Structural basis of phosphate export by human XPR1

Phosphorus is an essential element for all living organisms. While inorganic phosphate (Pi) cellular import is well documented, export is poorly characterized despite the recent discovery of the inositol pyrophosphate (PP-IP)-dependent Pi exporter XPR1. In this study, we determined the cryo-EM structures of XPR1. XPR1 forms a loose dimer, with each protomer containing 10 transmembrane helices (TMs) that can be divided into a peripheral domain (TM1, 3 and 4) and a core domain (TM2 and TM5-10) structurally related to ion-translocating rhodopsins. Bound Pi is observed in a tunnel formed by the α helical bundle inside the core domain at a narrowest point that separates the tunnel into an intracellular vestibule (IV) and an extracellular vestibule (EV). This narrowest point contains a cluster of Pi-coordinating basic residues, and their substitution by mutagenesis strongly impaired phosphate export. PP-IPs stimulate XPR1 activity by binding to its SPX domain. Loss of this interaction induces a conformational change of the wide open EV (conducting state) into a collapsed EV (closed state), mainly caused by structural movements of TM9 and the bulky sidechain of Trp573 right above the Pi binding site, which are essential for the Pi export activity of XPR1. Our structural and functional characterization paves the way for further in-depth studies of XPR1.