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Directly observing individual protein molecules in action at high spatiotemporal resolution has long been a holy grail for biological science. This is because we long have had to infer how proteins function from the static snapshots of their structures and dynamic behavior of optical makers attached to the molecules. This limitation has recently been removed to a large extent by the materialization of high-speed atomic force microscopy (HS-AFM). HS-AFM allows us to directly visualize the structure dynamics and dynamic processes of biological molecules in physiological solutions, at subsecond to sub-100-ms temporal resolution, without disturbing their function. In fact, dynamically acting molecules such as myosin V walking on an actin filament and bacteriorhodopsin in response to light are successfully visualized. In this review, we first describe theoretical considerations for the highest possible imaging rate of this new microscope, and then highlight recent imaging studies. Finally, the current limitation and future challenges to explore are described.
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Supplemental Video 1: Principle of tapping mode AFM. The cantilever is oscillated in the z-direction at its resonant frequency. The position of the laser beam reflected back from the cantilever is monitored to measure the cantilever deflection. The oscillation amplitude reduced by tip tapping on the sample surface is maintained constant by moving the sample stage in the z-direction through feedback control. The resulting sample stage movement traces the sample surface. Therefore, the sample surface topography can be reconstructed by the use of a signal by which the z-scanner is driven. Download video file (MOV)
Supplemental Video 2: Processive movement of myosin V (M5-HMM). The dynamic process in 1 µM ATP was captured at 7 fps. Scan range, 130 × 65 nm2 with 80 × 40 pixel. Seven frames are selected and shown in Figure 4b. Foot stomp event occurred at the trailing head is marked with a red triangle. Download video file (MOV)
Supplemental Video 3: Hand-over-hand movement of myosin V (M5-HMM) including foot stomp of the leading head. The dynamic process in 1 µM ATP was captured at 7 fps. Scan range, 150 × 75 nm2 with 80 × 40 pixel. Five frames are selected and shown in Figure 4c. Foot stomp events occurred at the leading head are marked with light-blue triangles. Download video file (MOV)
Supplemental Video 4: Long tracking of myosin V (M5-HMM) walking along actin filament. This typical movie showing long processive runs in 1 µM ATP was captured at 7 fps. To chase the M5-HMM molecule, the scan area was moved. Scan range, 150 × 75 nm2 with 80 × 40 pixel; the whole imaging area, 560 × 120 nm2; number of steps observed, 14. Foot stomp events at the leading head are marked with light-blue triangles. Download video file (MOV)
Supplemental Video 5: AFM movie of the C-terminal side of α3β3 in 3 µM ATP. Scan area, 18 × 14 nm2; frame rate, 12.5 fps. Download video file (MOV)