As a consequence, the spinach structure shows a single 25 residue-long helix rather than the two helices (2a and 2b) observed in CyanoQ. In addition, PsbQ contains a much longer N-terminal sequence, which might be important for binding to PSII (Kuwabara et al. 1986). All three crystallised proteins differ in their isoelectric points as calculated by Protparam (Gasteiger et al. 2005) with pI values of 4.5 for T. elongatus CyanoQ, 5.6 for Synechocystis CyanoQ and 9.25 for spinach PsbQ. This

is reflected in their surface charge distribution (Fig. 5). Both CyanoQ proteins show see more only a small patch of positively charged surface around T. elongatus Arg109, whereas the equivalent region of the PsbQ protein contains a large patch of lysine residues thought to be involved in binding to PSII (Meades et al. 2005) (Fig. 5, top). Fig. 5 Solvent accessible surface charges of CyanoQ from T. elongatus (3ZSU), Synechocystis (3LS0) and spinach PsbQ (1VYK and 1NZE). Colour range spans from -5 (red) to 5 (blue) kT/e. Differences between the two spinach structures result from the fact that fewer residues could be fitted in 1NZE. Arrows point at Cα of selected residues. Arg109 is resolved in dual conformation Significant differences in surface charge are also observed on the opposite faces of PsbQ and CyanoQ (Fig. 5): PsbQ is relatively uncharged whereas CyanoQ is negatively charged (Fig. 5, bottom row).

Given the differences in composition of the extrinsic PSII subunits in cyanobacteria and plants, this face of the protein may be involved in interactions LY2874455 order with these subunits or with assembly factors or possibly other protein components in the thylakoid membrane. Comparison of zinc-binding sites Zinc ions have been shown to bind to plant PsbQ (Calderone Tideglusib et al. 2003; Balsera

et al. 2005) and CyanoQ from Synechocystis (Jackson et al. 2010), although the binding sites are not conserved (Fig. S7). Zinc has also been shown to bind to plant PsbP (Kopecky et al. 2012) and CyanoP from T. elongatus (Michoux et al. 2010) and Synechocystis (Jackson et al. 2012). The physiological relevance of these metal binding sites is currently GSK126 concentration unknown. In Synechocystis CyanoQ two zinc ions are coordinated by six amino-acid residues (Fig. 3 and Fig. S7). Despite the fact that five out of the six corresponding positions are occupied by potential metal ligands in T. elongatus CyanoQ, no zinc cations are present in the crystal structure. Unlike Synechocystis CyanoQ, where it was possible to obtain both zinc-bound and metal-free structures, our attempts to crystallise T. elongatus CyanoQ with zinc failed. Although there were no bound Zn2+ ions in our structure, we were able to fit a sulphate ion into the electron density. This anion is coordinated by three consecutive residues, Ser126ValThr128, found at the beginning of helix 4, at the apex of the protein.