Cell Locomotion
G‐actin and F‐actin
G-Actin (PDB code: 1j6z).
ADP and the divalent cation
are highlighted.
Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. Thus, actin participates in many important cellular processes including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell t ki i
i l
d
ll
t ll
signaling, and the establishment and maintenance of cell junctions and cell shape.
F‐Actin; surface representation of 13 subunit repeat based on Ken Holmes' p
actin filament model
Polymerization of G‐actin
of G‐actin into F‐actin
Actin Crosslinking Proteins Bridging pairs of actin filaments
of actin filaments
a) When crosslinked by a short protein fascin actin
short protein, fascin, actin filaments form bundles
b) When crosslinked by long crosslinking proteins, flamin, actin filaments form crosslinks at various angles.
angles
The three phases of G actin polymerization in vitro
During the initial nucleation phase, ATP-G
actin
ti monomers (pink)
( i k) slowly
l l form
f
stable
t bl
complexes of actin (purple). These nuclei
are more rapidly elongated in the second
phase by addition of subunits to both ends
of the filament. In the third phase, the ends
of actin filaments are in a steady state with
monomeric ATP-G actin.
actin After their
incorporation into a filament, subunits
slowly hydrolyze ATP and become stable
ADP-F
ADP
F actin. ATP binding clefts, black
triangles of all the subunits are oriented in
the same direction in F-actin.
Myosin
The thick myosin bands are
not single myosin proteins
but are made of multiple
p
myosin molecules. Each
myosin molecule is
composed of two parts: the
globular "head" and the
elongated "tail". They are
arranged to form the thick
bands as shown