| Abstrakt: |
In this chapter we describe isolation of loosely stacked lamellar aggregates of LHCII that are capable of undergoing light-induced reversible structural changes, similar to those in granal thylakoid membranes (LHCII, the main chlorophyll a/b light-harvesting antenna complex of photosystem II). This unexpected structural flexibility of the antenna complexes depends largely on the lipid content retained during the isolation. The essence of the procedure is to adjust-for the plant material used-the proper conditions of detergent solubilization and purification mild enough for the associated lipids but also provide sufficient purity. Microcrystals and most other LHCII preparations, which are more delipidated, are not capable of similar changes. The light-induced structural reorganizations can be enhanced by the addition of different thylakoid lipids, which depending on the lipid species, also lead to the transformation of the lamellar structure. The preparation of different LHCII-lipid macro-assemblies is also described. In structurally flexible LHCII preparations and in thylakoids, the changes originate from a thermo-optic effect: fast local thermal transients, T-jumps, resulting from dissipation of the (excess) excitation energy that lead to elementary structural transitions in the vicinity of the dissipating centers. This can occur because thylakoids and structurally flexible LHCII assemblies but, for example, not the microcrystals, exhibit a thermal instability below the denaturation temperature, and thus (local) heating lead to reorganizations without the loss of the molecular architecture of the constituents. We also list the main biochemical and biophysical techniques that can be used for testing the structural flexibility of LHCII, and discuss the potential physiological significance of the structural changes in light adaptation and photoprotection of plants. |
