Fig. 6: Putative activation model for the InsPs and KIDINS220 modulated phosphate exporter in XPR1.

A schematic representation of the proposed XPR1 activation mechanism. Under non-activating conditions, the TMD is locked in a closed state (State 1). When KIDINS220 is absent, elevated InsPs in the cytoplasm bind between the two SPX domains of XPR1, locking XPR1 in a closed conformation (State 2). Mutation of the Glu622/Phe623 motif which blocks the intracellular cavity induces XPR1 to transition to an inward-facing conformation (State 3). In the first three states, the flexible C-terminal helix (631–655) resides near the SPX domain. In the presence of KIDINS220, the C-terminal helix binds with KIDINS220 (1–432) and induces a further conformational change of the SPX domain possibly due to steric hindrance. Although the dynamics of the SPX domain are too strong to observe its specific orientation, subsequent results indicate that its conformation undergoes a 180° flip compared to State 2, with XPR1 still remaining in a closed conformation (State 4). InsPs bind at the interface between the SPX domain and the TMD, maintaining XPR1 in a closed conformation. However, this binding enhances the dynamics of the Glu622/Phe623 motif, thereby eliminating one of the barriers to activation (State 5). Upon the introduction of substrate phosphate ions, they promptly bind to XPR1, thereby inducing the conformational alteration of XPR1 into an outward-open state. This conformational change, in turn, promotes the export of these phosphate ions (State 6). Although the two halves of the XPR1 dimer in different states were depicted with the same behavior in our figure drawing, we hypothesize that their actual functional states might not be in synchronization. States 3–6 were observed in this study. State 2 is derived from PDB ID: 8X5F and State 1 is hypothesized. Created in BioRender. Zuo, P. (2025) https://BioRender.com/v23k582.