Chlorophyll-binding subunits of photosystem I and II: Biosynthesis, chlorophyll incorporation and assembly

As an essential cofactor of photosystem I and photosystem II, chlorophyll plays a fundamental role in oxygenic photosynthesis. Chlorophyll molecules are responsible for both the absorption of visible light and its photochemical conversion during the process of charge separation. The vast majority of chlorophyll molecules located in photosystems is bound to six core subunits that appear to have a common evolutionary origin. Available data indicate that these large transmembrane proteins are synthesized on membrane-bound ribosomes and inserted into the thylakoid membrane with the assistance of SecY translocase and various protein factors. Newly synthesized chlorophyll-proteins associate with small transmembrane subunits, carotenoids, and other cofactors, and assemble in a stepwise manner into the final functional photosystems. This chapter summarizes our current knowledge of the individual events during photosystem biogenesis: apoprotein translation and membrane insertion, loading of chlorophyll molecules into the synthesized apoproteins, formation of assembly modules, and final assembly into the photosystems.