Arabidopsis ANTR1 Is a Thylakoid Na+-dependent Phosphate Transporter

Abstract

In this study, the putative anion transporter 1 (ANTR1) from Arabidopsis thaliana was shown to be localized to the chloroplast thylakoid membrane by Western blotting with two different peptide-specific antibodies. ANTR1 is homologous to the type I of mammalian Na ؉ -dependent inorganic phosphate (P i ) transporters. The function of ANTR1 as a Na ؉ -dependent P i transporter was demonstrated by heterologous expression and uptake of radioactive P i into Escherichia coli cells. The expression of ANTR1 conferred increased growth rates to the transformed cells and stimulated P i uptake in a pH-and Na ؉ -dependent manner as compared with the control cells. Among various tested effectors, P i was the preferred substrate. Although it competed with the uptake of P i , glutamate was not transported by ANTR1 into E. coli. In relation to its function as a P i transporter, several physiological roles for ANTR1 in the thylakoid membrane are proposed, such as export of P i produced during nucleotide metabolism in the thylakoid lumen back to the chloroplast stroma and balance of the trans-thylakoid H ؉ electrochemical gradient storage.Solute and metabolite transporters play essential roles in physiological processes, including nutrient uptake, cell homeostasis, signal transduction, growth, and stress responses in every living organism. In the plant chloroplast, the photosynthetic organelle, most transporters have been identified and biochemically characterized from the envelope membrane (1, 2). Among them, there are several translocators for inorganic phosphate (P i ), all functioning as antiport systems using P i or phosphorylated C3 and C6 compounds as counter substrates (3, 4). Much less information is available for transport processes across the chloroplast thylakoid membrane, which is mostly studied as the site of light-driven photosynthetic reactions coupled to ATP synthesis. Only a few thylakoid transporters have been identified and functionally characterized. Examples are ATP transport across spinach thylakoid membrane into the lumenal space and a thylakoid ATP/ADP carrier identified and characterized in Arabidopsis thaliana (5,6). An active nucleotide metabolism in the thylakoid lumen (5) implies the existence of additional, yet unidentified, transporters, such as those recycling P i to the soluble stroma.A few Na ϩ -coupled P i transporters (NaP i ) 6 have in recent years been reported in green algae and vascular plants (7,8). NaP i systems are known to be mostly active in mammals, whereas H ϩ -coupled P i transport is dominant in plants (4). Three NaP i types have been described in eukaryotes, NaP i -II and III being the main ones in mammals (9 -11). NaP i -II type plays the role of an intracellular P i accumulation system, whereas NaP i -III type has the characteristics of a housekeeping system (9). The molecular mechanisms controlling the NaP i -II and III uptake systems have recently been reviewed (11). NaP i -I represents a group of proteins for which the endogenous substrate, ionic coupling, and physiological fu...

A plastidial sodium-dependent pyruvate transporter

“…When the D 600 was approximately 0.5, 0.1 mM IPTG was added to the E. coli culture which was further incubated for 4 h at 25 uC (ref. 27). An artificial pH gradient was formed across the E. coli membrane as described previously 28 .…”

AT_CHLORO, a Comprehensive Chloroplast Proteome Database with Subplastidial Localization and Curated Information on Envelope Proteins

“…2 of Ref. 94). Again, in our study, ANTR1 was only detected in the envelope (supplemental Data 10, lane 205) and not detected during extensive proteomics analyses targeted to the thylakoid membranes (38,92,93).…”

“…Another recent study (94) described the identification, expression, and functional expression in Escherichia coli of ANTR1, a thylakoid Na ϩ -dependent phosphate transporter from Arabidopsis. In this report, ANTR1 was only detected in the thylakoids and not detected in purified envelope membranes (see Fig.…”

Increased tolerance to salt stress in the phosphate-accumulating Arabidopsis mutants siz1 and pho2

“…In mammals, Na + -coupled Pi transporters (NaPi) play a role in the Pi reabsorption process and the regulation of Pi homeostasis (Segawa et al 2009). Recently, a few NaPi have been identiWed in green algae and vascular plants (Mimura et al 2002;Rubio et al 2005;Pavón et al 2008). Biochemical and physiological studies have suggested an interaction between salt stress and Pi metabolism in plants.…”