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appears to interact with small myosins (myoB and myoC) and capping
protein, unearthing a potentially vital pathway which links many components
of the actin polymerization machinery of the cell.
Evolutionary implications
The evolutionary lineage of the social amoebas is now thought to have
diverged from that of mammals shortly after plants separated from fungi and
animals. Despite this ancient separation, Dictyostelium cells move in a highly
similar fashion to crawling cells from mammals and other metazoa, using the
same basic framework (actin filaments, nucleated by the Arp2/3 complex
under the control of WASp and Scar1; Devreotes and Zigmond, 1988) and
regulated by many of the same controls (Racs and heterotrimeric G-proteins
coupled to serpentine receptors). It is therefore very informative to consider
which systems are conserved in nearly all eukaryotes which crawl using actin,
and which have appeared more recently. Well-conserved proteins could be
considered to be core components, while proteins which are found in
mammals but not elsewhere would be more likely to represent refinements and
enlargements of the actin repertoire.
All of the components which were required to reconstitute Listeria motility
in vitro (Loisel et al., 1999) are present in Dictyostelium. Actin, all seven
members of the Arp2/3 complex, cofilin and capping protein are all present
and highly related to their mammalian homologues. Likewise, the three
additional proteins which enhanced the eciency of the reaction (profilin,
a -actinin and VASP; Loisel et al., 1999) are highly conserved. The core
components defined in vitro by Loisel et al. are thus consistent with our
evolutionary definition.
More interesting are the regulators of the Arp2/3 complex. Scar1 and all
four proteins defined by Eden et al. (PIR121, NAP1, HSPC300 and Abi2) are
strikingly conserved. The other protein which has been described as a Scar/
WAVE regulator, IRSp53 (Miki et al., 2000), has not been seen, despite
genome sequencing which has been estimated to have hit 95% of all genes. If,
as seems likely, it is not found once the genome is completed, it is unlikely to
be essential for the coupling between the well-conserved Scar and Arp2/3
complex. WASp is also present in Dictyostelium, although there is as yet no
sign of the putative WASp regulator WIP. However, WIP-related proteins are
extremely divergent: the analagous regulator for N-WASp, CR16, is only
barely recognizable as related (Ho et al., 2001). This is odd, considering the
strong conservation of the rest of the system and the presumed essential role
for WIP.
Of the less well-understood regulators of the Arp2/3 complex, CARMIL is
only known in amoebae, although it has clear homologues in metazoans. It
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