Abstract
In one of the final stages of cyanobacterial Photosystem II (PS II) assembly, binding of up to four extrinsic proteins to PS II stabilizes the oxygen-evolving complex (OEC). Growth of cyanobacterial mutants deficient in certain combinations of these thylakoid-lumen-associated polypeptides is sensitive to changes in environmental pH, despite the physical separation of the membrane-embedded PS II complex from the external environment. In this perspective we discuss the effect of environmental pH on OEC function and photoautotrophic growth in cyanobacteria with reference to pH-sensitive PS II mutants lacking extrinsic proteins. We consider the possibilities that, compared to pH 10.0, pH 7.5 increases susceptibility to PS II-generated reactive oxygen species (ROS) causing photoinhibition and reducing PS II assembly in some mutants, and that perturbations to channels in the lumenal regions of PS II might alter the accessibility of water to the active site as well as egress of oxygen and protons to the thylakoid lumen. Reduced levels of PS II in these mutants, and reduced OEC activity arising from the disruption of substrate/product channels, could reduce the trans-thylakoid pH gradient (ΔpH), leading to the impairment of photosynthesis. Growth of some PS II mutants at pH 7.5 can be rescued by elevating CO2 levels, suggesting that the pH-sensitive phenotype might primarily be an indirect result of back-pressure in the electron transport chain that results in heightened production of ROS by the impaired photosystem.
Highlights
Photosystem II (PS II) is a thylakoid membrane-bound protein complex that functions as a waterplastoquinone oxidoreductase in oxygenic phototrophs (Vinyard et al, 2013)
Considering that the cytosol and thylakoid lumen are well-buffered with respect to environmental pH, and the fact that PS II oxygen evolution is known to function optimally when the lumen pH is relatively low, between pH 5.0 and 6.5 (Kramer et al, 1999; Najafpour et al, 2016), it was surprising that a number of Synechocystis 6803 PS II mutants were unable to grow photoautotrophically at environmental pH 7.5, whereas growth was possible at pH 10.0 (Table 1; Figures 1D,E) (Eaton-Rye et al, 2003; Summerfield et al, 2005a,b, 2007)
A number of pH-sensitive Synechocystis 6803 PS II extrinsic protein mutants demonstrate that changes in environmental pH affect the function of the oxygen-evolving complex (OEC) and lumen-exposed PS II proteins
Summary
Photosystem II (PS II) is a thylakoid membrane-bound protein complex that functions as a waterplastoquinone oxidoreductase in oxygenic phototrophs (Vinyard et al, 2013). Environmental pH and Photosystem II chlorophyll-binding core antenna proteins CP43 and CP47, form a protective environment around the site of the Mn4CaO5 cluster or oxygen-evolving complex (OEC) that catalyzes the watersplitting reaction (Shen, 2015). The extrinsic proteins bind to the PS II monomer subsequent to assembly and photoactivation of the Mn4CaO5 cluster (Dasgupta et al, 2008; Nickelsen and Rengstl, 2013) Based on their locations in the X-ray-derived structure of PS II from Thermosynechococcus vulcanus (Umena et al, 2011; Suga et al, 2015), and extensive biochemical studies (reviewed in Bricker et al, 2012; Ifuku, 2015; Ifuku and Noguchi, 2016; Roose et al, 2016), it seems likely that PsbO and PsbV bind first: PsbO binds via interactions with loop E of CP47, loop E of CP43 and the C-terminus of both D1 and D2; PsbV binds via loop E of CP43 and the C-terminus of both D1 and D2. We offer a perspective regarding the effects of environmental pH on the function of PS II in cyanobacterial cells and propose a mechanism by which some mutations in the lumenal regions of PS II prevent photoautotrophic growth at pH 7.5
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