Accessibility of MTS reagents along the central pathway. (A) A sagittal section of a surface representation showing the mutated residues along the central pathway colored in red. Gray bars are predicted membrane leaflet. Representative residues are labeled. (B–E) Accessibility of two different-size MTS reagents in the open state. Superimposed scaled currents induced by ATP without (black) and with (blue) MTSET or MTS-TPAE applications (blue bars) show no modification for K293C (B), S94C (C), T60C (D), or I66C (E).