Functional Analysis of the Plant Vacuolar Proton-Pumping Pyrophosphatase

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Plant vacuolar membranes are unusual in containing two classes of proton pump: an ATPase (V-ATPase) and an inorganic pyrophosphatase (V-PPase). While the V-ATPase is ubiquitous among eukaryotes, the V-PPase is restricted in its distribution to plants, some photosynthetic bacteria and a few members of the Archaea. Vacuolar proton pumps are known to acidify the vacuolar lumen, thereby generating the protonmotive force (PMF) required for proton-coupled transport of solutes into the vacuole via antiporters.
In Arabidopsis, a family of three AVP genes encodes V-PPases. We have shown in avp1 mutants that PPi-driven proton currents are completely absent at the vacuoles of many cell types. [AVP2 is known to be Golgi-localised, while expression of AVP3 is limited to pollen.] The phenotype of avp1 mutants is subtle under nutritionally-replete conditions. However, mutants are hypersensitive to salinity. Surprisingly, this sensitivity relates to chloride ions rather than to sodium. In accord with the notion that a V-PPase-generated PMF might play a critical role in anion accumulation in vacuoles, avp1 mutants are also exhibit a marked phenotype if nitrogen is supplied as nitrate rather than ammonium.
Future work will address the mechanistic basis of the role of the V-PPase in anion nutrition through measurement of vacuolar pH in vivo with ion-selective electrodes.

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