Novel aspects of chlorophyll a/b-binding proteins

The light-harvesting proteins (LHC) constitute a multigene family including, in higher plants, at least 12 members whose location, within the photosynthetic membrane, relative abundance and putative function appear to be very different. The major light-harvesting complex of photosystem II (LHCII) is the most abundant membrane protein in the biosphere and fulfil a constitutive light-harvesting function for photosystem II while the early light-induced proteins (ELIPs) are expressed in low amounts under stress conditions. Primary sequence analysis suggests that all these proteins share a common structure which was resolved at 3.7 Å resolution by electron crystallography in the case of the major LHCII complex: Three transmembrane helices connected by hydrophilic loops coordinate seven chlorophyll a and five chlorophyll b molecules by histidine, glutamine, asparagine lateral chains as well as by charge compensated ionic pairs of glutamic acid and arginine residues; moreover, at least two xantophyll molecules are located at the centre of the structure in close contact with seven porphyrins, tentatively identified as chlorophyll a. The antenna system is also involved in the regulation of excitation energy transfer to reaction centre II. This function has been attributed to three members of the protein family, namely CP29, CP26 and CP24 (also called minor chlorophyll proteins) which have been recently characterised and shown to bind most of the xantophyll cycle carotenoids, thus suggesting that the non-photochemical quenching mechanism is acting in these proteins. Further support to this assignment comes from the recent identification of protonation sites in CP29 and CP26 by covalent dicyclohexhylcarbodiimide binding suggesting that these respond to low lumenal pH. In addition, CP29 is reversibly phosphorylated under light and cold stress conditions, undergoing conformational change, supporting the hypothesis that these subunits, present in low amounts in photosystem II, have a major regulatory role in the light-harvesting function and are thus important in environmental stress resistance.