1. Service de Biophysique des Fonctions Membranaires, URA2096/CNRS and DBJC/CEA, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
2. National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Road, Chaoyang District, Beijing 100101, China
3. Laboratory of Biophysics, Wageningen University, PO Box 8128, 6700 ET, Wageningen, The Netherlands
4. Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, UK

Correspondence to: Peter Horton⁴Bruno Robert¹ Correspondence and requests for materials should be addressed to P.H. (Email: p.horton@sheffield.ac.uk) or B.R. (Email: robert@dsvidf.cea.fr).

Abstract

In order to maximize their use of light energy in photosynthesis, plants have molecules that act as light-harvesting antennae, which collect light quanta and deliver them to the reaction centres, where energy conversion into a chemical form takes place. The functioning of the antenna responds to the extreme changes in the intensity of sunlight encountered in nature^{1, 2, 3}. In shade, light is efficiently harvested in photosynthesis. However, in full sunlight, much of the energy absorbed is not needed and there are vitally important switches to specific antenna states, which safely dissipate the excess energy as heat^{2, 3}. This is essential for plant survival⁴, because it provides protection against the potential photo-damage of the photosynthetic membrane⁵. But whereas the features that establish high photosynthetic efficiency have been highlighted⁶, almost nothing is known about the molecular nature of the dissipative states. Recently, the atomic structure of the major plant light-harvesting antenna protein, LHCII, has been determined by X-ray crystallography⁷. Here we demonstrate that this is the structure of a dissipative state of LHCII. We present a spectroscopic analysis of this crystal form, and identify the specific changes in configuration of its pigment population that give LHCII the intrinsic capability to regulate energy flow. This provides a molecular basis for understanding the control of photosynthetic light-harvesting.

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