Biology Reference
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The tail of Myo5p was shown to induce cytosol-dependent actin
polymerization around Sepharose beads. This assay appears to mimick the
formation of dynamic cortical actin patches (Geli et al., 2000; Idrissi et al.,
2002), structures that have been involved in endocytosis. Importantly, the
agreement between the biochemical requirements in vitro and the genetic
requirements in vivo appears striking and corroborate the working model of
an intricate link between actin dynamics and endocytosis. Both processes
depend on the Arp2/3 complex, Vrp1p and cofilin (Geli et al., 2000; Idrissi et
al., 2002). Interestingly, profilin seems to be dispensable, perhaps highlighting
that, as the WH2 domain (present in Las17p and also Vrp1p ) can bind G-
actin directly, it may be bypassing the need for the recruitment of monomeric
ATP-actin by profilin. These findings are similar but not identical to the
requirements for the in vitro reconstitution of the motility of Listeria from
purified components, which is commonly considered to be a model for
lamellipodium extension (Loisel et al., 1999).
In amoeba, class I myosins are also able to recruit the Arp2/3 complex, but
do so with a twist, involving an adapter protein. CARMIL and Acan125 were
isolated from extracts using recombinant GST fusions with the SH3 domains
of D. discoideum MyoB and MyoC (Jung et al., 2001), and Acanthamoeba
MyoA (Lee et al., 1999) and MyoC (Xu et al., 1995) as a
nity baits,
respectively. These homologous adapters of 116 and 125 kDa bear two C-
terminal PXXP motifs that are ligands of the SH3 domain and consist of
multiple leucine-rich repeat sequences (Xu et al., 1997). The native CARMIL
complex contains all seven components of the Arp2/3 complex and the a and b
subunits of capping protein (Jung et al., 2001), which blocks the barbed end of
actin filaments. Determining the significance of the presence of both an actin
nucleator and a polymerization terminator in the same complex will require
more investigation, but it invites further speculations about their functions in
shaping the very architecture of the cortical actin meshwork. MyoB, MyoC,
CARMIL and the Arp2/3 complex are concentrated in actin-driven cellular
protrusions, especially crown-shaped macropinocytic cups and the leading
edge of migrating cells. Most recently, preliminary data indicated that, when
fused to GST, the GPR loop of MyoK is able to bind not only F-actin, but
also the profilin-actin complex. In addition, a pull-down experiment with the
GST-GPR loop construct in cytosolic extracts detected a protein of about
55 kD. MALDI-TOF analysis identified it as the D. discoideum homologue of
the yeast and mammal Abp1p/SH3P7r2 protein (C. Kistler and T. Soldati,
unpublished results). This latter protein is conserved from yeast to human and
has been shown to recruit and activate the Arp2/3 complex (Goode et al.,
2001; Kessels et al., 2001). These data are very suggestive that the function of
MyoK in cortical management and regulation of motility and phagocytosis
are exerted through the recruitment and modulation of the actin nucleation
machinery (see model Figure 3.5).
