Functional architecture of the major light-harvesting complex from higher plants

Light-harvesting complexes (Lhc) catalyse sunlight harvesting for photosynthesis as well as other essential functions, including photoprotection by quenching of harmful chlorophyll triplet states and prevention of photoinhibition by dissipation of excitation energy in excess. In addition, folding of Lhc proteins depends on the availability of both xanthophylls and carotenoids, thus preventing the potential formation of harmful chlorophyll-protein complexes lacking photoprotectors. We have used the mutation analysis in order to study the association of the different functions to three protein domains, each composed of a xanthophyll molecule and of neighbour chlorophylls a and b, within the major antenna complex of photosystem II, i.e. LHCII. We have found that the xanthophyll to chlorophyll energy transfer is a shared property of the whole pigment-protein complex, and occurs with similar efficiency in each of the three structural domains. Photoprotection by quenching of chlorophyll triplets is catalysed mainly by lutein bound to site L1, and occurs via energy transfer from chlorophylls A1 and B1. This domain is essential for pigment-induced protein folding. The domains L2 and N1 weakly influence either the protein stability or the photoprotection; however, replacement of xanthophyll species bound to these structural domains modulates the fluorescence quantum yield of LHCII, and suggests that non-radiative dissipation of excess energy can be regulated through allosteric modification of the protein structure by exchanging xanthophylls in these sites.