Structural basis of phosphate export by human XPR1

Phosphate is essential for all life forms because of its indispensable roles in energy metabolism, nucleic acid and phospholipid synthesis, cellular signaling, and the formation of bones and teeth. Its homeostasis is maintained through balanced import and export processes. In humans, XPR1 has been proposed as the sole phosphate exporter, although some controversy remains. Here, we report the closed and open structures of human XPR1, purified in the absence and presence of exogenous InsP6, respectively. The exporter forms a symmetric homodimer, with transmembrane helix (TM) 1 serving as the dimeric interface. The transmembrane domain of each protomer contains a transport module (TM5-10) and a supporting module (TM1-4). Compared to the closed XPR1, the open structure demonstrates a significant displacement of the extracellular portion of TM9 towards the periphery of the phosphate transport pathway, creating an open portal to the extracellular milieu. Additionally, a potential phosphate ion coordination site is strategically located in the middle of the transport pathway. On the intracellular side, the pathway entrance is obstructed by a cluster of residues, the C-plug, positioned on the carboxyl side of TM10. Consistently, the removal of the C-terminus significantly increases the transport activity of XPR1 in reconstituted liposomes. Our findings provide a comprehensive understanding of the export mechanism of human XPR1 and hold promise for the development of potential therapeutics for primary familial brain calcification and ovarian cancer.