Figure 7.

Direct Access of Permeant Ions to the Selectivity Filter in the Absence of Membrane Stretch

Example of a giant excised patch recording for mechanosensitive TREK channels expressed in Xenopus oocytes. Currents show alternating activation by membrane stretch (−15 mmHg applied to the patch pipette; red bar) or replacement of intracellular K⁺ (gray bar) with Rb⁺ (blue bar). Rb⁺ as the permeant ion produces direct activation of a voltage-dependent gate within the filter, indicating free access from the cytoplasmic side. Membrane stretch (red bar) also activates channels to a similar extent. When applied together with Rb⁺ the effects are synergistic. If lipids directly prevented K⁺ permeation (Brohawn et al., 2014a), then Rb⁺ would not be able to access the filter in the absence of membrane stretch. Instead, Rb⁺ activates the filter gate both before and after membrane stretch, indicating free access to the filter. Inhibition of channel activity by 100 μM tetrapentylammonium (TPA) is shown in green as a control. This result clearly demonstrates that direct lipid occlusion of the inner pore does not represent the primary mechanism of mechanogating.