The actinome of Dictyostelium discoideum in comparison to actins and actin-related proteins from other organisms

Actin belongs to the most abundant proteins in eukaryotic cells
which harbor usually many conventional actin isoforms as well as
actin-related proteins (Arps). To get an overview over the
sometimes confusing multitude of actins and Arps, we analyzed the
Dictyostelium discoideum actinome in detail and compared it with
the genomes from other model organisms. The D. discoideum actinome
comprises 41 actins and actin-related proteins. The genome contains
17 actin genes which most likely arose from consecutive gene
duplications, are all active, in some cases developmentally
regulated and coding for identical proteins (Act8-group). According
to published data, the actin fraction in a D. discoideum cell
consists of more than 95% of these Act8-type proteins. The other 16
actin isoforms contain a conventional actin motif profile as well
but differ in their protein sequences. Seven actin genes are
potential pseudogenes. A homology search of the human genome using
the most typical D. discoideum actin (Act8) as query sequence finds
the major actin isoforms such as cytoplasmic beta-actin as best
hit. This suggests that the Act8-group represents a nearly perfect
actin throughout evolution. Interestingly, limited data from D.
fasciculatum, a more ancient member among the social amoebae, show
different relationships between conventional actins. The Act8-type
isoform is most conserved throughout evolution. Modeling of the
putative structures suggests that the majority of the actin-related
proteins is functionally unrelated to canonical actin. The data
suggest that the other actin variants are not necessary for the
cytoskeleton itself but rather regulators of its dynamical features
or subunits in larger protein complexes.