BioSystems 88 (2007) 273–282
Dynamic polymorphism of actin as activation
mechanism for cell motility
Jun Kozuka
a,b,∗
, Hiroaki Yokota
c
, Yoshiyuki Arai
a,b
,
Yoshiharu Ishii
a
, Toshio Yanagida
a,b,d
a
Formation of Soft Nanomachines Project, Core Research for Evolution Science and Technology,
Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
b
Department of Biophysical Engineering, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
c
Department of Molecular Physiology, The Tokyo Metropolitan Institute of Medical Science,
3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8613, Japan
d
Soft Biosystem Group, Laboratories for Nanobiology, Graduate School of Frontier Biosciences,
University of Osaka, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 3 March 2006; accepted 20 July 2006
Abstract
Actin filament dynamics are crucial in cell motility. Actin filaments, and their bundles, networks, and gels assemble and disas-
semble spontaneously according to thermodynamic rules. These dynamically changing structures of actin are harnessed for some
of its functions in cells. The actin systems respond to external signals, forces, or environments by biasing the fluctuation of actin
assembly structures. In this study, dynamic conformation of actin molecules was studied by monitoring conformational dynamics
of actin molecules at the single molecule level in real time. Actin conformation spontaneously fluctuates between multiple confor-
mational states. Regarding myosin motility, the dynamic equilibrium of actin conformation was interpreted as between states that
activates and inhibits the motility. The binding of myosin to actin filaments activates myosin motility by shifting the conformational
fluctuation of actin towards the state that activates the motility. Thus, the activation mechanism based on thermal fluctuation is
suggested at molecular level as well as at cellular level.
© 2006 Elsevier Ireland Ltd. All rights reserved.
Keywords: Actin; Single-molecule FRET; Dynamic polymorphism; Myosin motility; Allosteric regulation
1. Introduction
Cell motility including muscle contraction, cell
movement, cytokinesis, cytoplasmic streaming, and
∗
Corresponding author at: Graduate School of Frontier Biosciences,
University of Osaka, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
E-mail addresses: kozuka@phys1.med.osaka-u.ac.jp (J. Kozuka),
hiroaki
yokota@rinshoken.or.jp (H. Yokota),
arai@phys1.med.osaka-u.ac.jp (Y. Arai),
ishii@phys1.med.osaka-u.ac.jp (Y. Ishii),
yanagida@phys1.med.osaka-u.ac.jp (T. Yanagida).
vesicle transport, largely depends on actin. Actin is abun-
dant and versatile. Globular actins self-assemble into
linear filaments. Actin filaments associate and form bun-
dles, networks and gels in two- and three-dimensional
manner. The polymerization and depolymerization of
filaments and formation and deformation of their
assemblies occur dynamically, and these processes are
regulated by a variety of actin binding proteins, allow-
ing actin to efficiently function in response to external
stimuli in cells (Rafelski and Theriot, 2004). In Listeria
monocytogenes, an intracellular pathogenic bacterium,
in which actin filaments assemble into a polarized alley
0303-2647/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.biosystems.2006.07.012