NOVEL ASPECTS OF CHLOROPHYLL A B-BINDING PROTEINS/

The light-harvesting proteins (LHC) constitute a multigene family incl uding, in higher plants, at least 12 members whose location, within th e photosynthetic membrane, relative abundance and putative function ap pear to be very different. The major light-harvesting complex of photo system II (LHCII) is the most abundant membrane protein in the biosphe re and fulfil a constitutive light-harvesting function for photosystem II while the early light-induced proteins (ELIPs) are expressed in lo w amounts under stress conditions. Primary sequence analysis suggests that all these proteins share a common structure which was resolved at 3.7 Angstrom resolution by electron crystallography in the case of th e major LHCII complex: Three transmembrane helices connected by hydrop hilic loops coordinate seven chlorophyll a and five chlorophyll b mole cules 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 centr e of the structure in close contact with seven porphyrins, tentatively identified as chlorophyll a. The antenna system is also involved in t he regulation of excitation energy transfer to reaction centre II. Thi s function has been attributed to three members of the protein family, namely CP29, CP26 and CP24 (also called minor chlorophyll proteins) w hich have been recently characterised and shown to bind most of the xa ntophyll cycle carotenoids, thus suggesting that the non-photochemical quenching mechanism is acting in these proteins. Further support to t his assignment comes from the recent identification of protonation sit es in CP29 and CP26 by covalent dicyclohexhylcarbodiimide binding sugg esting that these respond to low lumenal pH. In addition, CP29 is reve rsibly phosphorylated under light and cold stress conditions, undergoi ng conformational change, supporting the hypothesis that these subunit s, present in low amounts in photosystem II, have a major regulatory r ole in the Light-harvesting function and are thus important in environ mental stress resistance.