Abstract
Understanding the mechanistic basis of balanced excitation energy distribution between photosystem II and photosystem I (PSII and PSI) requires detailed investigation of the thylakoid light‐harvesting system composed of energetically connected LHCII trimers. The exact mechanisms controlling the excitation energy distribution remain elusive, but reversible phosphorylation is known to be one important component. Here, we addressed the role of grana margins in regulation of excitation energy distribution, as these thylakoid domains host all the complexes of photosynthetic light reactions with dynamic response to environmental cues. First, the effect of detergents for the thylakoid membrane connectivity is explained. We show that a specific interaction between the separate LHCII trimers as well as between the LHCII trimers and the PSII and PSI–LHCI complexes is a prerequisite for energetically connected and functional thylakoid membrane. Second, we demonstrate that the optimization of light reactions under changing light conditions takes place in energetically connected LHCII lake and is attained by lateral rearrangements of the PSII–LHCII and PSI–LHCI–LHCII complexes depending especially on the phosphorylation status of the LHCII protein isoform LHCB2.
Highlights
Photosynthetic light reactions, involving two light-driven photosystems (PSII and PSI), convert solar energy into chemical energy in the thylakoid membrane of chloroplasts
Light energy is harvested by two specialized pigment–protein complex systems: the light-harvesting complex I (LHCI), collecting excitation energy only for PSI and the light-harvesting complex II (LHCII), either attached to PSII complexes or functioning as a shared antenna system for both photosystems (Grieco, Suorsa, Jajoo, Tikkanen, & Aro, 2015; Wientjes, van Amerongen, & Croce, 2013)
LHCIIs are found as trimers consisting of variable quantities of homologous proteins LHCB1–3 (Jansson, 1994), and the trimers are connected to PSII complexes via LHCB4– 6 proteins (Boekema, Van Roon, Van Breemen, & Dekker, 1999; Kouril, Dekker, & Boekema, 2012)
Summary
Photosynthetic light reactions, involving two light-driven photosystems (PSII and PSI), convert solar energy into chemical energy in the thylakoid membrane of chloroplasts. The underlying molecular mechanisms guiding the functional interactions of thylakoid protein complexes are not yet fully understood It is, well known that the reversible phosphorylation of PSII core and LHCII proteins plays an important role (Mekala, Suorsa, Rantala, Aro, & Tikkanen, 2015; Puthiyaveetil, van Oort, & Kirchhoff, 2017). Even the specific roles of LHCB1 and LHCB2 phosphorylation have been elucidated (Pietrzykowska et al, 2014; Rantala et al, 2017), the actual mechanisms optimizing the excitation energy distribution have remained under debate This regulation has been explained with the traditional state transition model relying on migrating LHCII antenna. Our specific goals were (i) to explain how detergents can be used to investigate the thylakoid membrane connectivity and the lateral distribution of thylakoid protein complexes and (ii) to apply this method to examine the light- and (de)phosphorylation-induced rearrangements of the thylakoid protein complexes upon light acclimation
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