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Insall, 2001), despite obvious filopodia which closely resemble those in
mammalian cells, raising questions about the precise signalling pathways
required for formation of such structures. Nevertheless, the underlying
biochemical processes of actin polymerization and its regulation appear to be
largely conserved in humans and the highly motile social amoebas.
The Arp2/3 complex and its regulation in Dictyostelium
The Arp2/3 complex has been identified and purified in Dictyostelium, and its
members have been shown to be closely related to their mammalian
counterparts (Insall et al., 2001). It redistributes in response to chemotactic
stimuli, and is implicated in phagocytosis, macropinocytosis and chemotaxis.
Discovery of Scar1
Scar was first discovered in Dictyostelium, as a suppressor of the develop-
mental phenotypes caused by deletion of a cAMP receptor. The gene encodes
a WASp-related protein capable of activating Arp2/3 complex, and thus actin
nucleation (Bear et al., 1998; Machesky and Insall, 1998; Machesky et al.,
1999). G-protein coupled cAMP receptors (cARs) transduce extracellular
cAMP chemoattractant signals. In cAR 7 strains of Dictyostelium, the
multicellular development of the organism arrests before tip formation, but
this phenotype can be rescued if Scar is also removed genetically from these
cells (hence Scar, for Suppressor of cAR2). Scar null cells are considerably
smaller than wild-type cells, have reduced levels of filamentous actin, and have
abnormal cell morphology and actin distribution during chemotaxis (Bear et
al., 1998).
Since this initial work, there has been great interest in the function of Scar
over the last few years. It has been found that Scar regulates many processes,
including macropinocytosis and phagocytosis in Dictyostelium (Seastone et
al., 2001).
Other Arp2/3 regulators
The WASp family proteins are present in Dictyostelium in the form of a single
Scar protein, and a single WASp protein. However, these proteins are not the
only factors responsible for promotion of Arp2/3-dependent actin polymer-
ization. The Dictyostelium CARMIL (Capping protein, Arp2/3 and Myosin I
Linker) protein contains an acidic domain, capable of activating Arp2/3 (Jung
et al., 2001). In addition to binding and activating Arp2/3, this protein also
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