Mechanistic aspects of the xanthophyll dynamics in higher plant thylakoids

Plant thylakoids have a highly conserved xanthophyll composition, consisting of b-carotene, lutein, neoxanthin and a pool of violaxanthin that can be converted to antheraxanthin and zeaxanthin in excess light conditions. Recent work has shown that xanthophylls undergo dynamic changes not only in their composition but also in their distribution among Lhc proteins. Xanthophylls are released from specific binding site in the major trimeric LHCII complex of photosystem II and subsequently bound to different sites into monomeric Lhcb proteins and dimeric Lhca proteins. In this work we review available evidence from in vivo and in vitro studies on the structural determinants that control xanthophyll exchange in Lhc proteins. We conclude that the xanthophyll exchange rate is determined by the structure of individual Lhc gene products and it is specifically controlled by the lumenal pH independently from the activation state of the violaxanthin de-epoxidase enzyme.