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 fo r photosynthesis. LHCs of chlorophytes, chromophytes, dinophytes, and rhodo phytes are similar in that they have three transmembrane regions and severa l highly conserved Chi-binding residues. All LHCs bind Chi a, but in specif ic taxa certain characteristic pigments accompany Chi a: Chi b and lutein i n chlorophytes, Chi c and fucoxanthin in chromophytes, Chi c and peridinin in dinophytes, and zeaxanthin in rhodophytes. The specificity of pigment bi nding was examined by in vitro reconstitution of various pigments with a si mple light-harvesting protein (LH-CaR1), from a red alga (Porphyridium crue ntum), that normally has eight Chi a and four zeaxanthin molecules. The pig ments typical of a chlorophyte (Spinacea oleracea), a chromophyte (Thallasi osira fluviatilis), and a dinophyte (Prorocentrum micans) were found to fun ctionally bind to this protein as evidenced by their participation in energ y transfer to Chi a, the terminal pigment. This is a demonstration of a fun ctional relatedness of rhodophyte and higher plant LHCs. The results sugges t that eight Chi-binding sites per polypeptide are an ancestral trait, and that the flexibility to bind various Chi and carotenoid pigments may have b een retained throughout the evolution of LHCs.