1: Proc Natl Acad Sci U S A  1997 May 27;94(11):5646-50 

Axial rotation of sliding actin filaments revealed by single-fluorophore
imaging.

Sase I, Miyata H, Ishiwata S, Kinosita K Jr.

Department of Physics, Faculty of Science and Technology, Keio University,
Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223, Japan.

In the actomyosin motor, myosin slides along an actin filament that has a
helical structure with a pitch of approximately 72 nm. Whether myosin precisely
follows this helical track is an unanswered question bearing directly on the
motor mechanism. Here, axial rotation of actin filaments sliding over myosin
molecules fixed on a glass surface was visualized through fluorescence
polarization imaging of individual tetramethylrhodamine fluorophores sparsely
bound to the filaments. The filaments underwent one revolution per sliding
distance of approximately 1 microm, 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 also indicates that each "kick"
of myosin is primarily along the axis of the actin filament. The successful,
real-time observation of the changes 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.

PMID: 9159126 [PubMed - indexed for MEDLINE]