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actin filaments (Evangelista et al., 2002; Sagot et al., 2002a). The classic actin
nucleator, Arp2/3 complex (see Introduction), is not necessary for the cable
formation, even though it is indispensable for the assembly of yeast actin
structures of another type, so called actin 'patches' (Winter et al., 1997). In
vitro experiments demonstrated that truncated constitutively active formin
nucleates the assembly of actin filaments (Pruyne et al., 2002; Sagot et al.,
2002b). While the nucleating Arp2/3 complexes are attached to the 'minus'
filament ends and connect them with other actin filaments creating an
integrated branching filament array (Pollard and Beltzner, 2002), the
nucleating formins remain at the 'plus' filament ends (Pruyne et al., 2002)
and promote the assembly of long non-branching filaments (Pruyne et al.,
2002; Sagot et al., 2002b). Both FH1 and FH2 formin domains are required
for actin filament nucleation. Formin-induced actin polymerization can be
strongly enhanced by cooperation with profilin (Sagot et al., 2002b), an actin
monomer binding protein that can associate with formins via a proline-rich
FH1 domain (Watanabe et al., 1997).
In mammalian cells, expression of a truncated, constitutively active mutant
of mDia1 formin also strongly augments the level of actin polymerization
(Watanabe et al., 1999). Moreover, in the cells expressing this mutant,
polymerized actin forms numerous bundles, all orientated in a single direction,
and the cells acquire a characteristic elongated bipolar morphology
(Figure 5.4). Since FH1 and FH2 formin domains are highly conserved, it is
most likely that mDia1, in a manner similar to yeast formins, strongly
promotes actin filament nucleation. However, the complex phenotypic effect
induced by active mDia1 in mammalian cells is di cult to explain only by
enhanced actin nucleation. Enhancement of Arp2/3 complex-dependent actin
nucleation (by overexpression of the Arp2/3 complex activator, scar, or its C-
terminal VCA domain) leads not to formation, but rather to disruption of all
normal actin structures and complete loss of cell polarization (Machesky and
Insall, 1998). At the same time, expression of active mDia1 leads to
development of a highly organized, albeit exaggerated, polarized phenotype.
Thus, it appears that the active mDia1, in fact, triggers not only actin
filament nucleation, but a series of events that together produce the polarized
phenotype. Among relevant events, the most interesting one is a possible effect
on microtubule dynamics and organization. In fact, formins were shown to
affect microtubules in addition to actin. For example, in fission yeast, either
knockout or overexpression of For3 formin leads to significant alterations in
the cytoplasmic microtubule array (Nakano et al., 2002). There are several
indications that, in higher eukaryotes, formins may also be involved in
microtubule regulation. First, it appears that in cells expressing active mDia1,
microtubules are aligned along the same direction as the numerous actin
bundles (Figure 5.4), and this effect depends on the FH2 domain of mDia1
(Ishizaki et al., 2001). It was further shown that activation of mDia leads to an
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