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Probing Conformational Changes of Gramicidin Ion Channels by Single-Molecule Patch-Clamp Fluorescence Microscopy

Greg S. Harms ^*, Galya Orr ^*, Mauricio Montal , Brian D. Thrall ^*, Steve D. Colson ^* and H. Peter Lu ^*

^* Pacific Northwest National Laboratory, Fundamental Science Division, Richland, Washington 99352; and Division of Biology, Section of Neurobiology, University of California at San Diego, La Jolla, California 92093

Correspondence: Address reprint requests to H. Peter Lu, Pacific Northwest National Laboratory, Fundamental Science Division, PO Box 999, Richland, WA 99352. E-mail: peter.lu{at}pnl.gov.

Complex conformational changes influence and regulate the dynamics of ion channels. Such conformational changes are stochastic and often inhomogeneous, which makes it extremely difficult, if not impossible, to characterize them by ensemble-averaged experiments or by single-channel recordings of the electric current that report the open-closed events but do not specifically probe the associated conformational changes. Here, we report our studies on ion channel conformational changes using a new approach, patch-clamp fluorescence microscopy, which simultaneously combines single-molecule fluorescence spectroscopy and single-channel current recordings to probe the open-closed transitions and the conformational dynamics of individual ion channels. We demonstrate patch-clamp fluorescence microscopy by measuring gramicidin ion channel conformational changes in a lipid bilayer formed at a patch-clamp micropipette tip under a buffer solution. By measuring single-pair fluorescence resonance energy transfer and fluorescence self-quenching from dye-labeled gramicidin channels, we observed that the efficiency of single-pair fluorescence resonance energy transfer and self-quenching is widely distributed, which reflects a broad distribution of conformations. Our results strongly suggest a hitherto undetectable correlation between the multiple conformational states of the gramicidin channel and its closed and open states in a lipid bilayer.

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