Characterization of pigment-protein complexes in the cyanobacterium Anacystis Nidulans R2
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Intrinsic membrane proteins associated with photosynthetic electron transfer have highly-conserved analogs in both cyanobacteria and chloroplasts. However, the protein complexes required for light-harvesting are quite different. Cyanobacteria contain extrinsic phycobilisomes, whereas chloroplasts have integral membrane light-harvesting Chl-protein complexes. We show that cyanobacteria are capable of synthesizing an intrinsic light-harvesting structure (termed CPVI-4) similar in many respects to that found in chloroplasts. In normally grown A. nidulans R2, the main light-harvesting structure is the extrinsic phyco- bilisome, whereas under iron stress, phycobilisome quantities decrease and the cells accumulate CPVI-4. CPVI-4 was biochemically purified and characterized with respect to its spectral and biochemical properties. Antisera were raised against an apoprotein of CPVI-4 (34 kD) as well as several other membrane components regulated by iron. The antibodies were used to follow the kinetics of the disappearance of each of these components following addition of iron to iron-depleted cultures. Phycobilisome linker and anchor polypeptides were found to be specifically glycosylated, and purified phyco- bilisomes were found to contain significant quantities of carbohydrates (particularly glucose). The glycosylated sub-units contained differential quantities of glucose and N acetylgalactosamine, and these sugars were present on regions of the linker proteins previously shown to be important for PBsome assembly. These findings have broad implications regarding both the function and assembly of phycobilisomes as well as the response of A. nidulans R2 to a variety of different stress conditions.
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