Preliminary Structural Studies of Photosystem I

Photosystem I core complex (CCI) of the thermophilic cyanobacterium Mastigocladus laminosus has been purified and characterized in two oligomers; a monomeric form and a trimeric form. The purified preparations of both the monomer and the trimer were used for crystallization and preliminary crystallographic analysis. Both preparations produced three dimensional single crystals of several crystal habits in various crystallization conditions. An examination of the crystals in comparison to the related pre-crystallization preparations demonstrated that both forms of the CCI complex are not altered by the crystallization process. Two of the monomeric crystal habits showed X-ray diffraction suitable for crystallographic analysis. The “hexagonal plate” crystals have an hexagonal unit cell with dimensions of 192×192×163 Å and diffract X-ray to about 5.5 Å resolution. The “boat” crystals have an hexagonal unit cell with dimensions of 221×221×149 Å and diffract X-ray to about 6Å resolution. Both of these hexagonal crystal forms contain six CCI monomers, in a probably similar crystal packing. The differences in cell dimensions could be a result of a different content of solvent and detergent in the crystal. Assuming that in both forms the c axis is parallel to the transmembranal molecular axis, and that there are two layers of monomers along this crystallographic axis, it is estimated that the upper limit thickness of the CCI monomeric complex is 79 Å. A preliminary model of ferredoxin-like subunit (PsaC or VII) of CCI was also constructed. The initial model was built from segments of the backbone structure of two related ferredoxins of known structure, and the amino acid sequence of subunit VII of the spinach CCI. The structure of the initial model was then refined with conventional molecular mechanics techniques. The energy minimization converged to an optimized structure which can be used as a working model for biologically relevant questions.