Evidence for a non-overlapping subcellular localization of the family I isoforms of soluble inorganic pyrophosphatase in Arabidopsis thaliana.

Pyrophosphate is a byproduct of macromolecular biosynthesis and its degradation gives a thermodynamic impulse to cell growth. Soluble inorganic pyrophosphatases (PPa) are present in all living cells, but in plants and other Eukaryotes membrane-bound H+-pumping pyrophosphatases may compete with these soluble counterparts for the substrate. In Arabidopsis thaliana there are six genes encoding for classic family I PPa isoforms, five cytoplasmic, and one considered to be organellar. Here, six transgenic stable A. thaliana lines, each expressing one of the PPa isoforms from this same plant species in fusion with a fluorescent protein, were obtained and analyzed under confocal and immunogold transmission electron microscopy. The results confirmed the cytoplasmic localization for isoforms 1-5, and showed an exclusive chloroplastic localization for isoform 6. In contrast to previous reports, the data presented here revealed a differential distribution pattern for the isoforms 1 and 5, in comparison to isoforms 2 and 3, and also the presence of isoform 4 in the intercellular space and cell wall, in addition to its presence in cytoplasm. To the best of our knowledge, this is the first report of a PPa family I protein localized in the intercellular space in plants.