Zhu, Q.; Yaggi, M. F.; Jork, N.; Jessen, H. J.; Diver, M. M.

Inorganic phosphate (Pi) has essential metabolic and structural roles in living organisms. The Pi exporter, XPR1/SLC53A1, is critical for maintaining cellular Pi homeostasis. When intercellular Pi is high, cells synthesize inositol pyrophosphate (1,5-InsP8) - a signaling molecule that is required for XPR1 function. Inactivating mutations of XPR1 lead to brain calcifications causing neurological symptoms that include migraine, movements disorders, psychosis, and dementia. Distinct cryo-electron microscopy structures of dimeric XPR1 and functional characterization define the substrate translocation pathway and delineate how binding of InsP8 initiates the transport cycle. InsP8 binding rigidifies the intracellular SPX domains with InsP8 acting as a bridge between dimers and the SPX and transmembrane domains. When locked in this state, the C-terminal tail is sequestered revealing the entrance to the transport pathway, thus explaining the obligate roles of the SPX domain and InsP8. Together, these findings advance our understanding of XPR1 transport activity and expand opportunities for rationalizing disease mechanisms and therapeutic intervention.