Abstract
We used cryoelectron tomography to reveal the arrangements of photosystem II (PSII) and ATP synthase in vitreous sections of intact chloroplasts and plunge-frozen suspensions of isolated thylakoid membranes. We found that stroma and grana thylakoids are connected at the grana margins by staggered lamellar membrane protrusions. The stacking repeat of grana membranes in frozen-hydrated chloroplasts is 15.7 nm, with a 4.5-nm lumenal space and a 3.2-nm distance between the flat stromal surfaces. The chloroplast ATP synthase is confined to minimally curved regions at the grana end membranes and stroma lamellae, where it covers 20% of the surface area. In total, 85% of the ATP synthases are monomers and the remainder form random assemblies of two or more copies. Supercomplexes of PSII and light-harvesting complex II (LHCII) occasionally form ordered arrays in appressed grana thylakoids, whereas this order is lost in destacked membranes. In the ordered arrays, each membrane on either side of the stromal gap contains a two-dimensional crystal of supercomplexes, with the two lattices arranged such that PSII cores, LHCII trimers, and minor LHCs each face a complex of the same kind in the opposite membrane. Grana formation is likely to result from electrostatic interactions between these complexes across the stromal gap.
Highlights
The site of photosynthesis, by which green plants and algae convert sunlight into chemical energy and evolve oxygen is the thylakoid membrane in the chloroplast interior
Tomographic volumes of vitreous thin sections and isolated thylakoid membranes have revealed the organization of photosystem II (PSII) in thylakoid grana
We show that PSII in grana stacks is mostly, if not entirely, dimeric and that the dimeric PSII/light-harvesting complex II (LHCII) supercomplexes form regular crystalline arrays in normal, high light–treated chloroplasts
Summary
The site of photosynthesis, by which green plants and algae convert sunlight into chemical energy and evolve oxygen is the thylakoid membrane in the chloroplast interior. The thylakoid membrane segregates laterally into stacked grana disks and nonstacked stroma lamellae (Andersson and Anderson, 1980; Arvidsson and Sundby, 1999; Wehrmeyer, 1964), which contain different sets of membrane protein complexes (Dekker and Boekema, 2005). Five major membrane protein complexes (the light-harvesting complexes I and II [LHCI and LHCII], photosystems I and II [PSI and PSII], and the cytochrome b6/f complex) cooperate to generate a proton gradient across the thylakoid membrane by light-driven charge separation (for a review, see Nelson and BenShem, 2004). A low-resolution electron microscopy (EM) map of the cF1Fo complex (Mellwig and Bottcher, 2003) indicates the same basic structure as in the homologous bacterial and mitochondrial enzymes (Stock et al, 1999), consisting of an Fo rotor assembly in the membrane and the catalytic F1 part that protrudes into the chloroplast stroma
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