Abstract
Three independent three-dimensional reconstructions of the spinach photosystem II-light-harvesting complex supercomplex were derived from single particle analyses of non-stained, vitrified samples imaged by electron microscopy. Each reconstruction was found to differ significantly in the composition of the lumenal oxygen-evolving complex extrinsic proteins. From difference mapping, aided by electron microscopy of negatively stained selectively washed samples, regions of density were assigned to the PsbO and PsbP/PsbQ proteins. Interpretation of the density assigned to the PsbO protein was explored using computer-aided structural predictions. PsbO is calculated to be mainly a beta-protein (38% beta) composed of two domains within an overall elongated shape (Pazos, F., Heredia, P., Valencia, A., and De Las Rivas, J. (2001) Proteins Struct. Funct. Genet. 45, 372-381). The positioning and fitting of the proposed structural model for the PsbO protein within the three-dimensional map indicated that there is a single copy per reaction center. Moreover, the structural model derived for PsbO, together with difference mapping, indicates that this protein stretches across the surface of the reaction center with its N- and C-terminal domains located toward the CP47 and CP43 side, respectively. This structural assignment is discussed in terms of the recent x-ray-derived cyanobacterial model of PSII (Zouni, A., Witt, H.-T., Kern, J., Fromme, P., Krauss, N., Saenger, W., and Orth, P. (2001) Nature 409, 739-743).
Highlights
Three independent three-dimensional reconstructions of the spinach photosystem II-light-harvesting complex supercomplex were derived from single particle analyses of non-stained, vitrified samples imaged by electron microscopy
The structural model derived for PsbO, together with difference mapping, indicates that this protein stretches across the surface of the reaction center with its N- and C-terminal domains located toward the CP47 and CP43 side, respectively
Using cryo-electron microscopy (EM) and difference mapping, we have attempted to identify the positioning of the 33-kDa protein within the threedimensional structure of the LHCII-Photosystem II (PSII) supercomplex of spinach
Summary
The structural model derived for PsbO, together with difference mapping, indicates that this protein stretches across the surface of the reaction center with its N- and C-terminal domains located toward the CP47 and CP43 side, respectively This structural assignment is discussed in terms of the recent x-ray-derived cyanobacterial model of PSII Less is known about the structure of the PsbP and PsbQ extrinsic OEC proteins present in higher plants and algae [5] This is true for the PsbU extrinsic protein of cyanobacteria, but a high resolution three-dimensional structure of PsbV (Cyt-549) of Synechocystis has recently been obtained [16], and its assignment within the x-ray map of PSII of S. elongatus is unequivocal [15]. The results obtained are discussed in terms of structural predictions for the PsbO protein based on computational methodologies [14] and the x-ray structure of cyanobacterial PSII published recently in Ref. 16
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