Lhc proteins and the regulation of photosynthetic light harvesting function by xanthophylls

Photoprotection of the chloroplast is an important component of abiotic str ess resistance in plants. Carotenoids have a central role in photoprotectio n. We review here the recent evidence, derived mainly from in vitro reconst itution of recombinant Lhc proteins with different carotenoids and from car otenoid biosynthesis mutants, for the existence of different mechanisms of photoprotection and regulation based on xanthophyll binding to Lhc proteins into multiple sites and the exchange of chromophores between different Lhc proteins during exposure of plants to high light stress and the operation of the xanthophyll cycle. The use of recombinant Lhc proteins has revealed up to four binding sites in members of Lhc families with distinct selectivi ty for xanthophyll species which are here hypothesised to have different fu nctions. Site L1 is selective for lutein and is here proposed to be essenti al for catalysing the protection from singlet oxygen by quenching chlorophy ll triplets. Site L2 and N1 are here proposed to act as allosteric sites in volved in the regulation of chlorophyll singlet excited states by exchangin g ligand during the operation of the xanthophyll cycle. Site V1 of the majo r antenna complex LHC II is here hypothesised to be a deposit for readily a vailable substrate for violaxanthin de-epoxidase rather than a light harves ting pigment. Moreover, xanthophylls bound to Lhc proteins can be released into the lipid bilayer where they contribute to the scavenging of reactive oxygen species produced in excess light.