Structural characterization of a complex of photosystem I and light-harvesting complex II of Arabidopsis thaliana.

Chloroplasts are central to the provision of energy for green plants. Their photosynthetic membrane consists of two major complexes converting sunlight: photosystem I (PSI) and photosystem II (PSII). The energy flow toward both photosystems is regulated by light-harvesting complex II (LHCII), which after phosphorylation can move from PSII to PSI in the so-called state 1 to state 2 transition and can move back to PSII after dephosphorylation. To investigate the changes of PSI and PSII during state transitions, we studied the structures and frequencies of all major membrane complexes from Arabidopsis thaliana chloroplasts at conditions favoring either state 1 or state 2. We solubilized thylakoid membranes with digitonin and analyzed the complete set of complexes immediately after solubilization by electron microscopy and image analysis. Classification indicated the presence of a PSI-LHCII supercomplex consisting of one PSI-LHCI complex and one LHCII trimer, which was more abundant in state 2 conditions. The presence of LHCII was confirmed by excitation spectra of the PSI emission of membranes in state 1 or state 2. The PSI-LHCII complex could be averaged with a resolution of 16 Å, showing that LHCII has a specific binding site at the PSI-A,-H,-L, and-K subunits. Oxygenic photosynthesis relies on a balanced system of light harvesting and the conversion of the light energy into chemical energy in the photosynthetic reaction centers of PSI 1 and PSII. PSI and PSII have different absorption spectra. The light energy fluctuates in intensity and quality, and plants can adapt to the changing light conditions by directing the absorbed light to either PSI or PSII to keep the energy conversion efficient. The response of the photosynthetic apparatus to light fluctuations is called state transitions (1-3). State transitions occur in three main steps. Upon preferential excitation of PSII, the linear electron flow between PSI and PSII may become unbalanced, resulting in an over-reduction of the plastoquinone (PQ) pool and the