Phosphorylation in State Transition : Less cause more effect

Abstract: Study of the Arabidopsis thaliana knockout mutant lacking Lhcb3 (koLhcb3) have revealed a close similarity to the wild type plants. Growth rate, NPQ, qP, Φ(PSII), circular dichroism spectra, pigment composition and content of LCHII trimers have been found to be unaffected by this mutation. The proteomic analysis shows only some minor increases in the amount of Lhcb1 and Lhcb2. PAM fluorometry revealed a significant increase in the rate of the state 1 to state 2 state transition in the koLhcb3. None the less, the extent of state transition is identical to wild type. Alterations in the PSII-LHCII supercomplex structure have been demonstrated as well. The M-trimer was found to be rotated ~21° CCW. This altered binding of the LHCII M-trimer is likely the cause of the altered affinity resulting in the increased rate of state transition. Proteomic analysis of the phosphorylation of LHCII revealed a significant increase in state 1 and 2 LHCII phosphorylation relative to wild type. Investigation whether phosphorylation or the altered LHCII binding is the cause of the accelerated rate of state transition have not been conclusive so far. A Lhcb6 depleted mutant (koLhcb6) showed a significant alteration of the PSII-LHCII supercomplex structure and photosynthetic acclimation processes. The LHCII M-trimer is depleted in the PSII-LHCII supercomplexes causing the state transition process to be “stuck” in state 2 and the mutants ability to preform NPQ is inhibited as well. The Lhcb6 protein was concluded to be essential for the binding of the LHCII M-trimer to the PSII core as well as energy transfer. The depletion of LHCII M-trimer was linked to the reduced ability to photoacclimate using NPQ as well.