Plants lacking the main light-harvesting complex retain photosystem II macro-organization

Photosystem II (PSII) is a key component of photosynthesis, the process of converting sunlight into the chemical energy of life. In plant cells, it forms a unique oligomeric macrostructure in membranes of the chloroplasts. Several light-harvesting antenna complexes are organized precisely in the PSII macrostructure—the major trimeric complexes (LHCII) that bind 70% of PSII chlorophyll and three minor monomeric complexes—which together form PSII supercomplexes. The antenna complexes are essential for collecting sunlight and regulating photosynthesis, but the relationship between these functions and their molecular architecture is unresolved. Here we report that antisense Arabidopsis plants lacking the proteins that form LHCII trimers have PSII supercomplexes with almost identical abundance and structure to those found in wild-type plants. The place of LHCII is taken by a normally minor and monomeric complex, CP26, which is synthesized in large amounts and organized into trimers. Trimerization is clearly not a specific attribute of LHCII. Our results highlight the importance of the PSII macrostructure: in the absence of one of its main components, another protein is recruited to allow it to assemble and function

Structure and membrane organization of photosystem II in green plants

Photosystem II (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSII is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 Angstrom and so provide information mainly on the localization heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 Angstrom has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.</p

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. 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.

Supramolecular organization of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

We present an extended analysis of the organization of green plant photosystem II and its associated light-harvesting antenna using electron microscopy and image analysis. The analysis is based on a large dataset of 16 600 projections of negatively stained PSII-LHCII supercomplexes and megacomplexes prepared by means of three different pretreatments. In addition to our previous work on this system [Boekema, E.J., van Roon, H., Calkoen, F., Bassi, R. and Dekker, J.P. (1999) Biochemistry 38, 2233-2239], the following results were obtained. The rotational orientation of trimeric LHCII at the S, M and L binding positions was determined. It was found that compared to the S trimer, the M and L trimers are rotationally shifted by about -20 degrees and -50 degrees, respectively. The number of projections with empty CP29, CP26 and CP24 binding sites was found to be about 0, 18 and 4%, respectively. We suggest that CP26 and CP24 are not required for the binding of trimeric LHCII at any of the three binding positions. A new type of megacomplex was observed with a characteristic windmill-like shape. This type III megacomplex consists of two C2S2 supercomplexes connected at their CP26 tips. Structural variation in the region of the central dimeric photosystem II complex was found to occur at one specific position near the periphery of the complex. We attribute this variation to the partial absence of an extrinsic polypeptide or one or more small intrinsic membrane proteins

Heptameric association of light-harvesting complex II trimers in partially solubilized photosystem II membranes

We report a structural characterization by electron microscopy and image analysis of a supramolecular complex consisting of seven trimeric light-harvesting complex II proteins, The complex was readily observed in partially-solubilized Tris-washed photosystem II membranes from spinach but was also found to occur, with a low frequency, in oxygen-evolving photosystem II membranes. The structure reveals sis peripheral trimers with the same rotational orientation and a central trimer with the opposite orientation. We conclude that the heptamer represents a naturally occurring aggregation state of part of the light-harvesting complex II trimers in the thylakoid membranes. (C) 1999 Federation of European Biochemical Societies.</p

One-step isolation and biochemical characterization of a highlyactive plant PSII monomeric core

We describe a one-step detergent solubilization protocol for isolating a highly active form of Photosystem II (PSII) from Pisum sativum L. Detailed characterization of the preparation showed that the complex was a monomer having no light harvesting proteins attached. This core reaction centre complex had, however, a range of low molecular mass intrinsic proteins as well as the chlorophyll binding proteins CP43 and CP47 and the reaction centre proteins D1 and D2. Of particular note was the presence of a stoichiometric level of PsbW, a low molecular weight protein not present in PSII of cyanobacteria. Despite the high oxygen evolution rate, the core complex did not retain the PsbQ extrinsic protein although there was close to a full complement of PsbO and PsbR and partial level of PsbP. However, reconstitution of PsbP and PsbPQ was possible. The presence of PsbP in absence of LHCII and other chlorophyll a/b binding proteins confirms that LHCII proteins are not a strict requirement for the assembly of this extrinsic polypeptide to the PSII core in contrast with the conclusion of Caffarri et al. (2009)

The native architecture of a photosynthetic membrane

In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membrane-associated chlorophyll–protein complexes. Although the detailed structure of each complex has been determined, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found for the peripheral light-harvesting LH2 complex. In the first, groups of 10–20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)–light-harvesting 1 (RC–LH1–PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow 'energy conduits' of LH2 just two or three complexes wide

Isolation of novel PSII-LHCII megacomplexes from pea plants characterized by a combination of proteomics and electron microscopy

This work was supported by the Italian Ministry of Education, University and Research, “Futuro in Ricerca 2013” program RBFR1334SB to CP