Chapter 15: From Cell Growth to the 3.0 Å Resolution Crystal Structure of Cyanobacterial Photosystem II

This chapter describes the structure of PS II that was obtained on the basis of an improved protocol for the preparation, purification and crystallization of the dimeric Photosystem II core complex (PS IIcc) from Thermosynechococcus elongatus (T. elongatus). The individual components of this PS IIcc were biochemically characterized and the aggregation behavior of dimeric PS IIcc, which is crucial for the design of crystallization experiments, was studied using dynamic light scattering, analytical ultracentrifugation and gel permeation chromatography. The new protocols for PS IIcc purification and crystallization are prerequisites for a significantly improved resolution of the crystal structure to 3.0 Å. Critical aspects for the interpretation of the electron density map of PS IIcc at medium resolution (3.8–3.2 Å) are pointed out. The 3.0 Å resolution structure of Photosystem II is an important step towards detailed information on cofactor arrangement. These findings offer a basis for a deeper mechanistic understanding of electron and energy transfer. Likewise, the new assignment of eleven β-carotenes is important for considerations on photoprotection. Fourteen integrally bound lipids were resolved for the first time and their structural and functional importance for flexibility and assembly of PS IIcc are discussed. Of special interest is the discovery of a lipophilic diffusion pathway for plastoquinone Q_B. Limitations of our current structure information are outlined, especially with respect to the nature of the manganese cluster of the water oxidizing complex.