Does SLC17A9 (VNUT) function as a stand-alone nucleotide and/or phosphate transporter?

The solute carrier SLC17A9 was previously identified as a vesicular nucleotide transporter that is primarily responsible for transporting ATP and related nucleotides into intracellular vesicles, and its deletion in mice resulted in a loss of ATP from intracellular storage organelles. Using recombinant SLC17A9 and functional reconstitution in liposomes we have now attempted to characterize the molecular features of its transport activity. While the purified protein includes the entire open reading frame, forms dimers, and is apparently properly folded and correctly oriented in the liposomes, we were unable to measure any nucleotide transport activity although the electrochemical potential was intact, the liposomes were not leaky, and the related vesicular glutamate transporter (SLC17A57) reconstituted in parallel under the same conditions was active. We also tested whether SLC17A9, similar to related transporters in the SLC17 family, functions as a Na⁺-dependent phosphate transporter. However, no such activity was detectable, and expression of the transporter in HeLa cells did not result in consistent increases of phosphate uptake. We conclude that the role of SLC17A9 in vesicular nucleotide transport needs further corroboration, possibly requiring additional, hitherto unknown factors.