Structural rearrangements in single ion channels detected optically in living cells

Total internal reflection fluorescence microscopy was used to detect single fluorescently labeled voltage-gated Shaker K⁺ channels in the plasma membrane of living cells. Tetramethylrhodamine (TMR) attached to specific amino acid positions in the voltage-sensing S4 segment changed fluorescence intensity in response to the voltage-driven protein motions of the channel. The voltage dependence of the fluorescence of single TMRs was similar to that seen in macroscopic epi-illumination microscopy, but the exclusion of nonchannel fluorescence revealed that the dimming of TMR upon voltage sensor rearrangement was much larger than previously thought, and is due to an extreme, ≈20-fold suppression of the elementary fluorescence. The total internal reflection voltage-clamp method reveals protein motions that do not directly open or close the ion channel and which have therefore not been detected before at the single-channel level. The method should be applicable to a wide assortment of membrane-associated proteins and should make it possible to relate the structural rearrangements of single proteins to simultaneously measured physiological cell-signaling events.