The major light-harvesting complex of photosystem II (LHC-II) serves as the principal solar energy collector in the photosynthesis of green plants and presumably also functions in photoprotection under high-light conditions. Here we report the first X-ray structure of LHC-II in icosahedral proteoliposome assembly at atomic detail. One asymmetric unit of a large R32 unit cell contains ten LHC-II monomers. The 14 chlorophylls (Chl) in each monomer can be unambiguously distinguished as eight Chla and six Chlb molecules. Assignment of the orientation of the transition dipole moment of each chlorophyll has been achieved. All Chlb are located around the interface between adjacent monomers, and together with Chla they are the basis for efficient light harvesting. Four carotenoid-binding sites per monomer have been observed. The xanthophyll-cycle carotenoid at the monomer-monomer interface may be involved in the non-radiative dissipation of excessive energy, one of the photoprotective strategies that have evolved in plants.

During photosynthesis, the plant photosystem II core complex receives excitation energy from the peripheral light-harvesting complex II (LHCII). The pathways along which excitation energy is transferred between them, and their assembly mechanisms, remain to be deciphered through high-resolution structural studies. Here we report the structure of a 1.1-megadalton spinach photosystem II-LHCII supercomplex solved at 3.2 Å resolution through single-particle cryo-electron microscopy. The structure reveals a homodimeric supramolecular system in which each monomer contains 25 protein subunits, 105 chlorophylls, 28 carotenoids and other cofactors. Three extrinsic subunits (PsbO, PsbP and PsbQ), which are essential for optimal oxygen-evolving activity of photosystem II, form a triangular crown that shields the Mn4CaO5-binding domains of CP43 and D1. One major trimeric and two minor monomeric LHCIIs associate with each core-complex monomer, and the antenna-core interactions are reinforced by three small intrinsic subunits (PsbW, PsbH and PsbZ). By analysing the closely connected interfacial chlorophylls, we have obtained detailed insights into the energy-transfer pathways between the antenna and core complexes.

In order to maximize their use of light energy in photosynthesis, plants have molecules that act as light-harvesting antennae, which collect light quanta and deliver them to the reaction centres, where energy conversion into a chemical form takes place. The functioning of the antenna responds to the extreme changes in the intensity of sunlight encountered in nature. In shade, light is efficiently harvested in photosynthesis. However, in full sunlight, much of the energy absorbed is not needed and there are vitally important switches to specific antenna states, which safely dissipate the excess energy as heat. This is essential for plant survival, because it provides protection against the potential photo-damage of the photosynthetic membrane. But whereas the features that establish high photosynthetic efficiency have been highlighted, almost nothing is known about the molecular nature of the dissipative states. Recently, the atomic structure of the major plant light-harvesting antenna protein, LHCII, has been determined by X-ray crystallography. Here we demonstrate that this is the structure of a dissipative state of LHCII. We present a spectroscopic analysis of this crystal form, and identify the specific changes in configuration of its pigment population that give LHCII the intrinsic capability to regulate energy flow. This provides a molecular basis for understanding the control of photosynthetic light-harvesting.

The first structure of an aldehyde dehydrogenase (ALDH) is described at 2.6 A resolution. Each subunit of the dimeric enzyme contains an NAD-binding domain, a catalytic domain and a bridging domain. At the interface of these domains is a 15 A long funnel-shaped passage with a 6 x 12 A opening leading to a putative catalytic pocket. A new mode of NAD binding, which differs substantially from the classic beta-alpha-beta binding mode associated with the 'Rossmann fold', is observed which we term the beta-alpha,beta mode. Sequence comparisons of the class 3 ALDH with other ALDHs indicate a similar polypeptide fold, novel NAD-binding mode and catalytic site for this family. A mechanism for enzymatic specificity and activity is postulated.

CP29, one of the minor light-harvesting complexes of higher-plant photosystem II, absorbs and transfers solar energy for photosynthesis and also has important roles in photoprotection. We have solved the crystal structure of spinach CP29 at 2.80-Å resolution. Each CP29 monomer contains 13 chlorophyll and 3 carotenoid molecules, which differs considerably from the major light-harvesting complex LHCII and the previously proposed CP29 model. The 13 chlorophyll-binding sites are assigned as eight chlorophyll a sites, four chlorophyll b and one putative mixed site occupied by both chlorophylls a and b. Based on the present X-ray structure, an integrated pigment network in CP29 is constructed. Two special clusters of pigment molecules, namely a615-a611-a612-Lut and Vio(Zea)-a603-a609, have been identified and might function as potential energy-quenching centers and as the exit or entrance in energy-transfer pathways.

