Assembly of light harvesting pigment-protein complexes in photosynthetic eukaryotes

In photosynthetic eukaryotes, the antenna system includes members of a protein family of Light-harvesting complexes encoded by the Lhc genes. These proteins bind 8-14 chlorophylls (Chls) and 2-4 carotenoid molecules per 22-28 kDa polypeptide; further, the pigments are needed for the assembly of monomeric proteins. Some members form dimers (LHCI) or trimers (LHCII). Chl a is needed for the assembly of all the Lhc proteins, while two members, Lhca1 and Lhca3, can refold in vitro without Chl b. Among carotenoids, lutein is bound to site L1 in all the Lhc proteins, whose occupancy is essential for protein assembly. Violaxanthin and zeaxanthin can also drive protein folding, although with a lower efficiency with respect to lutein. Current knowledge on the assembly mechanisms is also reviewed in this chapter: in vitro experiments have shown how Lhc folding is triggered by the binding of Chl a and of lutein; in contrast Chl b is only bound in a second phase and functions to stabilize the pigment-protein complexes. Together with the reaction centers, antenna complexes are organized as supercomplexes in the thylakoid membranes. The structural organization of the antenna of Photosystem I (PS I) is quite different from that of Photosystem II (PS II): the PS II antenna system is flexible and its size is modulated according to environmental conditions, while in PS I antenna protein content is maintained constant with respect to the reaction centre.