The PHO1 protein family in phosphate export and beyond

The Arabidopsis thaliana PHOSPHATE 1 (PHO1) was first identified as an inorganic phosphate (Pi) exporter mediating Pi loading into the root xylem. PHO1 is the prototypical member of a family of eukaryotic proteins possessing both an SPX and EXS domain. Recent cryo-EM structures of both the Arabidopsis PHO1 homologue PHO1;H1 and its mammalian orthologue Xenotropic and Polytropic Retrovirus Receptor 1 (XPR1), revealed that these proteins work as Pi channels that use a double-gated mechanism controlled by inositol polyphosphate. Key amino acids involved in binding Pi and gating the channel are highly conserved in PHO1 homologues throughout eukaryote superphyla, including in unicellular organisms, indicating a possible conservation of function during evolution. Beyond their structural homology, both plant and animal PHO1 orthologues are primarily localized to intracellular vesicles, raising interesting questions about the cell biology of Pi export. Arabidopsis PHO1 also contributes to the response of guard cells and seeds to abscisic acid (ABA) through yet unknown mechanisms. In this review, we will highlight how the recently gained knowledge about PHO1 and XPR1 structure and its cell biology is shaping our views about how these proteins contribute to Pi homeostasis at the cellular and organismal levels.