VACUOLAR H+-TRANSLOCATING PYROPHOSPHATASES - A NEW CATEGORY OF ION TRANSLOCASE

The membrane surrounding the central vacuole of plant cells contains an H+-translocating ATPase (H+-ATPase) and an H+-translocating inorganic pyrophosphatase (H+-PPase). Both enzymes are abundant and ubiquitous in plants but the H+-PPase is unusual in its exclusive use of inorganic pyrophosphate (PP(i)) as an energy source. The lack of sequence identity between the vacuolar H+-PPase and any other characterized ion pump implies a different evolutionary origin for this translocase. The existence of the vacuolar H+-PPase, in conjunction with increasing recognition of PP(i) as a key metabolite in plant systems, necessitates reconsideration of ATP as the primary energy source for membrane transport in plant cells.