The actinome of Dictyostelium amoebae

The highly conserved protein actin is the building block in the
cytoskeleton of eukaryotic cells and provides a structural
framework known as the microfilament system.The molecular principle
of actin-based amoeboid movement was so successful in evolution
that it was kept nearly identically from lower (e.g. amoebae) to
higher (e.g. neutrophils) eukaryotes.To understand this type of
cellular movement one has first to identify and to characterize the
proteins which play a major role during the dynamic rearrangement
of actin. The collection of actin isoforms, of actin-variants and
actin related proteins (Arps) in a given cell is known as the
'actinome' whose number of proteins can be quite different from one
organism to the next. Therefore, the present work describes studies
on the actinome of thesocial amoeba Dictyostelium discoideum,
compares the findings with actinomes from other organisms, and
discusses similarities and alterations that might have happened
during evolution. D. discoideum is among the oldest organisms which
exhibit actin-based amoeboid movement, the genome is completely
sequenced and the system can be easily studied by molecular and
biochemical approaches. The study was started using bioinformatics
and the computational methods provided a global view on the D.
discoideum actinome. It turned out that the D. discoideum genome
conprises a total of 33 actin and 8 Arp genes, seven actin genes
are putative pseudogenes. Interestingly, there are 17
distinguishable actin genes which code for identical proteins.
Phylogenetic analyses helped to understand the putative duplication
events during evolution. Modelling of the three-dimensional
structures showed that the typical actin-fold, the ATP-binding
pocket, and other functional domains are highly conserved.
Homologues of the members of the D. discoideum actinome across
various model organisms clearly demonstrated which amino acids in
conserved domains are of special importance. All Arp subfamilies
that are found in mammals are also present in D. discoideum. Two of
the actin related proteins, Arp5 and Arp6, were selected for
molecular and cellular studies. Using fluorescently labeled fusion
proteins first data indicated that both Arps are present also in
the nucleus, suggesting an involvement in chromatin reorganization.