I. Sase et al., AXIAL ROTATION OF SLIDING ACTIN-FILAMENTS REVEALED BY SINGLE-FLUOROPHORE IMAGING, Proceedings of the National Academy of Sciences of the United Statesof America, 94(11), 1997, pp. 5646-5650
In the actomyosin motor, myosin slides along an actin filament that ha
s a helical structure with a pitch of approximate to 72 nm, Whether my
osin precisely follows this helical track is an unanswered question be
aring directly on the motor mechanism, Here, axial rotation of actin f
ilaments sliding over myosin molecules fixed on a glass surface was vi
sualized through fluorescence polarization imaging of individual tetra
methylrhodamine fluorophores sparsely bound to the filaments. The fila
ments underwent one revolution per sliding distance of approximate to
1 mu m, which is much greater than the 72 nm pitch, Thus, myosin does
not ''walk'' on the helical array of actin protomers; rather it ''runs
,'' skipping many protomers, Possible mechanisms involving sequential
interaction of myosin with successive actin protomers are ruled out at
least for the preparation described here in which the actin filaments
ran rather slowly compared with other in vitro systems, The result al
so indicates that each ''kick'' of myosin is primarily along the axis
of the actin filament, The successful, real-time observation of the ch
anges in the orientation of a single fluorophore opens the possibility
of detecting a conformational change(s) of a single protein molecule
at the moment it functions.