Supermolecular organization of photosystem II and its associated light-harvesting antenna in Arabidopsis thaliana

The organization of Arabidopsis thaliana photosystem II (PSII) and its asso ciated light-harvesting antenna (LHCII) was studied in isolated PSII-LHCII supercomplexes and native membrane-bound crystals by transmission electron microscopy and image analysis. Over 4000 single-particle projections of PSI I-LHCII supercomplexes were analyzed. In comparison to spinach supercomplex es [Boekema, E.J., van Roon, H., van Breemen, J.F.L. & Dekker, J.P (1999) E ur. J Biochem. 266, 444-452] some striking differences were revealed: a muc h larger number of supercomplexes from Arabidopsis contain copies of M-type LHCII trimers. M-type trimers can also bind in the absence of the more com mon S-type trimers. No binding of L-type trimers could be detected. Analysi s of native membrane-bound PSII crystals revealed a novel type of crystal w ith a unit cell of 25.6 x 21.4 nn (angle 77 degrees), which is larger than any of the PSII lattices observed before. The data show that the unit cell is built up from C2S2M2 supercomplexes, rather than from C2S2M supercomplex es observed in native membrane crystals from spinach [Boekema, E.J., Van Br eemen, J.F.L., Van Roon, H. & Dekker, J.P. (2000) J. Mol. Biol 301, 1123-11 33]. It is concluded from both the single particle analysis and the crystal analysis that the M-type trimers bind more strongly to PSII core complexes in Arabidopsis than in spinach.