Freestanding lipid bilayer tensiometer for the study of mechanosensitive ion channels

Mechanical forces modify the cell membrane potential by opening mechanosensitive ion channels. We report the design and construction of a lipid bilayer tensiometer to study channels that respond to lateral membrane tension, y , in the range 0.2 to 1.4 kBT nm2 (0.8 to 5.7 mmN). The instrument consists of a black-lipid-membrane bilayer, a custom-built microscope, and a high-resolution manometer. Values of y are obtained from the determination of the bilayer curvature as a function of applied pressure by means of the Young-Laplace equation. We demonstrate that y can be determined by calculating the bilayer radius of curvature from fluorescence microscopy imaging or from measurements of the bilayer's electrical capacitance, both yielding similar results. Using electrical capacitance, we show that the mechanosensitive potassium channel TRAAK responds to y , not curvature. TRAAK channel open probability increases as y is increased from 0.2 to 1.4 kB T nm2 but open probability never reaches 0.5. Thus, TRAAK opens over a wide range of y , but with a tension sensitivity about one-fifth that of the bacterial mechanosensitive channel MscL.