Photosystem II Supercomplexes Of Higher Plants: Isolation And Determination Of The Structural And Functional Organization

Abstract

Photosystem II is a supercomplex composed of 27-28 different subunits and it represents the most important machinery of the plants photosynthetic appara- tus, having the ability to split water into oxygen, protons and electrons. In the last few years the structures of most of the photosynthetic complexes have been resolved, allowing to organize in a ‘‘visual framework’’ the large body of information obtained by genetics, biochemical and spectroscopic methods about the function and organization of the complexes. Only the struc- ture of PSII-LHCII from higher plants is still lacking due to the impossibility to obtain a homogeneous and stable preparation of the supercomplex, which has also prevented functional and spectroscopic studies. In this work homogeneous and stable Photosystem II supercomplexes with dif- ferent antenna size were isolated. A full gallery of complexes, from the core to the largest C2S2M2, was characterized by electron microscopy and biochemi- cal and spectroscopic methods, allowing to relate for the first time the supramo- lecular organization to the protein and pigment content and the energy transfer processes. A new complex containing a monomeric core, a trimeric LHCII (S) and a monomeric CP26 was isolated, showing that the antenna proteins can bind to the monomeric core in contrast to the current belief. The comparison of the supercomplexes obtained from WT plants and knock out mutants of sev- eral Lhcb proteins allowed determining the hierarchy of the assembly and to suggest a role for the individual subunits. The data also provides information about the organization of the oxygen evolving complex. For the first time it was possible to study the energy transfer process in the supercomplexes with the use of picosecond fluorescence spectroscopy. The functional implication of these results on photoinhibition, state transition and energy transfer are discussed.