Three-dimensional Structure of Higher Plant Photosystem I Determined by Electron Crystallography

Abstract

We describe the three-dimensional structure of higher plant photosystem I (PSI) as obtained by electron microscopy of two-dimensional crystals formed at the grana margins of thylakoid membranes. The negatively stained crystalline areas displayed unit cell dimensions a ‫؍‬ 26.6 nm, b ‫؍‬ 27.7 nm, and ␥ ‫؍‬ 90 o , and p22 1 2 1 plane group symmetry consisting of two monomers facing upward and two monomers facing downward with respect to the membrane plane. Higher plant PSI shows several structural similarities to the cyanobacterial PSI complex, with a prominent ridge on the stromal side of the complex. The stromal ridge is resolved into at least three separate domains that are interpreted as representing the three well characterized stromal subunits, the psa C, D, and E gene products. The lumenal surface is relatively flat but exhibits a distinct central depression that may be the binding site for plastocyanin. Higher plant PSI is of dimensions 15-16 ؋ 11-12.5 nm, and thus leaves a larger footprint in the membrane than its cyanobacterial equivalent (13 ؋ 10.5 nm). It is expected that additional membrane-bound polypeptides will be present in the higher plant PSI. Both higher plant and cyanobacterial complexes span about 8 -9 nm in the direction orthogonal to the membrane. This report represents the first three-dimensional structure for the higher plant PSI complex.Photosystem (PS) 1 I is a multiprotein complex found in the photosynthetic membranes of plants and cyanobacteria. It carries out the second light-driven electron transfer step in the linear Z scheme of photosynthesis (1). In higher plants, a significant proportion of the PSI complexes is located in the stromal lamellae (2). Cyclic electron transfer may be carried out by this pool of PSI for at least part of the time. It is also thought that higher plant PSI complexes may be located around the periphery of the grana stacks (the grana margins) where they interact with photosystem II (PSII) complexes, and probably take part in linear electron transfer (2). On the basis of spectroscopic data, it has been proposed that there is heterogeneity in the structure of higher plant PSI (3), with stromally located PSI complexes predicted to have a smaller photosynthetic unit size (i.e. a reduced number of light-harvesting pigments) than their granally located sisters. Until now, there has been little direct structural investigation of higher plant PSI. The paucity of structural data was largely due to the lack of two-or threedimensional crystals of the higher plant complex. In the absence of crystals, the analysis of freeze-fractured or freezeetched thylakoid membranes by shadowing and electron microscopy was carried out (e.g. see Dunahay and Staehelin (4), but only the overall dimensions of the complexes were determined, and uncertainty over their identification as PSI was not eliminated. In a second approach, higher plant PSI complexes have been purified after detergent solubilization and then examined by electron microscopy after negative staining (5). These studies...

The Location of the Mobile Electron Carrier Ferredoxin in Vascular Plant Photosystem I

“…These data are compatible with the values derived from earlier studies with an identical a/b axis ratio of 0.96 (2,19). Each unit cell is composed of four PS-I complexes; two with the luminal side up and two with the stromal face up.…”

“…Well ordered two-dimensional crystals from spinach thylakoids have been shown to contain PS-I and have yielded twodimensional data to 2.7 nm resolution (2) and refined threedimensional data to 2.5 nm resolution (19). However, neither of these studies retained the mobile electron carrier Fd.…”

The Location of Plastocyanin in Vascular Plant Photosystem I

“…Unlike cyanobacteria, spinach PS-I does not appear to form trimers but does show many common structural features (2,9). In neither of the studies on spinach (2,9) were the mobile electron carriers Pc and Fd retained. However, studies on the binding site for Fd, the terminal mobile electron acceptor in PS-I, have recently been possible (10) using a cross-linking and image analysis approach.…”

“…Two-dimensional crystals of a P22 1 2 1 plane group occurring at the edges of grana in spinach thylakoids have been shown to contain PS-I and have yielded structural data to 2.7 nm (2) and refined three-dimensional data to 2.5 nm resolution (9). Unlike cyanobacteria, spinach PS-I does not appear to form trimers but does show many common structural features (2,9).…”

Characterization and Evolution of Tetrameric Photosystem I from the Thermophilic Cyanobacterium Chroococcidiopsis sp TS-821

“…Cyanobacterial PSI trimers have been reported in many published works (Boekema et al, 1987(Boekema et al, , 2001Almog et al, 1991;Kruip et al, 1993Kruip et al, , 1994Kruip et al, , 1997Tsiotis et al, 1995;Garczarek et al, 1998;Tucker and Sherman, 2000;Mangels et al, 2002;Bibby et al, 2003;Brecht et al, 2012), and the only existing crystal structure of a cyanobacterial PSI, from Thermosynechococcus elongatus BP-1 (Jordan et al, 2001), suggests that cyanobacteria PSI preferentially forms a trimer. Although there have been reports of trimeric forms of PSI in plants (Heinemeyer et al, 2004;Kouril et al, 2005), it is now widely accepted that plant PSI forms only monomeric complexes (Kitmitto et al, 1998;Kouril et al, 2005). Furthermore, it is known that PSI is a monomer in green and red algae (Gardian et al, 2007) as well as in diatoms (Veith and Büchel, 2007).…”