Structural and functional differentiation of the light-harvesting protein Lhcb4 during land plant diversification

About 475 million years ago, plants originated from an ancestral green alga and evolved first as non-vascular and later as vascular plants, becoming the primary producers of biomass on lands. During that time, the light-harvesting complex II (LHCII), responsible for sunlight absorption and excitation energy transfer to the photosystem II (PSII) core, underwent extensive differentiation. Lhcb4 is an ancestral LHCII that, in flowering plants, differentiated into up to three isoforms, Lhcb4.1, Lhcb4.2 and Lhcb4.3. The pivotal position of Lhcb4 in the PSII-LHCII supercomplex (PSII-LHCIIsc) allows functioning as linker for either S- or M-trimers of LHCII to the PSII core. The increased accumulation of Lhcb4.3 observed in PSII-LHCIIsc of plants acclimated to moderate and high light intensities induced us to investigate, whether this isoform has a preferential localization in a specific PSII-LHCIIsc conformation that might explain its light-dependent accumulation. In this work, by combining an improved method for separation of different forms of PSII-LHCIIsc from thylakoids of Pisum sativum L. grown at increasing irradiances with quantitative proteomics, we assessed that Lhcb4.3 is abundant in PSII-LHCIIsc of type C₂ S₂ , and, interestingly, similar results were found for the PsbR subunit. Phylogenetic comparative analysis on different taxa of the Viridiplantae lineage and structural modeling further pointed out to an effect of the evolution of different Lhcb4 isoforms on the light-dependent modulation of the PSII-LHCIIsc organization. This information provides new insight on the properties of the Lhcb4 and its isoforms and their role on the structure, function and regulation of PSII.