Structural and functional heterogeneity in the major light-harvesting complexes of higher plants

The major light-harvesting complex (LHC IIb) of higher plants plays a crucial role in capturing light energy for photosynthesis and in regulating the flow of energy within the photosynthetic apparatus. Multiple isoforms of the protein bind chlorophyll and xanthophyll chromophores, but it is commonly believed that the pigment-binding properties of different LHC IIb complexes are conserved within and between species. We have investigated the structure and function of different LHC IIb complexes isolated from Arabidopsis thaliana grown under different light conditions. LHC IIb isolated from low light-grown plants shows increased amounts of the Lhcb2 gene product, increased binding of chlorophyll a, and altered energy transfer characteristics. We suggest that Lhcb2 specifically binds at least one additional chlorophyll a compared to the Lhcb1 gene product, and that differences in the functioning of LHC IIb from high and low light-grown plants are a direct consequence of the change in polypeptide composition. We show that changes in LHC IIb composition are accompanied by changes in photosynthetic function in vivo and discuss the possible functional significance of LHC IIb heterogeneity.