Abstract
PsbS plays a major role in activating the photoprotection mechanism known as "non-photochemical quenching," which dissipates chlorophyll excited states exceeding the capacity for photosynthetic electron transport. PsbS activity is known to be triggered by low lumenal pH. However, the molecular mechanism by which this subunit regulates light harvesting efficiency is still unknown. Here we show that PsbS controls the association/dissociation of a five-subunit membrane complex, composed of two monomeric Lhcb proteins (CP29 and CP24) and the trimeric LHCII-M. Dissociation of this supercomplex is indispensable for the onset of non-photochemical fluorescence quenching in high light, strongly suggesting that protein subunits catalyzing the reaction of heat dissipation are buried into the complex and thus not available for interaction with PsbS. Consistently, we showed that knock-out mutants on two subunits participating to the B4C complex were strongly affected in heat dissipation. Direct observation by electron microscopy and image analysis showed that B4C dissociation leads to the redistribution of PSII within grana membranes. We interpreted these results to mean that the dissociation of B4C makes quenching sites, possibly CP29 and CP24, available for the switch to an energy-quenching conformation. These changes are reversible and do not require protein synthesis/degradation, thus allowing for changes in PSII antenna size and adaptation to rapidly changing environmental conditions.
Highlights
From the Italian Ministry of Research Special Fund for Basic Research
This structural organization responds to the requirements of light harvesting regulation; in high light, when absorbed energy does not limit growth, the photosystem II (PSII) antenna loses the components of the external antenna layer, namely CP24, LHCII-M, and LHCII-L [12], whereas the internal antenna components, CP26, CP29, and LHCII-S, are always retained in a 1:1 stoichiometry with the PSII core complex
A Pentameric Lhcb Complex (B4C) Is Dissociated upon Light Treatment—PSII antenna organization has been shown to be fundamental for the full establishment of non-photochemical quenching (NPQ), which, is significantly impaired in plants depleted of antenna proteins or where the antenna organization is affected [23,24,25]
Summary
From the Italian Ministry of Research Special Fund for Basic Research. To whom correspondence should be addressed: Dipartimento Scientifico e Tecnologico, Universitadi Verona, Strada Le Grazie 15, I-37134 Verona, Italy. Differences between the mentioned isoforms have been largely conserved in all higher plants during at the last 350 million years of evolution, strongly indicating that each pigment-protein complex has a specific function [4], the specific role of each gene product in light harvesting and/or photoprotection is still under debate [5]. Their topological organization into the supercomplex has been analyzed by electron microscopy and biochemical methods (3, 6 – 8) showing that Lhcb subunits are organized into two layers around the PSII core. These results suggest that the NPQ process includes a rapid and reversible change in the organization of grana membranes with disconnection of a subset of Lhcb proteins from the PSII reaction center
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