than 0.1 subunit per second. Capping protein is expected to reduce
treadmilling. The combination of barbed end caps and a high concentration
of profilin allows cells to maintain a high concentration of unpolymerized
ATP-actin ready for elongation of barbed ends when they appear.
But new barbed ends rarely appear without a positive stimulus. First,
profilin inhibits the initiation of new actin filaments by spontaneous
nucleation. Secondly, the Arp2/3 complex is inactive without nucleation-
promoting factors. Thirdly, nucleation-promoting factors such as WASp
(Wiskott-Aldrich Syndrome protein) are strongly auto-inhibited and thus
inactive in the absence of positive signals. Thus the system is poised far from
equilibrium ready to grow new actin filaments in response to positive stimuli.
No signalling pathway from chemoattractants or repellents to actin is fully
defined, but it is clear that multiple receptor types participate. Some are from
familiar families: receptor tyrosine kinases (such as the EGF receptor), seven
helix receptors coupled to trimeric G-proteins (such as Dictyostelium cAMP
receptors and human leukocyte receptor for f-Met-Leu-Phe), and integrins
coupled to cytoplasmic tyrosine kinases. Other receptors with yet-to-be-
defined transduction mechanisms also direct actin assembly. Examples are
DCC, the receptor for the growth cone attractant netrin, and Robo, the
receptor for the growth cone repellent Slit (Stein and Tessier-Lavigne, 2001).
Endogenous activation mechanisms independent of external stimuli are also
likely to exist.
Each family of receptors has its own downstream transduction hardware, so
the opportunities for complexity are immense, but several of these pathways
lead to a modest number of 'nucleation-promoting factors' that activate the
Arp2/3 complex (reviewed by Higgs and Pollard, 2001). It is important to note
that alternative pathways are likely to exist: de novo formation of new ends by
formins (Sagot et al., 2002; Pruyne et al., 2002) or other proteins, or
multiplication of ends of existing filaments by uncapping (Glogauer et al.,
2000) or severing (Zebda et al., 2000).
The first nucleation-promoting factor for the Arp2/3 complex to be
identified was ActA from Listeria monocytogenes (Welch et al., 1998). This
transmembrane protein su ces for the bacterium to usurp the cytoplasmic
actin system to assemble a comet tail. The first eukaryotic proteins shown to
activate the Arp2/3 complex were the WASp/Scar family (Machesky and
Insall, 1998; Machesky et al., 1999; Rohatgi et al., 1999; Yarar et al., 1999;
Winter et al., 1999; Egile et al., 1999). WASp is the product of the gene
mutated in the X-linked immunodeficiency and bleeding disorder called
Wiskott-Aldrich syndrome. Newly recognized nucleation-promoting factors