Function of the ER located ATP carrier ER-ANT1
Final Report Abstract
ER-ANT1 was shown to act an Endoplasmic Reticulum (ER) located ATP/ADP-antiporter, specific occurring in vascular plants. Since structurally different transporters mediate energy provision to the ER, the cellular function of ER-ANT1 is still unknown. Mutants lacking ER-ANT1 (er-ant1 plants) exhibit a photorespiratory phenotype, accompanied by high glycine levels and stunted growth, pointing to an inhibition of glycine decarboxylase (GDC). To reveal whether it is possible to suppress this marked mutant phenotype we exploited the power of a forward-genetic screen. It turned out, that absence of a so far uncharacterized member of the haloacid dehalogenase (HAD)-like hydrolase strongly suppressed the dwarf phenotype of er-ant1 plants. Localization studies suggest that the corresponding protein locates to chloroplasts and activity assays showed that the enzyme dephosphorylates, with high specificity, the B6 vitamer pyridoxal phosphate. Additional physiological experiments identified imbalances in vitamin B6 homeostasis in erant1 mutants. It seems likely that the correspondingly impaired chloroplast metabolism, but not decreased GDC activity, is causative for the er-ant1 mutant dwarf phenotype. We forward a hypothesis setting transport of pyridoxal phosphate by ER-ANT1 and chloroplastic pyridoxal phosphate dephosphorylation by the HAD-type PLPP protein into the cellular context. With the identification of this HAD-type pyridoxal phosphate phosphorylase (PLPP) we provide further insight into B6 vitamer homeostasis. Overexpression of a bacterial superoxide reductase does not prevent dwarf growth of er-ant1 mutants further supporting our conclusions that decreased HAD-type PLPP activity in chloroplasts, accompanied with increased stromal PLP concentrations is causative for suppression of er-ant1 dwarf growth. We forward a novel hypothesis in which ER-ANT1 activity in the Endoplasmic Reticulum (ER) is required to connect a PLP pool in the ER lumen to PLP dependent reactions in other cellular compartments. With this analysis, we shed further light on a “transcompartmentality” of pyridoxal-phosphate metabolism in Arabidopsis.
Publications
- Identification and characterisation of suppressor mutants complementing a photorespiratory defect in the Arabidopsis mutant er-ant1
Jacqueline Altensell
