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(RxxPxxP) or class II (PxxPxR) (Mayer, 2001). Recently, ligands of
S. cerevisiae and D. discoideum class I myosin SH3 domains have been
identified (see below), and shown to link these molecular motors with the
endocytic machinery, the Arp2/3 complex and the machinery responsible for
actin dynamics. In addition, the discovery of complex networks of molecules
associating through a cascade of interactions between SH3 and proline-rich
domains (PRD) (Holt and Koffer, 2001; Tong et al., 2002) sheds new light on
the functional significance of TH2 and SH3 domains of class I myosins. For
example, TH2 domains bear strong resemblance with the PRD of WASp,
SCAR and formins, all involved in regulation of actin nucleation (Mullins,
2000; Pruyne et al., 2002; Evangelista et al., 2002). It is also important to note
that, beside being able to recruit and stimulate Arp2/3 nucleating activity via a
C-terminal acidic domain, the proteins of the WASp/Scar family have a
WASp homology 2 (WH2) domain that can recruit G-actin (Mullins, 2000). In
addition, in yeast and mammalian cells Las17p/WASp interacts with verprolin
(Vrp1p)/WIP (WASp-interacting protein) (Vaduva et al., 1997; Vaduva et al.,
1999). Many of the proteins shown to interact with WASp have both PRD
and SH3 domains, emphasizing the potentially high complexity of
resulting networks (Tong et al., 2002).
The two S. cerevisiae type I myosins, Myo3p and Myo5p, are linked to the
Arp2/3 complex by multiple interactions. First, these myosins bind directly to
proline-rich motifs in both Vrp1p and Las17p by means of their SH3 domain
(Evangelista et al., 2000; Lechler et al., 2000; Geli et al., 2000). The interaction
with Vrp1p is required for their proper localization to cortical patches that
mark sites of polarized cell growth (Anderson et al., 1998). In addition, Vrp1p
binds Las17p which recruits the Arp2/3 complex in a pathway of actin
nucleation parallel to the one dependent on class I myosins (Lechler et al.,
2001). Moreover, both myosins directly interact with the Arp2/3 complex via a
C-terminal acidic motif (Evangelista et al., 2000; Lechler et al., 2000; Lee et
al., 2000). Related acidic sequences are similarly located at the C-terminus of
other fungal class I myosins (Candida albicans, S. pombe and Aspergillus)of
Las17p and of human WASp. In S. cerevisiae, Myo3p, Myo5p and Las17p
function in a redundant manner to activate the Arp2/3 complex, as removal of
the acidic sequence from class I myosins or Las17p has little effect, but
deletion of all acidic domains virtually eliminates actin filament assembly in
cortical patches (Lee et al., 2000). In an assay based on permeabilized urea-
treated cells, Myo3p or Myo5p were shown to promote actin polymerization
only when their motor activity was intact. Nucleation activity was impaired by
dephosphorylation or mutation of the TEDS site (Lechler et al., 2000).
Contrary to WASp, yeast Las17p lacks a Cdc42-binding domain. Therefore,
these latter results suggest that the primary pathway by which this GTPase
induces actin polymerization in S. cerevisiae is through the activation of
myosin I by Cdc42-dependent kinases of the PAK family.
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