The family of light-harvesting-related proteins (LHCs, ELIPs, HLIPs): was the harvesting of light their primary function?

Light-harvesting complex proteins (LHCs) and early light-induced proteins ( ELIPs) are essential pigment-binding components of the thylakoid membrane a nd are encoded by one of the largest and most complex higher plant gene fam ilies. The functional diversification of these proteins corresponded to the transition from extrinsic (phycobilisome-based) to intrinsic (LHC-based) l ight-harvesting antenna systems during the evolution of chloroplasts from c yanobacteria, yet the functional basis of this diversification has been elu sive. Here, we propose that the original function of LHCs and ELIPs was not to collect light and to transfer its energy content to the reaction center s but to disperse the absorbed energy of light in the form of heat or fluor escence. These energy-dispersing proteins are believed to have originated i n cyanobacteria as one-helix, highly light-inducible proteins (HLIPs) that later acquired four helices through two successive gene duplication steps. We suggest that the ELIPs arose first in this succession, with a primary fu nction in energy dispersion for protection of photosynthetic pigments from photo-oxidation. We consider the LHC I and II families as more recent and v ery successful evolutionary additions to this family that ultimately attain ed a new function, thereby replacing the ancestral extrinsic light-harvesti ng system. Our model accounts for the non-photochemical quenching role rece ntly shown for higher plant psbS proteins. (C) 2000 Elsevier Science B.V. A ll rights reserved.