(2009). For densitometry

gels were analysed by Image Studio Lite (LI-COR, Inc). Results and discussion CyanoQ associates with PSII complexes isolated from T. elongatus The CyanoP and CyanoQ orthologues in T. elongatus click here are encoded by tlr2075 (Michoux et al. 2010) and tll2057, respectively. Despite detailed analysis of the subunit composition of His-tagged PSII complexes isolated from T. elongatus by mass spectrometry (Sugiura et al. 2010), neither CyanoQ nor CyanoP has been detected. To investigate whether CyanoQ or CyanoP are able to associate with PSII isolated from T. elongatus, we first performed pull-down experiments by binding solubilised membrane extracts obtained from a His-tagged CP43 strain of T. elongatus (CP43-His) to a cobalt resin and analysing bound proteins released by 100-mM imidazole. Immunoblotting experiments revealed that a significant proportion of CyanoQ co-purified with CP43-His (Fig. 1). By contrast, no detectable CyanoQ bound to the cobalt resin when a non-tagged WT sample was tested. As expected, the D1 and PsbO subunits of PSII co-purified with His-tagged CP43, as did significant amounts of Psb27, which is known to be a component of non-oxygen-evolving PSII Selumetinib cell line complexes (Nowaczyk et al. 2006; Grasse et al. 2011). In contrast only trace amounts of CyanoP co-purified with CP47-His under the experimental conditions used. Fig. 1 Association of CyanoQ

with His-tagged CP43. Detergent solubilised membrane extracts from either WT or His-tagged CP43 strains of T. elongatus (CP43-His)

were mixed with cobalt resin and the bound proteins eluted by 100-mM imidazole (100 mM) followed by SDS solubilising buffer (SDS) for analysis by a SDS-PAGE and silver staining and b immunoblotting. Pre solubilised extract added to resin; Post solubilised extract after incubation with cobalt resin; Wash last wash before elution; Ctrl control in which resin lacking Co was used A commonly used method to isolate highly active oxygen-evolving dimeric PSII complexes from T. elongatus for structural studies involves a two-step anion-exchange chromatography protocol (Kern et al. 2005). This type of preparation has been successfully used to generate high-quality PSII crystals yielding diffraction data Doxorubicin in vitro of up to 3 Å resolution (Loll et al. 2005; Murray et al. 2008a, b). The PSII preparation analysed here (which produced 400-µm-long PSII crystals) also contained detectable levels of the alpha subunit of the ATPase (Tlr0435) and, interestingly, a predicted thioredoxin peroxidase/peroxiredoxin (Tll1454), which is homologous to a peroxiredoxin (2-CysPrx) thought to interact with PSII in chloroplasts (Muthuramalingam et al. 2009) (Fig. 2). Immunoblotting of the PSII complex revealed that CyanoQ was indeed present and had been purified to about the same degree as the D1 subunit (approximate 10-fold enrichment on chlorophyll basis compared with thylakoid membranes).