Chlorophyll and carotenoid binding in a simple red algal light-harvesting complex crosses phylogenetic lines

The membrane proteins of peripheral light-harvesting complexes (LHCs) bind chlorophylls and carotenoids and transfer energy to the reaction centers for photosynthesis. LHCs of chlorophytes, chromophytes, dinophytes, and rhodophytes are similar in that they have three transmembrane regions and several highly conserved Chi-binding residues. All LHCs bind Chi a, but in specific taxa certain characteristic pigments accompany Chi a: Chi b and lutein in chlorophytes, Chi c and fucoxanthin in chromophytes, Chi c and peridinin in dinophytes, and zeaxanthin in rhodophytes. The specificity of pigment binding was examined by in vitro reconstitution of various pigments with a simple light-harvesting protein (LH-CaR1), from a red alga (Porphyridium cruentum), that normally has eight Chi a and four zeaxanthin molecules. The pigments typical of a chlorophyte (Spinacea oleracea), a chromophyte (Thallasiosira fluviatilis), and a dinophyte (Prorocentrum micans) were found to functionally bind to this protein as evidenced by their participation in energy transfer to Chi a, the terminal pigment. This is a demonstration of a functional relatedness of rhodophyte and higher plant LHCs. The results suggest that eight Chi-binding sites per polypeptide are an ancestral trait, and that the flexibility to bind various Chi and carotenoid pigments may have been retained throughout the evolution of LHCs.