Evolution and function of light-harvesting antenna in oxygenic photosynthesis

The light-harvesting proteins of oxygenic photosynthesis include mainly the phycobiliproteins and the light-harvesting complex (Lhc) family. Whereas the former are found in cyanobacteria, glaucophytes, red algae and cryptophytes, the latter are present in all eukaryotic organisms. Phycobiliproteins covalently bind bilins and are arranged in phycobilisomes in most organisms and form huge extramembranous protein aggregates. Only in cryptophytes they are lumenally localized. Lhc are membrane intrinsic proteins and fall in two distinct groups: the Lhca and Lhcb, serving as photosystem I (PSI) and photosystem II (PSII) antenna, respectively, are proteins of the green lineage of organisms (e.g., all green organisms from green algae to higher plants as well as chlorarachniophytes and euglenophytes). These proteins non-covalently bind Chl b, besides Chl a and carotenoids (Car). The other group consists of Lhcf, Lhcr, Lhcy, Lhcz, RedCaps and the photoprotective Lhcx. They bind a huge variety of carotenoids and often Chl c besides Chl a, depending on the taxon. These Lhc proteins are found mainly in the red lineage (red algae and all organisms derived thereof by secondary endosymbiosis, like, e.g., diatoms). Only Lhcx (also called LhcSR) is present in the green lineage as well and binds also Chl b instead of Chl c. Lhcf and Lhcr are the main light-harvesting proteins in the red lineage, whereby Lhcr often constitutes the PSI antenna. Besides these main Lhc families, another photoprotective protein, PsbS, is expressed in the green lineage that contains no pigments and consists of four helices. Only dinoflagellates contain an additional, water-soluble light-harvesting protein, the so-called peridinin-Chl protein.