Electron crystallographic study of photosystem II of the cyanobacterium Synechococcus elongatus

The determination of the structure of PSII at high resolution is required in order to fully understand its reaction mechanisms. Two-dimensional crystals of purified highly active Synechococcus elongatus PSII dimers were obtained by in vitro reconstitution. Images of these crystals were recorded by electron cryo-microscopy, and their analysis revealed they belong to the two-sided plane group p22(1)2(1), with unit cell parameters a = 121 A, b = 333 A, and alpha = 90 degrees. From these crystals, a projection map was calculated to a resolution of approximately 16 A. The reliability of this projection map is confirmed by its close agreement with the recently presented three-dimensional model of the same complex obtained by X-ray crystallography. Comparison of the projection map of the Synechococcus elongatus PSII complex with data obtained by electron crystallography of the spinach PSII core dimer reveals a similar organization of the main transmembrane subunits. However, some differences in density distribution between the cyanobacterial and higher plant PSII complexes exist, especially in the outer region of the complex between CP43 and cytochrome b(559) and adjacent to the B-helix of the D1 protein. These differences are discussed in terms of the number and organization of some of the PSII low molecular weight subunits.

Three-dimensional structure of the photosystem II core dimer of higher plants determined by electron microscopy

Here we report the first three-dimensional structure of a higher plant photosystem II core dimer determined by electron crystallography at a resolution sufficient to assign the organization of its transmembrane helices. The locations of 34 transmembrane helices in each half of the dimer have been deduced, 22 of which are assigned to the major subunits D1 (5), D2 (5), CP47 (6), and CP43 (6). CP47 and CP43, located on opposite sides of the D1/D2 heterodimer, are structurally similar to each other, consisting of 3 pairs of transmembrane helices arranged in a ring. Both CP47 and CP43 have densities protruding from the lumenal surface, which are assigned to the loops joining helices 5 and 6 of each protein. The remaining 12 helices within each half of the dimer are attributed to low-molecular-weight proteins having single transmembrane helices. Comparison of the subunit organization of the higher plant photosystem II core dimer reported here with that of its thermophilic cyanobacterial counterpart recently determined by X-ray crystallography shows significant similarities, indicative of a common evolutionary origin. Some differences are, however, observed, and these may relate to variations between the two classes of organisms in antenna linkage or thermostability.

Subunit positioning and transmembrane helix organisation in the core dimer of photosystem II

Recently 3D structural models of the photosystem II (PSII) core dimer complexes of higher plants (spinach) and cyanobacteria (Synechococcus elongatus) have been derived by electron [Rhee et al. (1998) Nature 396, 283-286; Hankamer et al. (2001) J. Struct. Biol., in press] and X-ray [Zouni et al. (2001) Nature 409, 739-743] crystallography respectively. The intermediate resolutions of these structures do not allow direct identification of side chains and therefore many of the individual subunits within the structure are unassigned. Here we review the structure of the higher plant PSII core dimer and provide evidence for the tentative assignment of the low molecular weight subunits. In so doing we highlight the similarities and differences between the higher plant and cyanobacterial structures.

Relationship between excitation energy transfer, trapping, and antenna size in photosystem II

We present a systematic study of the effect of antenna size on energy transfer and trapping in photosystem II. Time-resolved fluorescence experiments have been used to probe a range of particles isolated from both higher plants and the cyanobacterium Synechocystis 6803. The isolated reaction center dynamics are represented by a quasi-phenomenological model that fits the extensive time-resolved data from photosystem II reaction centers and reaction center mutants. This representation of the photosystem II "trapping engine" is found to correctly predict the extent of, and time scale for, charge separation in a range of photosystem II particles of varying antenna size (8-250 chlorins). This work shows that the presence of the shallow trap and slow charge separation kinetics, observed in isolated D1/D2/cyt b559 reaction centers, are indeed retained in larger particles and that these properties are reflected in the trapping dynamics of all larger photosystem II preparations. A shallow equilibrium between the antennae and reaction center in photosystem II will certainly facilitate regulation via nonphotochemical quenching, and one possible interpretation of these findings is therefore that photosystem II is optimized for regulation rather than for efficiency.

Phosphatidylglycerol is involved in the dimerization of photosystem II

Photosystem II core dimers (450 kDa) and monomers (230 kDa) consisting of CP47, CP43, the D1 and D2 proteins, the extrinsic 33-kDa subunit, and the low molecular weight polypeptides PsbE, PsbF, PsbH, PsbI, PsbK, PsbL, PsbTc, and PsbW were isolated by sucrose density gradient centrifugation. The photosystem II core dimers were treated with phospholipase A2 (PL-A2), which cuts phosphatidylglycerol (PG) and phosphatidylcholine molecules at the sn-2 position. The PL-A2-treated dimers dissociated into two core monomers and further, yielding a CP47-D1-D2 subcomplex and CP43. Thin layer chromatography showed that photosystem II dimers contained four times more PG than their monomeric counterparts but with similar levels of phosphatidylcholine. Consistent with this was the finding that, compared with monomers, the dimers contained a higher level of trans-hexadecanoic fatty acid (C16:1Delta3tr), which is specific to PG of the thylakoid membrane. Moreover, treatment of dimers with PL-A2 increased the free level of this fatty acid specific to PG compared with untreated dimers. Further evidence that PG is involved in stabilizing the dimeric state of photosystem II comes from reconstitution experiments. Using size exclusion chromatography, it was shown that PG containing C16:1Delta3tr, but not other lipid classes, induced significant dimerization of isolated photosystem II monomers. Moreover, this dimerization was observed by electron crystallography when monomers were reconstituted into thylakoid lipids containing PG. The unit cell parameters, p2 symmetry axis, and projection map of the reconstituted dimer was similar to that observed for two-dimensional crystals of the native dimer.

