Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2007
Publication Date: 12/21/2007
Publication URL: www.jbc.org/cgi/doi/10.1074/jbc.M707007200
Citation: Wu, G., Ortiz-Flores, G., Ortiz-Lopez, A., Ort, D.R. 2007. A point mutation in atpC1 raises the redox potential of the Arabidopsis chloroplast ATP synthase gamma-subunit regulatory disulfide above the range of thioredoxin modulation. Journal of Biological Chemistry. 282(51):36782-36789. Interpretive Summary: The regulation of key enzymes in photosynthesis by light is a central factor in ensuring efficient energy storage in the rapidly changing light environment of natural habitats. The chloroplast ATP synthase is one of the important enzymes regulated by light ensuring that ATP synthesis can occur efficiently when amble light is available but preventing the wasteful hydrolysis of ATP at night or when light is very low. There are still central features about the light-regulation of ATP synthesis activity that remained to be established and mutants of the light regulation process is one useful approach to further understand this process in leaves. In this paper we report on the analysis of an Arabidopsis mutant that we produced that interfered with the operation of the ATP synthase. This study is relevant to agricultural and plant scientists interested in the mechanism and purpose of the regulation of photosynthesis by light.
Technical Abstract: The light-dependent regulation of chloroplast ATP synthase activity depends on an intricate but ill-defined interplay between the proton electrochemical potential across the thylakoid membrane and thioredoxin-mediated redox modulation of a cysteine bridge located on the ATP synthase gamma-subunit. The abnormal light-dependent regulation of the chloroplast ATP synthase in the Arabidopsis thaliana cfq (coupling factor quick recovery) mutant was caused by a point mutation (G to A) in the atpC1 gene, which caused an amino acid substitution (E244K) in the vicinity of the redox modulation domain in gamma-subunit of the ATP synthase. Equilibrium redox titration revealed that this mutation made the regulatory sulfhydryl group energetically much more difficult to reduce relative to wild type (i.e., raised the Em,7.9 by 39 mV). Enzymatic studies using isolated chloroplasts showed significantly lower light-induced ATPase and ATP synthase activity in the mutant compared to wild type. The lower ATP synthesis capacity in turn restricted overall rates of leaf photosynthesis in the cfq mutant under low light. This work provides in situ validation of the concept that thioredoxin-dependent reduction of the gamma-subunit regulatory disulfide modulates the proton electrochemical potential energy requirement for activation of the chloroplast ATP synthase and that the activation state of the ATP synthase can limit leaf level photosynthesis.