant thylakoid membranes unveiled by structural mass-spectrometry

Pascal Albanese,Guido Saracco,Richard A Scheltema,Sem Tamara,Cristina Pagliano

doi:10.1038/s41467-020-15184-1

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.

Highlights

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
Their discrimination is difficult even by biochemical methods, a recent mass spectrometry study performed on preparations of LHCII trimers with different configurations revealed that the M-trimer is enriched in Lhcb[1], while Lhcb[2] is almost absent in this trimer compared with Strimers[22]
We used paired PSII–LHCIIsc purified from stacked thylakoid membranes isolated from plants grown at three different light intensities (Fig. 1)

Summary

Introduction

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. 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 Lhcb[4] subunits facing each other in adjacent membranes. The atomic structures available for isolated LHCII trimers do not allow discrimination between these two Lhcb proteins[18,19,20], as they show high sequence similarity and differentiate mostly at their N-terminus[21], a feature which is missing in the available high-resolution structures Their discrimination is difficult even by biochemical methods, a recent mass spectrometry study performed on preparations of LHCII trimers with different configurations revealed that the M-trimer is enriched in Lhcb[1], while Lhcb[2] is almost absent in this trimer compared with Strimers[22]. All these structural and biochemical evidences suggest that: (1) Lhcb[3] is present in one copy per M-

Methods

Results

Conclusion