Functions and Evolution of Lhcb Isoforms Composing LHCII, the Major Light Harvesting Complex of Photosystem II of Green Eukaryotic Organisms

Abstract

Oxygenic photosynthesis provides energy and oxygen for almost all forms of life on earth. This process is based on the energy of photons, which is used to split water and use its electrons to reduce carbon atoms to create organic molecules and thus fix the light energy into a chemical form. Two photosytems working in series are involved in light harvesting and conversion. Both are multi-protein supercomplexes composed of a core complex, where the photochemical reaction takes place, and an antenna system involved in light harvesting. In plants and green algae, the antenna of photosystem II, the photosynthetic complex involved in water splitting, comprises the Light Harvesting Complex II (LHCII) trimers, the most abundant membrane protein on earth. LHCII is composed of highly conserved Lhcb isoforms and all green organisms count a high number of Lhcb. In vascular plants they are classified in three distinct subclasses, Lhcb1, 2 and 3, while in algae and non-vascular plants, these isoforms are less differentiated and called Lhcbm proteins. In this review, we compare LHCII proteins of different organisms, from green algae to angiosperms, and discuss the role of the modifications that occurred through evolution. We highlight the various functions of the different isoforms in photosynthesis, ranging from light harvesting, a common role to all these proteins, to regulations of photosynthesis that rely on specific isoforms.

Keywords: LHCII isoforms; Lhcb3; light harvesting complexes in plants and green algae; organic molecules; photosynthesis; state transitions..