Structure and function of the extrinsic lipoproteins of Photosystem II in Synechocystis sp. PCC 6803

Abstract

The cyanobacterium Synechocystis sp. PCC6803 contains six extrinsic Photosystem II proteins located in the thylakoid lumen. Three of these, CyanoP, CyanoQ and Psb27 (homologues of the higher plant PsbP, PsbQ and Psb27 subunits) are characterised by post translational N-terminal lipid modification, thus forming a unique group of PS II extrinsic lipoproteins. These subunits are absent from all available crystal structures of PS II and their functions have not yet been fully defined. To further the understanding of these proteins and their functions the structures of CyanoQ and CyanoP have been determined by X-ray crystallography and solution state NMR spectroscopy, respectively.

The CyanoQ protein folds as a four-helix bundle in an up-down-up-down configuration, similar to the C-terminal domain of spinach PsbQ. A potentially significant divalent metal binding site located adjacent to a conserved hydrophobic pocket has been identified, which might be important for the function of CyanoQ. The CyanoP protein folds as an αβα-sandwich domain which is consistent with the structure of higher plant PsbP. A surface cleft containing a large number of conserved residues found only in CyanoP and PsbP-like homologues has been identified and analysis suggests that one of the potential cation binding sites found in CyanoP may be functionally significant. Evidence for the evolution and divergence of the PsbP super family is presented from a structural perspective including identification of residues which distinguish the PsbP family from unrelated proteins with a similar domain fold. Overall the high structural similarity of the CyanoP and CyanoQ subunits to their higher plant homologues disputes earlier theories that PsbP and PsbQ are evolutionary replacements for the cyanobacterial PsbU and PsbV subunits. The function of the three lipoproteins has also been investigated in vivo by physiological characterisation of extrinsic lipoprotein knockout mutants. Evidence is presented to support a similar functional relationship between PS II and the extrinsic homologues in cyanobacteria and higher plants, consistent with their similar structures. Several previously unexplored approaches were taken, which required the design, modification and construction of instruments for the measurement of 77 K fluorescence and thermoluminescence. In addition, a construct to interrupt the CtpA protease gene has been made and used to generate novel double mutants in the ΔCyanoP, ΔCyanoQ and ΔPsb27 strains. Observations made suggest the CyanoQ and Psb27 subunits influence energy coupling and innate photoprotective mechanisms in PS II early assembly intermediates, whilst CyanoP appears to be involved in photoactivation or repair mechanisms.