Regulation of light harvesting in multimeric and monomeric photosystem II supercomplexes

An intriguing architecture called semi-crystalline photosystem II (PSII) array has been observed in the thylakoid membranes in vascular plants. It is an array of PSIIlight harvesting complex II (LHCII) supercomplexes only appears in the low-light, whose functional role has not been clarified. We identified PSIILHCII supercomplexes in their monomeric and multimeric forms in the low-light acclimated spinach leaves and prepared them using sucrose density gradient-ultracentrifugation in the presence of amphipol A8-35. When the leaves were acclimated to high-light, however, only monomeric forms were present. Single particle electron microscopy identified that the multimeric PSIILHCII supercomplexes were composed of two (megacomplex) or three (arraycomplex) units of PSIILHCII supercomplexes, which aligned like a fraction of the semi-crystalline array. Further characterization using fluorescence analysis revealed that multimeric forms have a higher light-harvesting capability, but a lower thermal dissipation capability than the monomeric form, suggesting such a configurational conversion of PSIILHCII supercomplexes possibly serves as a structural basis for the plants acclimation to environmental light.