Supramolecular organization of thylakoid membrane proteins in green plants

Abstract

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…This preparation was characterized before by electron microscopy 33 and consists of two slightly different types of particles which however contain the same number of LHCI subunits. 34 These types of complexes bind most likely 9 LHCI subunits per PSI core complex 35 and contain about 230 Chls (M. Hippler, personal communication; ref 26). The OD of the sample at the maximum of Q y absorption band (at ∼679 nm) was ∼0.7 cm -1 for the absorption measurements, ∼0.11 cm -1 for the self-absorption-free fluorescence experiments, and ∼5.5 cm -1 for polarized experiments, the high OD being applied in order to improve the signal-to-noise ratio of the collected spectra and calculate anisotropy precisely.…”

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

“…The PSI structure of pea (Ben-Shem et al 2003) shows an empty pocket at the PSI-A, -K and -L units, which was proposed to comprise the docking site for LHCII (Ben-Shem et al 2004;Dekker and Boekema 2005). Recently, electron microscopy of solubilized thylakoid membranes of Arabidopsis thaliana, prepared either in state 1 or in state 2, revealed the presence of LHCII-LHCI-PSI supercomplexes, which were more abundant in state 2 (Kouril et al 2005).…”

Biomolecular transport and separation in nanotubular networks.

“…Of the other results not reported here we have (1) demonstrated the use of motor proteins (kinesin) and cytoskeletal filaments (microtubules) to create lipid nanotubes from giant vesicles that expanded the range of membranes that allowed tubulation to bending rigidities of up to 24 x 10 -20 J (DMPC), (2) shown that we can orient domains and lipid tubules on giant vesicles using electric fields, (3) begun to build systems that capture and probe giant vesicles in a fluidic channel, (4) established cell culture platforms for imaging of cells growing in culture to examine cellular nanotube formation, and (5) designed and synthesized novel lipids and membrane compositions to yield unique membrane structure and properties to tailor domain curvature in giant vesicles. Although we have discovered much about how lipid nanotube formation can be driven by protein affinity and activity we remain curious as to how transport is driven between cells connected by nanotubular networks and how we can exploit those processes to create new nanofluidic systems based on directed assembly.…”

“…In cells, nanotubes have typically been observed as membrane extensions from regions facilitating molecular transport or energy conversion, such as the endoplasmic reticulum 1 and the thylakoid membranes in chloroplasts. 2 Recently, lipid nanotubes have even been observed serving as cytoplasmic bridges between live cells. 3 In these instances, tubule diameters of fifty to a few hundred nanometers have been reported with lengths as long as tens of microns.…”