The absorption of sunlight is the first step in the process of photosynthesis and is performed by a special group of proteins, called antenna proteins. Ligation of chlorophyll to the pigmentbinding proteins is a central step in the assembly of the photosynthetic apparatus. This process is complicated by the facts that i) free chlorophyll has a potentially damaging photooxidative activity in the light and ii) the pigmentbinding proteins are stabilized by chlorophyll, but in the absence of this pigment they are rapidly degraded. Therefore this process has to be highly coordinated, possibly by the use of special pigmentcarrier proteins.
Christiane Funk 1150 766func_1Microscopic picture showing cells of the photosynthetic cyanobacterium Synechocystis sp. PCC 6803. Their blue-green (cyano) color is a result of the light-harvesting pigments. Blue=phycobilins, green=chlorophyllFunctional photosynthetic complexes only have a short lifespan, as a means of quality control. Turnover of pigmentbinding proteins and changes in the composition of lightharvesting and/or reaction centre pigmentprotein complexes are also the major tools for light adaptation. When the protein is degraded, pigments become free and may damage the cell. Under these conditions, pigmentcarrier proteins are extremely important. A special scenario for protein degradation is leaf senescence, which starts with a decrease in photosynthesis.
Carrier proteins – no matter if they function during assembly of new antenna proteins or during turnover of proteins - should be able to bind pigments transiently; uptake as well as handing over the chlorophylls must also be easy. Photooxidative damage by chlorophyll has to be prevented, either by quenchers like carotenoids or a special protein structure. Therefore, carrier proteins will not have the same features as normal antenna proteins. However, the hypothetical pigmentcarrier proteins known today have high structural homology to the antenna proteins. Despite this similarity, their regulation is very different.
Three model organisms are being studied: the tree Populus trichocarpa, the annual plant Arabidopsis thaliana and the cyanobacterium Synechocystis sp. PCC 6803. Using these organisms, interesting and relevant comparative studies are possible.
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