Revealing the structure of the oxygen-evolving core dimer of photosystem II by cryoelectron crystallography

Here we present cryoelectron crystallographic analysis of an isolated dimeric oxygen-evolving complex of photosystem II (at a resolution of approximately 0.9 nm), revealing that the D1-D2 reaction center (RC) proteins are centrally located between the chlorophyll-binding proteins, CP43 and CP47. This conclusion supports the hypothesis that photosystems I and II have similar structural features and share a common evolutionary origin. Additional density connecting the two halves of the dimer, which was not observed in a recently described CP47-RC complex that did not include CP43, may be attributed to the small subunits that are involved in regulating secondary electron transfer, such as PsbH. These subunits are possibly also required for stabilization of the dimeric photosystem II complex. This complex, containing at least 29 transmembrane helices in its asymmetric unit, represents one of the largest membrane protein complexes studied at this resolution.

Localization of the 23-kDa subunit of the oxygen-evolving complex of photosystem II by electron microscopy

A dimeric photosystem II light-harvesting II super complex (PSII-LHCII SC), isolated by sucrose density gradient centrifugation, was previously structurally characterized [Boekema, E. J., Hankamer, B., Bald, D., Kruip, J., Nield, J., Boonstra, A. F., Barber, J. & Rögner, M. (1995) Proc. Natl Acad. Sci. USA 92, 175-179]. This PSII-LHCII SC bound the 33-kDa subunit of the oxygen-evolving complex (OEC), but lacked the 23-kDa and 17-kDa subunits of the OEC. Here the isolation procedure was modified by adding 1 M glycine betaine (1-carboxy-N,N,N-trimethylmethanaminium hydroxide inner salt) to the sucrose gradient mixture. This procedure yielded PSII-LHCII SC that contained both the 33-kDa and the 23-kDa subunits and had twice the oxygen-evolving capacity of the super complexes lacking the 23-kDa polypeptide. Addition of CaCl2 to PSII-LHCII SC with the 23-kDa subunit attached did not increase the oxygen-evolution rate. This suggests that the 23-kDa subunit is bound in a functional manner and is present in significant amounts. Over 5000 particle projections extracted from electron microscope images of negatively stained PSII-LHCII SC, isolated in the presence and absence of glycine betaine, were analyzed using single-particle image-averaging techniques. Both the 23-kDa and 33-kDa subunits could be visualized in top-view and side-view projections. In the side view the 23-kDa subunit is seen to protrude 0.5-1 nm further than the 33-kDa subunit, giving the PSII particle a maximal height of 9.5 nm. Measured from the centres of the masses, the two 33-kDa subunits associated with the dimeric PSII-LHCII SC are separated by 6.3 nm. The corresponding distance between the two 23-kDa subunits is 8.8 nm.

The three-dimensional structure of a photosystem II core complex determined by electron crystallography

BACKGROUND

Photosystem II (PSII) is a multisubunit protein complex which is embedded in the photosynthetic membranes of plants. It uses light energy to split water into molecular oxygen and reducing equivalents. PSII can be isolated with varying degrees of complexity in terms of its subunit composition and activity. To date, no three-dimensional (3-D) structure of the PSII complex has been determined which allows location of the proteins within the PSII complex and their orientation in relation to the thylakoid membrane.

RESULTS

Two-dimensional (2-D) PSII core complex crystals composed of the two reaction centre proteins, D1 and D2, two chlorophyll-binding proteins, CP47 and CP43, cytb559 and associated low molecular weight proteins were formed after reconstituting the isolated complex into purified thylakoid lipids. Electron micrographs of negatively stained crystals were used for 2-D and 3-D image analyses. In the resulting maps, the PSII complex is composed of two halves related by twofold rotational symmetry, thus, confirming the dimeric nature of the complex; each monomer appears to contain five domains. Comparison of the 3-D images with platinum shadowed images of the crystals allowed the likely lumenal and stromal surfaces of the complex to be identified and regions contained within the membrane to be inferred. The projection structure of 2-D crystals of a smaller CP47-D1-D2-cytb559 complex was used to identify the domains apparently associated with CP43.

CONCLUSION

The results indicate that PSII probably exists as a dimer in vivo. The extensive proteinaceous protrusions from the lumenal surface have been tentatively assigned to hydrophilic loops of CP47 and CP43; the positioning of these loops possibly implies their involvement in the water-splitting process.

