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Item does not contain fulltext Self-aggregation of isolated plant light-harvesting complexes (LHCs) upon detergent extraction is associated with fluorescence quenching and is used as an in vitro model to study the photophysical processes of nonphotochemical quenching (NPQ). In the NPQ state, in vivo induced under excess solar light conditions, harmful excitation energy is safely dissipated as heat. To prevent self-aggregation and probe the conformations of LHCs in a lipid environment devoid from detergent interactions, we assembled LHCII trimer complexes into lipid nanodiscs consisting of a bilayer lipid matrix surrounded by a membrane scaffold protein (MSP). The LHCII nanodiscs were characterized by fluorescence spectroscopy and found to be in an unquenched, fluorescent state. Remarkably, the absorbance spectra of LHCII in lipid nanodiscs show fine structure in the carotenoid and Q(y) region that is different from unquenched, detergent-solubilized LHCII but similar to that of self-aggregated, quenched LHCII in low-detergent buffer without magnesium ions. The nanodisc data presented here suggest that 1), LHCII pigment-protein complexes undergo conformational changes upon assembly in nanodiscs that are not correlated with downregulation of its light-harvesting function; and 2), these effects can be separated from quenching and aggregation-related phenomena. This will expand our present view of the conformational flexibility of LHCII in different microenvironments.

Iron availability is a major limiting factor for photosynthesis and hence for life in most of the aquatic environments on earth. Cyanobacteria are important primary producers and prevail over Fe- deficiency by de-repressing the isiAB operon, which codes for the antenna protein IsiA and flavodoxin. We demonstrate that under nanomolar iron concentrations, a giant IsiA-Photosystem I supercomplex is formed, consisting of a Photosystem I trimer encircled by two complete IsiA rings with 18 and 25 copies in the inner ring and outer ring, respectively. The IsiA-Photosystem I supercomplex contains more than 850 chlorophylls and has a mass of 3.2 MDa, making it the most complex membrane protein that has been isolated to date. Ultrafast fluorescence spectroscopic results show fast and efficient excitation transfer and trapping in the supercomplex. The electron transfer throughput of Photosystem I is increased by 300%, an evolutionary adaptation that has allowed cyanobacteria to avoid oxidative stress. This adaptation of the photosynthetic apparatus confers an enormous, as-yet unrecognized, evolutionary advantage to cyanobacteria living under conditions of severe Fe stress and thereby have adapted to the modern low iron concentration in aquatic environments.1. Eldridge, M. et al. Aquatic Microbial Ecology 35, 79-91 (2004).2. Martin, J. & Fitzwater, S. Nature 331, 341-343 (1988).3. Wilhelm, S. Aquatic Microbial Ecology 9, 295-303 (1995).4. Bibby, T. S., Nield, J. & Barber, J. Nature 412, 743-5 (2001).5. Boekema, E. J. et al. Nature 412, 745-8 (2001).6. Singh, A., McIntyre, L. & Sherman, L. Plant Physiology 132, 1825 (2003).

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.