LHCII - a protein like a 'Swiss Army knife' with many mechanisms and functions

The review highlights the relationship between the molecular organization of the light-harvesting complex of photosystem II (LHCII) and sunlight utilization by higher plants. The molecular form of LHCII switches rapidly and reversibly during diurnal changes of light intensity, from low (ca. 10) to high [ca. 1,000 μmol(photon) m^-2 s^-1], so the sensitivity of LHCII to light may control the balance between light harvesting and photoprotection state. Our understanding and concept of this mechanism are based on the knowledge of the structure and photophysics of different LHCII molecular forms: monomer, dimer, trimer, and aggregate. It is proposed that LHCII monomers, dimers, and lateral aggregates are fundamental blocks of excess light-dissipation machinery. Trimer is exceptionally well suited to play a physiological role of an antenna complex. A correlation between the LHCII molecular form and the presence of xanthophyll cycle pigment violaxanthin and zeaxanthin in the complex structure is also shown. Moreover, the role of LHCII protein phosphorylation in thylakoid membrane architecture is also discussed. The dual function of LHCII has been studied in the natural thylakoid membranes of chloroplasts, in the artificial lipid-LHCII model membranes, and by suspension of LHCII in a detergent solution.