Isolation and biochemical characterisation of monomeric and dimeric photosystem II complexes from spinach and their relevance to the organisation of photosystem II in vivo

Membranes enriched in photosystem II were isolated from spinach and further solubilised using n-octyl beta-D-glucopyranoside (OctGlc) and n-dodecyl beta-D-maltoside (DodGlc2). The OctGlc preparation had high rates of oxygen evolution and when subjected to size-exclusion HPLC and sucrose density gradient centrifugation, in the presence of DodGlc2, separated into dimeric (430 kDa), monomeric (236 kDa) photosystem II cores and a fraction containing photosystem II light-harvesting complex (Lhcb) proteins. The dimeric core fraction was more stable, contained higher levels of chlorophyll, beta-carotene and plastoquinone per photosystem II reaction centre and had a higher oxygen-evolving activity than the monomeric cores. Their subunit composition was similar (CP43, CP47, D1, D2, cytochrome b 559 and several lower-molecular-mass components) except that the level of 33-kDa extrinsic protein was lower in the monomeric fraction. Direct solubilisation of photosystem-II-enriched membranes with DodGlc2, followed by sucrose density gradient centrifugation, yielded a super complex (700 kDa) containing the dimeric form of the photosystem II core and Lhcb proteins: Lhcb1, Lhcb2, Lhcb4 (CP29), and Lhcb5 (CP26). Like the dimeric and monomeric photosystem II core complexes, the photosystem II-LHCII complex had lost the 23-kDa and 17-kDa extrinsic proteins, but maintained the 33-kDa protein and the ability to evolve oxygen. It is suggested, with a proposed model, that the isolated photosystem II-LHCII super complex represents an in vivo organisation that can sometimes form a lattice in granal membranes of the type detected by freeze-etch electron microscopy [Seibert, M., DeWit, M. & Staehelin, L. A. (1987) J. Cell Biol. 105, 2257-2265].

Heterogeneity and pigment composition of isolated photosystem II reaction centers

Photosystem II reaction centers (RC) isolated from peas (Pisum sativum L) purified by ionexchange chromatography were shown, by high-performance liquid chromatography (HPLC) size-exclusion analyses, to consist of a mixture of monomers (180 +/- 20 kDa) and dimers (390 +/- 35 kDa). Both fractions were resolved and purified by sucrose density gradient centrifugation and their homogeneity was demonstrated in size-exclusion HPLC elution profiles. Also present in the nonresolved preparation and the monomeric fraction were low levels of CP47 apoprotein (1.8% and 0.9% apoprotein of that found in a CP47-RC preparation). This CP47 contamination could maximally account for 0.41 and 0.22 Chl/RC, respectively, based on 22 chlorophylls being bound to each CP47 protein. The level of CP47 apoprotein was undetectable in the dimeric fractions. Pigment analysis using reverse-phase HPLC confirmed that contamination by chlorophyll bound to the CP47 apoprotein in the nonresolved RC preparation was low and that the ratio of chlorophyll a to pheophytin a remained 6 when the preparation was separated into its monomeric and dimeric components. We conclude, in agreement with earlier work, that the reaction center of PSII, when isolated using mild detergents and ion-exchange chromatography, contains 6 chlorophyll a/2 pheophytin a. We therefore do not concur with the recent published work of Pueyo et al. [(1995) Biochemistry 34, 15214-15218) that this type of preparation contains 4 chlorophyll a/2 pheophytin a and that the remaining 2 chlorophyll a are due to contamination by CP47.

Supramolecular structure of the photosystem II complex from green plants and cyanobacteria

Photosystem II (PSII) complexes, isolated from spinach and the thermophilic cyanobacterium Synechococcus elongatus, were characterized by electron microscopy and single-particle image-averaging analyses. Oxygen-evolving core complexes from spinach and Synechococcus having molecular masses of about 450 kDa and dimensions of approximately 17.2 x 9.7 nm showed twofold symmetry indicative of a dimeric organization. Confirmation of this came from image analysis of oxygen-evolving monomeric cores of PSII isolated from spinach and Synechococcus having a mass of approximately 240 kDa. Washing with Tris at pH 8.0 and analysis of side-view projections indicated the possible position of the 33-kDa extrinsic manganese-stabilizing protein. A larger complex was isolated that contained the light-harvesting complex II (LHC-II) and other chlorophyll a/b-binding proteins, CP29, CP26, and CP24. This LHC-II-PSII complex had a mass of about 700 kDa, and electron microscopy revealed it also to be a dimer having dimensions of about 26.8 and 12.3 nm. From comparison with the dimeric core complex, it was deduced that the latter is located in the center of the larger particle, with additional peripheral regions accommodating the chlorophyll a/b-binding proteins. It is suggested that two LHC-II trimers are present in each dimeric LHC-II-PSII complex and that each trimer is linked to the reaction center core complex by CP24, CP26, and CP29. The results also suggest that PSII may exist as a dimer in vivo.