Regulation of PPi Levels Through the Vacuolar Membrane H+-Pyrophosphatase

Abstract

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 expres- sion 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 acidifica- tion. 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.