Biomedical Engineering Reference
In-Depth Information
2.3.2
Mechanical Forces Generated by Actomyosin
Contraction
Intracellular mechanical forces are generally generated from the interaction between
myosin II and actin fi laments (Fig.
2.4a
). Myosin II motors are elongated proteins
that consist of two heavy chains and two copies each of two light chains that form a
head-neck-tail structural arrangement (Alberts et al.
2008
). The globular head
domain of each of the heavy chains contains the force-generating machinery, and
enables myosin II to bind to and walk toward the plus ends of actin fi laments, using
the energy of ATP hydrolysis (Alberts et al.
2008
). Myosin II molecules move
toward the plus end of newly assembled actin fi laments, and they form bipolar
fi laments where actin fi laments intersect (Fig.
2.4b
) (Kolega
2006
). Myosin II bipolar
fi laments perform work on actin fi laments through a general three-step process of
binding, power stroke and unbinding.
In the lamellipodia of motile cells, myosin II motors slide toward the plus ends
of divergent actin fi laments while simultaneously pulling the fi laments back, resulting
in tension generation (Fig.
2.4b
) (Mitchison and Cramer
1996
; Svitkina et al.
1997
;
a
b
Myosin II moves
toward plus end
Formation of bipolar filament
Bipolar filament
formed
Myosin II
+
+
+
Network
force
Direction of
filament sliding
+
+
+
Movement
direction
c
SF region
Middle region
Posterior region
+
+
+
Contractile
force
+
Stress fiber (SF)
Network contraction increases torward the back
Fig. 2.4
Schematics of myosin II interaction with actin fi laments (Okeyo et al.
2010
). (
a
) Variation
in fi lament alignment from the leading edge to the back of the lamellipodia. Filaments become
increasingly aligned parallel to the leading edge due to actomyosin interactions. (
b
) Motion and
behavior of myosin II on actin fi laments. Single myosin II molecules move toward the plus end of
newly assembled actin fi laments (
orange fi laments
in
b
and
c
), and they form bipolar fi laments
where actin fi laments cross. (
c
) Dynamic contraction of actin network mediated by actomyosin-
generated network forces. Network contraction increases toward the back of the lamellipodia
where contractile stress fi bers rich in myosin II are located. Increased contractility and binding by
myosin II,
-actinin and other crosslinking proteins results in the formation of SFs (Adapted with
permission from JSME: [Journal of Biomechanical Science and Engineering], copyright (2012))
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