Edited by Arieh Warshel, University of Southern California, Los Angeles, CA, and approved January 26, 2021 (received for review August 13, 2020)Ion selectivity is a fundamental property of ion channels. Potassium channels exhibit an unusual selectivity, allowing passage of larger K+ ions (ionic radius of 1.3 Å) over smaller Na+ ions (1.0 Å); the mechanism underlying the small ion rejecting selectivity remains unknown. Here, we applied Na+ as a probe for examining the pore of a prototypical potassium channel and found slight Na+ conduction across the pore. Na+ traced a tortuous trajectory in...…Edited by Arieh Warshel, University of Southern California, Los Angeles, CA, and approved January 26, 2021 (received for review August 13, 2020)Ion selectivity is a fundamental property of ion channels. Potassium channels exhibit an unusual selectivity, allowing passage of larger K+ ions (ionic radius of 1.3 Å) over smaller Na+ ions (1.0 Å); the mechanism underlying the small ion rejecting selectivity remains unknown. Here, we applied Na+ as a probe for examining the pore of a prototypical potassium channel and found slight Na+ conduction across the pore. Na+ traced a tortuous trajectory in...WW…