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Figure 1.1 Dendritic nucleation, array-treadmilling model for the leading edge of motile
cells. (1) Chemoattractants activate plasma membrane receptors. (2) Signalling from the
receptors generates activated Rho-family GTPases and PIP 2 , which (3) activate WASp/Scar
proteins on the inner surface of the plasma membrane. (4) WASp/Scar assembles a complex
consisting of the Arp2/3 complex and an actin monomer on the side of an actin filament,
initiating a branch. (5) Actin-profilin elongates new barbed ends, which (6) push the
membrane forward until they are (7) capped. (8) Hydrolysis of ATP bound to actin
subunits and dissociation of the g-phosphate makes the filaments targets for (9) ADP/
cofilin, which promotes severing and depolymerization of the filaments. (10) Profilin
promotes exchange of ADP for ATP, (11) refilling the cytoplasmic pool of subunits. (12)
Parallel signalling through PAK and LIM-kinase tends to stabilize filaments through
phosphorylation, which inhibits ADF/cofilin. Not shown here are the events which lead to
remodelling of the branched network into long, unbranched filaments. (Modified from
Pollard et al., 2000). Reprinted with permission from the Annual Review of Biophysics and
Biomolecular Structure, Volume 29 u 2000 by Annual Reviews
complex, a barbed end capping protein and ADF/cofilin (Table 1.1). Profilin
increases the rate of movement. An actin filament crosslinking protein,
a-actinin, also promotes movement. These are all ancient proteins with genes
that arose before the earliest branches of the eukaryotic lineage. Their active
sites are conserved to such a remarkable degree that components from any
eukaryote interact with their partners from any other eukaryote. Thus the
entire system is ancient and highly conserved. It is noteworthy that genetic
experiments have established that the assembly and motility of actin filament
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