Assembly and Degradation of pigment-binding Proteins
In photosynthetic systems of cyanobacteria and plants, pigment-binding proteins are responsible for the absorption of light energy. Antenna pigments (chlorophyll and carotenoids) harvest light energy, and rapidly transfer it to nearby pigments, eventually it will reach the reaction center.
The main purpose of the proteins is to keep the pigments in their proper location and orientation, so that energy transfer is efficient. Ligation of chlorophyll to the pigment-binding proteins is a central step in the assembly of the photosynthetic apparatus. This process is complicated by the facts that a) free chlorophyll might damage the cell (photooxidative activity in the light) and b) the pigment-binding proteins are stabilized by chlorophyll, but in the absence of pigments they are rapidly degraded. Therefore this process has to be highly coordinated, possibly by the use of special pigment-carrier proteins.
In plants a family of chlorophyll a/b binding (Cab) proteins serves as the main light harvesting antenna, however, cyanobacteria lack these Cab antenna proteins. However, stress-induced relatives to the Cab proteins have been identified in all photosynthetic organisms. In the complete sequenced genome of the cyanobacterium Synechocystis sp. PCC 6803 we identified five genes that are predicted to code for small polypeptides similar to parts of the Cab proteins. We called these proteins small Cab-like proteins (SCPs).
It is still not known how the chlorophyll is transported after its biosynthesis to the different chlorophyll-binding proteins. Some members of the Cab proteins, e.g. the early light induced proteins (ELIPs), have been suggested to act as chlorophyll-carriers. In cyanobacteria the small relatives, the SCPs could serve in a similar function. Also when proteins are degraded, pigments become free, that might damage the cell. Under these conditions pigment-carrier 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 turn-over of proteins - should be able to bind pigments transiently: uptake as well as handing over the chlorophylls must be easy; photooxidative damage performed by chlorophyll has to be prevented. Therefore carrier proteins will not have the same features as normal antenna proteins. However, the hypothetical pigment-carrier proteins known today have high structural homology to the antenna proteins. Despite this similarity their regulation is very different. Three model organisms are studied: the tree Populus trichocarpa, the annual plant Arabidopsis thaliana and the cyanobacterium Synechocystis sp. PCC 6803. In this way highly interesting and relevant comparative studies are possible.