Cell movement requires cycles of polymerization and depolymerization of
actin. These are regulated by Rho family GTPases, which relay signals (both
endogenous or exogenous) to the actin polymerizing machinery of the cell, in
particular the Arp2/3 complex (Machesky and Gould, 1999). Activation of
Arp2/3 induces polymerization of new actin filaments adjacent to the plasma
membrane, usually at the leading edge. These new actin filaments push the
membrane forwards and determine the direction of cell locomotion.
Our understanding of cell motility has developed rapidly within the last
decade. The discovery and characterization of the actin nucleating machinery
of the cell, including the Arp2/3 complex and its regulators, has been a
particularly important advance.
The Arp2/3 complex and WASp family proteins
In living cells, the Arp2/3 complex is the major factor responsible for de novo
actin nucleation. From its initial purification and characterization (Machesky
et al., 1994; Mullins et al., 1997), to its connection first with Listeria motility
(Welch et al., 1997) and more recently Rho-family GTPases (Machesky and
Insall, 1998), the Arp2/3 complex has been found to play a pivotal role in actin
assembly. The complex consists of seven proteins: two actin related proteins,
Arp2 and Arp3, a WD repeat protein, p40, and four novel proteins termed
p34, p20, p21 and p16 (Kelleher et al., 1995). The crystal structure of this
complex has now been resolved (Robinson et al., 2001), providing further
insights into the molecular relationships between these subunits.
The rate-determining step for actin polymerization is nucleation, the initial
formation of a small filament. Once a short filament is formed, it extends at a
rapid and largely unregulated rate (Pollard et al., 2000). The Arp2/3 complex
acts as a catalyst for nucleation, and thereby controls the overall rate of actin
polymerization. The two actin related proteins in the complex are thought to
form a dimer similar to an actin dimer, which can act as a stable nucleus for
actin polymerization (Kelleher et al., 1995; Machesky et al., 1994; Mullins et
al., 1997). Arp2/3 alone does not stimulate actin filament production. Rather,
it is activated by a range of accessory proteins when cells are stimulated.
Members of the WASp (Wiskott-Aldrich Syndrome protein) family of
proteins are the best understood Arp2/3 activators. These interact with the
p21 subunit of the complex (Machesky and Insall, 1998), inducing a
conformational change which enables the complex to nucleate actin (Zalevsky
et al., 2001a). WASp family proteins in general are thought to help coordinate
actin reorganization by coupling Rho family GTPases and other upstream
signalling molecules to the activation and mobilization of the Arp2/3 complex
(Machesky and Gould, 1999).