2.A.94.1.6
The Xenotropic and polytropic retrovirus receptor 1, the XPR1 or SLC53A1 protein, also called SYG1 or XR of 696 aas and 10 - 12 TMSs. XPR1 exports inorganic phosphate in metazoans (Giovannini et al. 2013) and is associated with Primary Familial Brain Calcification (PFBC, formerly known as Fahr disease or Idiopathic Basal Ganglia Calcification in idiopathic forms) (Legati et al. 2015 and Anheim et al. 2016). It binds inositol hexakisphosphate (Ins6P) and similar inositol polyphosphates, such as 5-diphospho-inositol pentakisphosphate (5-InsP7), important intracellular signaling molecules (Wild et al. 2016). It has been associated with Primary Familial Brain Calcification (PFBC) (Monfrini et al. 2023).  XPR1 senses intracellular Pi levels via its SPX domain and downregulates cellular Pi uptake via the C-terminal domain; it may not be a transporter (Burns et al. 2024).  However, Zhang et al. 2025 provided structural insights into the mechanism of phosphate recognition and transport by XPR1.