Abstract
Photosystem II (PSII) is a multiprotein complex that splits water and initiates electron transfer in photosynthesis. The central part of PSII, the PSII core, is surrounded by light-harvesting complex II proteins (LHCIIs). In higher plants, two or three LHCII trimers are seen on each side of the PSII core whereas only one is seen in the corresponding positions in Chlamydomonas reinhardtii, probably due to the absence of CP24, a minor monomeric LHCII. Here, we re-examined the supramolecular organization of the C. reinhardtii PSII-LHCII supercomplex by determining the effect of different solubilizing detergents. When we solubilized the thylakoid membranes with n-dodecyl-β-D-maltoside (β-DM) or n-dodecyl-α-D-maltoside (α-DM) and subjected them to gel filtration, we observed a clear difference in molecular mass. The α-DM-solubilized PSII-LHCII supercomplex bound twice more LHCII than the β-DM-solubilized supercomplex and retained higher oxygen-evolving activity. Single-particle image analysis from electron micrographs of the α-DM-solubilized and negatively stained supercomplex revealed that the PSII-LHCII supercomplex had a novel supramolecular organization, with three LHCII trimers attached to each side of the core.
Highlights
Light-harvesting complex II (LHCII) proteins associate with photosystem II (PSII) to form a supercomplex
Isolation of the PSII-LHCII Supercomplexes from C. reinhardtii—In the present study, we solubilized the thylakoid membranes of wild-type C. reinhardtii with several different concentrations of either - or ␣-DM
Their more recent study in A. thaliana, in which the C2S2M2 PSII-LHCII supercomplex was the one with the largest area, did not show the L-trimer associations [16]
Summary
Light-harvesting complex II (LHCII) proteins associate with photosystem II (PSII) to form a supercomplex. Single-particle image analysis from electron micrographs of the ␣-DM-solubilized and negatively stained supercomplex revealed that the PSII-LHCII supercomplex had a novel supramolecular organization, with three LHCII trimers attached to each side of the core. When Arabidopsis thaliana thylakoid membranes are solubilized with ␣-DM and fractionated by gel filtration [15] or sucrose density gradient [16], the C2S2M2 organization is the largest type seen These single-particle images of the detergent-solubilized PSIILHCII supercomplexes were recently confirmed in the membranes by means of cryoelectron tomography [17, 18]. We examined the effects of ␣- and -DM on the solubilization of the thylakoid membranes from C. reinhardtii and demonstrated that the green algal PSII could form the C2S2M2L2 supercomplex in the ␣-DM solubilized membranes This largest PSII-LHCII supercomplex retained a high oxygenevolving activity
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