Structure-Function Studies of the Photosystem II Extrinsic Subunits PsbQ and PsbP from the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

The oxygen-evolving centre of Photosystem II (PS II) is located on the lumenal side of the PS II complex and is surrounded by a group of polypeptides known as the extrinsic proteins. In PS II of the cyanobacterium Synechocystis sp. PCC 6803 six extrinsic proteins have been identified: PsbO, PsbP, PsbQ, PsbU, PsbV and Psb27. We have obtained two X-ray crystallographic structures of PsbQ, from crystals grown in either the presence or absence of Zn²⁺ ions. The structures were solved by multiple wavelength anomalous dispersion phasing using data obtained from a selenomethionine derivative and have essentially identical structures. The protein was found to consist of a four-helix bundle with an up-down-up-down fold. His76 (present in a unique HisGlyPro motif which forms a kink in helix 2 of cyanobacterial PsbQ) together with Asp116 (helix 3), coordinates Zn adjacent to a hydrophobic cavity on the H2/H3 face. We hypothesize this metal binding site and cavity may play a role in a protein-protein interaction with another PS II subunit. Similar structure-function studies are underway for the PsbP subunit; to facilitate solving the structure of PsbP in solution we have determined the NMR backbone chemical shift values of isotopically labelled recombinant PsbP.