Regulation of PPi Levels Through the Vacuolar Membrane H+-Pyrophosphatase
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Inorganic pyrophosphate (PPi) is a high-energy compound, although the free energy change of its hydrolysis is approximately 60 % that of ATP. PPi is generated as a by-product of macromolecule biosyntheses in plants, especially in proliferating cells. In living cells, the accumulation of PPi causes the suppression of these metabolic processes and the formation of insoluble Ca–PPi complexes. To avoid these negative effects, the vacuolar H+-pyrophosphatase (H+-PPase) hydrolyzes PPi and pumps H+ across the vacuolar membrane to maintain their acidic state. Importantly, recent studies on fugu5, the H+-PPase loss-of-function mutants, have clearly demonstrated that their phenotype is rescued by the expression of the yeast cytosolic PPase IPP1, which hydrolyzes cytosolic PPi but has no effect on vacuolar acidification, thus strongly suggesting that the role of the H+-PPase lies in the consumption of the inhibitory PPi rather than vacuolar acidification. In this chapter we describe the chemical properties and metabolic role of PPi, in addition to the physiological functions of H+-PPase and soluble PPase revealed by using several mutant lines.
KeywordsArabidopsis H+-PPase PPi homeostasis Proton pump Metabolism Germination Gluconeogenesis Sucrose IPP1 Vacuolar pH Storage lipid mobilization Leaf development Cotyledon Cell proliferation Cell expansion Compensation fugu5 mutants
This work was supported by Grants-in-Aid from the Japan Society for the Promotion of Science (Grant 16-04179 to A.F.), Grant-in-Aid for Young Scientists (B) (21770036 and 24770039 to A.F.), Scientific Research (23248017 and 24114706, to M.M.), and the Steel Foundation for Environmental Protection Technology (to M.M.). The contribution to the fugu5-related research project of past and present members of Tsukaya laboratory (The University of Tokyo), Horiguchi laboratory (Rikkyo University), Ferjani laboratory (Tokyo Gakugei University), and Maeshima laboratory (Nagoya University) is gratefully acknowledged.
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