On some of the in organello processes involved in the biogenesis of chlorophyll-protein complexes

The biogenesis and assembly of chlorophyll-protein complexes consist of many steps. These are initiated with the transcription and translation of the different polypeptide components constituting the complexes. For the nuclear-encoded subunits the synthesis takes place in the cytoplasm, and they are synthesized as precursors, which are later imported into the chloroplast. Within the organelle, the precursors are inserted into the thylakoid membranes, as well as being processed to their mature forms. The different nuclear- and chloroplast-encoded subunits assemble together, and bind the pigments and other cofactors to form the active pigmented-complex. In the present article, we discuss only the in organello processes of the biogenesis. We describe the pathways taken by two nuclear-encoded thylakoid proteins, the precursor of the main light-harvesting chlorophyll-protein of photosystem II (pLHCP) and the precursor of photosystem I subunit II (pre subunit II). These polypeptide subunits that are located in two different photosynthetic complexes, differ from each other. While pLHCP is an integral membrane protein, which binds pigments, photosystem I-subunit II is a peripheral membrane protein, located on the stromal side of the thylakoids, and is not predicted to span it. The differences and the common features of the in organello biogenesis pathways of these two proteins are discussed.