Abstract
Three plant xanthophylls are components of the xanthophyll cycle in which, upon exposure of leaves to high light, the enzyme violaxanthin de-epoxidase (VDE) transforms violaxanthin into zeaxanthin via the intermediate antheraxanthin. Previous work () showed that xanthophylls are bound to Lhc proteins and that substitution of violaxanthin with zeaxanthin induces conformational changes and fluorescence quenching by thermal dissipation. We have analyzed the efficiency of different Lhc proteins to exchange violaxanthin with zeaxanthin both in vivo and in vitro. Light stress of Zea mays leaves activates VDE, and the newly formed zeaxanthin is found primarily in CP26 and CP24, whereas other Lhc proteins show a lower exchange capacity. The de-epoxidation system has been reconstituted in vitro by using recombinant Lhc proteins, recombinant VDE, and monogalactosyl diacylglycerol (MGDG) to determine the intrinsic capacity for violaxanthin-to-zeaxanthin exchange of individual Lhc gene products. Again, CP26 was the most efficient in xanthophyll exchange. Biochemical and spectroscopic analysis of individual Lhc proteins after de-epoxidation in vitro showed that xanthophyll exchange occurs at the L2-binding site. Xanthophyll exchange depends on low pH, implying that access to the binding site is controlled by a conformational change via lumenal pH. These findings suggest that the xanthophyll cycle participates in a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the antenna system of higher plants.
Highlights
Exceeds the capacity for electron transport from water to NADPϩ, excess energy can be diverted to molecular oxygen with the formation of reactive species harmful for the chloroplast, leading to photoinhibition of photosystems [5]
These findings suggest that the xanthophyll cycle participates in a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the antenna system of higher plants
The pigment composition of thylakoid membranes is modified by the operation of the xanthophyll cycle, consisting of the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin by the lumenal enzyme violaxanthin de-epoxidase (VDE), which binds to thylakoids upon activation by low lumenal pH
Summary
Exceeds the capacity for electron transport from water to NADPϩ, excess energy can be diverted to molecular oxygen with the formation of reactive species harmful for the chloroplast, leading to photoinhibition of photosystems [5] In these conditions photoprotection mechanisms are activated leading to the thermal dissipation of excess chlorophyll singlet states [6]. Biochemical and spectroscopic analysis of Lhc proteins upon in vitro de-epoxidation showed that xanthophyll exchange occurs at the L2-binding site This site was previously shown [10] to act as an allosteric regulator of thermal dissipation activity in Lhc proteins by controlling the transition between two conformations of Lhc proteins [1]. These data suggest that the xanthophyll cycle is part of a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the photosystems of higher plants
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