Abstract
The structure of the major light-harvesting chlorophyll a/b complex (LHCII) was analyzed by pulsed EPR measurements and compared with the crystal structure. Site-specific spin labeling of the recombinant protein allowed the measurement of distance distributions over several intra- and intermolecular distances in monomeric and trimeric LHCII, yielding information on the protein structure and its local flexibility. A spin label rotamer library based on a molecular dynamics simulation was used to take the local mobility of spin labels into account. The core of LHCII in solution adopts a structure very similar or identical to the one seen in crystallized LHCII trimers with little motional freedom as indicated by narrow distance distributions along and between α helices. However, distances comprising the lumenal loop domain show broader distance distributions, indicating some mobility of this loop structure. Positions in the hydrophilic N-terminal domain, upstream of the first trans-membrane α helix, exhibit more and more mobility the closer they are to the N terminus. The nine amino acids at the very N terminus that have not been resolved in any of the crystal structure analyses give rise to very broad and possibly bimodal distance distributions, which may represent two families of preferred conformations.
Highlights
Structural changes of LHCII may be required for its roles beyond light harvesting
A reduced spin labeling efficiency will only affect the intensity of the expected Double electron-electron resonance (DEER) signal but not give rise to false signals
Interspin distances were measured between PROXYL labels, attached to each of the trans-membrane helices of LHCII, and these distances were compared with distances determined in a simulation based on label rotamers in the structure of trimeric LHCII crystals [28]
Summary
Structural changes of LHCII may be required for its roles beyond light harvesting. Results: EPR distance mapping reveals a high flexibility of the N-proximal protein domain. Structure of Solubilized LHCII Measured by EPR site composed of xanthophyll cycle carotenoids and a Chl a dimer [2] According to another model of nonphotochemical quenching, LHCII interacts with the protein PsbS in the photosynthetic membrane at low lumen pH, which results in a conformational change of LHCII, an altered configuration of its pigments, and a switch to the dissipative state [5]. Besides this energy-dissipating function, LHCII is involved in the balanced distribution of excitation energy between the photosystems by dissociating from PSII and binding to PSI in high light conditions This short term process is referred to as state transition and is regulated by phosphorylation of the N-terminal protein domain in LHCII by the thylakoid kinase STN7 (10 –12). Based on the EPR distance map, we compared the structure of LHCII in detergent solution with the structure predicted from crystal data [3], including a PROXYL rotamer library based on a molecular dynamics simulation [28]
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