The H(+)-pumping inorganic pyrophosphatase of the vacuolar membrane of higher plants.

The vacuolar H(+)-translocating pyrophosphatase (H(+)-PPase) of plants is a member of a new class of energized ion translocases. The development of our understanding of this enzyme is briefly reviewed, including the evidence for its physiological role in H(+)-pumping and K+ transport into the vacuole, the identity of the polypeptides components, the cloning and sequencing of a cDNA encoding the catalytic subunit, and the partitioning of function between cytosolic and membrane domains of the protein. Lack of information about the identity of the substrate, activators and inhibitors of the H(+)-PPase has been a major barrier to the latter work. The various analyses that have been done of the kinetics of the enzyme predict different sets of activators and inhibitors, but work with residue-specific covalent inhibitors is now being done to resolve this. The results suggest that Mg2PPi is the substrate and that Mg2+ is an activator, but whether other PPi complexes inhibit the enzyme is still to be established.