Figure: The phylogenetic relationships among the nuclear ARPs from yeast (Sc, maroon), Arabidopsis (At, dark green), rice (Os, light green), and human (Hs, red) are shown. The four clades of nuclear ARPs that are clearly conserved in all eukaryotes and the orphaned ARP7 clade are named after the closest yeast ARP homolog and indicated in blue font (Meagher et al., 2004; Deal et al., 2005; Meagher et al., 2005). The phylogram presented used Parsimony to create the tree’s topography based on sequence similarity and a heuristic search. Human ARP7 & 9 are not shown. Numbers on branches are boot strap values indicating the statistical confidence in this topography. |
1. There are at least six distinct classes of nuclear ARPs that have been
characterized in yeast, plants, protists, and animals
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2. Most of these distinct
ARP classes are ancient, predating the divergence of the
four eukaryotic kingdoms. Hence these ARPs are found in most
yeast, plants, protists, and animals (see adjacent figure showing phylogeny of nuclear ARPs ).
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3. ARPs 4, 5, 6, 7, 8 and 9 were shown to be nuclear proteins in yeast. We have localized Arabidopsis ARP4, 6, 7 and 8 to the nucleus with several ARP-specific monoclonal antibodies
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4. ARPs
are generally expressed at 10- to 50- fold lower levels than conventional
actins.
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5. ARPs
contain the "actin" fold and ATP binding pocket, and hence, may have some of the flexibility of conventional actins.
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6. The likely ability of ARPs and actins to undergo a conformational change may help them "lock" the numerous subunit of large chromatin-remodeling complexes into functinal machines.
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