In plants, the photosynthetic machinery photosystem II (PSII) consists of a core complex associated with variable numbers of light-harvesting complexes II (LHCIIs). The supercomplex, comprising a dimeric core and two strongly bound and two moderately bound LHCIIs (CSM), is the dominant form in plants acclimated to limited light. Here we report cryo-electron microscopy structures of two forms of CSM (termed stacked and unstacked) from at 2.7- and 3.2-angstrom resolution, respectively. In each CSM, the moderately bound LHCII assembles specifically with a peripheral antenna complex CP24-CP29 heterodimer and the strongly bound LHCII, to establish a pigment network that facilitates light harvesting at the periphery and energy transfer into the core. The high mobility of peripheral antennae, including the moderately bound LHCII and CP24, provides insights into functional regulation of plant PSII.

Plants regulate photosynthetic light harvesting to maintain balanced energy flux into photosystems I and II (PSI and PSII). Under light conditions favoring PSII excitation, the PSII antenna, light-harvesting complex II (LHCII), is phosphorylated and forms a supercomplex with PSI core and the PSI antenna, light-harvesting complex I (LHCI). Both LHCI and LHCII then transfer excitation energy to the PSI core. We report the structure of maize PSI-LHCI-LHCII solved by cryo-electron microscopy, revealing the recognition site between LHCII and PSI. The PSI subunits PsaN and PsaO are observed at the PSI-LHCI interface and the PSI-LHCII interface, respectively. Each subunit relays excitation to PSI core through a pair of chlorophyll molecules, thus revealing previously unseen paths for energy transfer between the antennas and the PSI core.

The crystal structure of a dipeptide complex of bovine neurophysin H has been solved at 2.8 A resolutionsolely by using single-wavelength anomalous scattering data from a single iodinated derivative. The asymmetric unit is an elongated tetramer of dimensions 110 x 40 X 30 A, composed of two dimers related by pseudo twofold symmetry. Each monomer consists of two homologous layers, each with four antiparaflel 3-strands. The two regions are connected by a helix followed by a long loop. Monomer-monomer contacts involve antiparallel 13-sheet interactions, which form a dimer with two layers of eight P-strands. (10), one of which is isomorphous with the crystals of a porcine NP-I complex (11). These contained 4, 8, and 12 molecules per asymmetric unit, respectively, suggesting that the basic aggregates of the NP-dipeptide complex in the crystals are tetramers that can self-associate to higher oligomers (10). In view of the probable importance of NP self-association in packaging of the precursor in the NSG (1, 4) and evidence that NP complexes can be present in the NSG as the precipitated or crystalline state (12, 13), analysis of the crystal structure of these complexes offers an opportunity for providing detailed structural information not only on NP folding and protein-peptide interactions but also on how the complexes might be packaged in NSG.The crystal structure of a bovine NP-II-dipeptide complex"l reported here was determined using only the singlewavelength anomalous scattering (SAS) data from an iodinated derivative crystal. Iodine was chosen as a probe because of its relatively large anomalous scattering signal (Alf' = 6.8) and its ease of incorporation into the dipeptide.The resulting derivative crystals contained iodine atoms covalently linked to the Phe residues of the bound hormone analogs.Here we report the structure of a NP molecule, as well as aspects of the methodology used in solving the structure. METHODSCocrystaflization. Bovine NP-I was purified as described (14). The dipeptide para-iodo-L-phenylalanyltyrosine amide (I-Phe-Tyr-NH2) was custom-synthesized by Peninsula Laboratories and was demonstrated by using circular dichroism (8) to bind to the hormone-binding site with high affinity. Crystals of the NP-dipeptide complex were grown at pH 7.5 by using a modification of the procedure of Yoo et al. (9); crystals of the complex of the corresponding noniodinated peptide served as seeds. The crystals obtained were nearly isomorphous with those of the complex of the uniodinated peptide, having four molecules (chains) per asymmetric unit and space group P212121 with a = 120.0 A, b = 69.4 A, and c = 62.4 A.