Photosynthesis in higher plants takes place in the thylakoid membranes. These internal membranes of chloroplasts, a unique assembly of protein, pigment and lipid molecules, accommodate all light harvesting and energy transducing functions. While a wealth of information has been accumulated in the literature on the high- resolution structure and the function of photosynthetic complexes, much less is known about the structure and roles of molecular assemblies of higher hierarchical order. In Photosystem II, solar energy is collected by chlorophyll- and carotenoid-binding light harvesting complexes, which are encoded by the lhc multigene family. The LHC proteins of PSII in higher plants (LHCII) comprises up to 75% of the granum thylakoid membrane.
These pigment' protein complexes are organized around the reaction centre to form a supra-molecular complex, which also form large domains with long-range order. The self- organization of these macro-arrays is governed by the nature of the constituent proteins, and lip ids, but there are only speculations concerning the details of the molecular interactions. The diversity of these supra-molecular structures indicates their role in the adaptation and acclimation of plants. In addition to their primary function, the LHCs participate in the regulation of light energy conversion. The exact mechanisms are not known but it has been well established that some regulatory functions depend on the macrostructure. Conformational changes of the LHC-containing macrostructures have been shown to be associated with the formation of pH and redox signals. Also, isolated LHCII and the granal thylakoids possess the ability to undergo light-induced reversible structural changes, driven by a novel, biological thermo-optic mechanism. The present proposal focuses on the role of LHCs of PSII on the organization of complexes into larger assemblies, and in their roles in the multilevel regulatory mechanisms of plants.
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