Supramolecular organization of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

We present an extended analysis of the organization of green plant photosys tem II and its associated light-harvesting antenna using electron microscop y and image analysis. The analysis is based on a large dataset of 16 600 pr ojections of negatively stained PSII-LHCII supercomplexes and megacomplexes prepared by means of three different pretreatments. In addition to our pre vious work on this system [Boekema, E.J., van Roon, H., Calkoen, F., Bassi, R. and Dekker, J.P. (1999) Biochemistry 38, 2233-2239], the following resu lts were obtained. The rotational orientation of trimeric LHCII at the S, M and L binding posi tions was determined. It was found that compared to the S trimer, the M and L trimers are rotationally shifted by about -20 degrees and -50 degrees, r espectively. The number of projections with empty CP29, CP26 and CP24 binding sites was found to be about 0, 18 and 4%, respectively. We suggest that CP26 and CP24 are not required for the binding of trimeric LHCII at any of the three bin ding positions. A new type of megacomplex was observed with a characteristic windmill-like shape. This type III megacomplex consists of two C2S2 supercomplexes connec ted at their CP26 tips. Structural variation in the region of the central dimeric photosystem II co mplex was found to occur at one specific position near the periphery of the complex. We attribute this variation to the partial absence of an extrinsi c polypeptide or one or more small intrinsic membrane proteins.