Crystal Structure of PsbQ from <i>Synechocystis</i> sp. PCC 6803 at 1.8 Å: Implications for Binding and Function in Cyanobacterial Photosystem II JacksonSimon A. FagerlundRobert D. WilbanksSigurd M. Eaton-RyeJulian J. 2010 In <i>Synechocystis</i> sp. PCC 6803, PsbQ is associated with photosystem II (PSII) complexes with the highest activity and stability. However, this subunit is not found in PSII X-ray crystallographic structures from <i>Thermosynechococcus elongatus</i> or <i>Thermosynechococcus vulcanus</i>. We present the crystal structure of cyanobacterial PsbQ determined in the presence and absence of Zn<sup>2+</sup>. The protein has a well-defined helical core, containing four helices arranged in an up−down−up−down fold. A conserved potential interaction site composed of a divalent metal binding site and adjacent hydrophobic pocket has been identified.