Fig. 4: Putative ion coordination sites.
From: The identification of XPR1 as a voltage- and phosphate-activated phosphate-permeable ion channel
a The density map of TM5-10 of Pi/InsP6-hXPR1, with each TM helix colored individually at a contour level of 10.96σ. The non-protein isolated densities within the pore are colored in pink red. Densities from TM5 and TM10 (right), or TM7 and TM8 (left), are removed to expose the pore. b The string of putative ion densities in gray depicted at a 10.96σ contour level with the cartoon representation of Pi/InsP6-hXPR1 TM5-10 structure. Densities corresponding to the two putative ion coordination sites are boxed in red and blue. c, d Close-up views of the two putative ion coordination sites indicated in the colored boxes in (b), with the ion density shown at a 5.35σ contour level. e Relative Pi transport of the alanine mutations of three arginine residues within the red-colored putative Pi binding in (c). The relative transport was measured at the 20-min time point with the addition of valinomycin. Means ± SEM plotted (n = 4 independent assays). f XPR1 R570A currents evoked from a GUV patch by voltage ramps are decreased as internal Pi is increased from 10 mM (black) to 75 mM (red), using 10 Cl K-MSA internal solutions with external 0 Pi, 10 NMDG-Cl. Each curve represents the average of 5 traces (as in Fig. 2g).
