Tonoplast energization: Two H⁺ pumps, one membrane

Abstract

The vacuolar membrane (tonoplast) of plant cells contains two functionally and physically distinct phosphohydrolases, which catalyse electrogenic H + ‐translocation: An ATPase (tp‐ATPase; EC 3.6.1.3) and an inorganic pyrophosphatase (tp‐PPase; 3.6.1.1). Neither enzyme belongs to the F 0 F 1 – or E 1 E 2 ‐categories of primary cation pumps, but instead belong to a third and fourth category of enzyme, respectively. Research priorities for the tp‐ATPase are studies directed at understanding the roles of the 70 and 60 kDa subunits in catalysis and regulation; the involvement of the 16 kDa subunit in transmembrane H + conduction; and investigations of F 0 F 1 ‐ like structure/function partitioning. In the longer term, comparisons of sequence homology between the N,N′‐ dicyclohexylcarbodiimide ‐binding (16 kDa) proteins from different sources may enable elucidation of the evolutionary relationship of the tp‐ATPase with other putative third‐category H + – translocases. The tp‐PPase, on the other hand, represents an exciting but largely unexplored biochemical entity, which necessitates a reconsideration of accepted views concerning the involvement of inorganic pyrophosphate (PPi) in transmembrane energy conservation. Just why the tonoplast should be endowed with two H + ‐translocases is a problem that can only be approached once consideration is given to the paramount question of H + /PPi stoichiometry. Once the stoichiometry is known, it should be possible to establish the physiological poise of the tp‐PPase, and hence to speculate on its role in the metabolism of plant cells.