Progressive walking demonstrated by FRET assay. FAM (donor fluorophore, green star) labelled walker moves on the anchorage site labelled by FRET acceptor fluorophore TAMRA (red) and Cy5 (blue). Fluorescence intensity (662nm for Cy5, 575nm for TAMRA, λex= 488nm) was monitored during a) S4→S1 and b) S1→S4 tracks. The light source was obtained using the fluorometer, with light power of 66μW/cm2 at 350nm (spectral bandwidth=10nm, interval time=0.1s).