The Arp2/3-complex consists of two actin-related proteins (2 and 3 respectively) and five non-Arp subunits, each playing distinct roles in the functionality of the complex. Presumably, the complex as a whole is crucial in actin nucleation and the formation of branches at a characteristic 70° angle from the ‘mother filament’, in which the two Arp-subunits seem to form the first two subunits of the new branch and the other subunits play roles in the binding to other components necessary in the process.

Analysis of the ‘flat files’ revealed the possible origin of the subunit-components of the complex and possible isoforms: multiple sequence alignments further allowed identification of the differences between the two Arp2-isoforms, indicating that, remarkably, a change in the nucleotide-binding domain occurs, presumably changing the type of molecules the different isoforms can interact with.

BLAST-searches for each of the consecutive subunits indicated that the non-Arp subunits show very little sequence homology with other known proteins, with the exception of the p41-subunit which has the characteristic propeller-like shape of a ‘WD40-protein’. Several kinase-binding domains could be identified using the ‘Scansite’-tool, as well as a couple of domains of which the origin and nature have not been determined as of yet. Additionally it was found that the Arp2-subunit carries some posttranslational modifications, most notably acetylation of lysine residues, that will neutralize this amino acids’ positive charge hence affects the range of molecules the subunit can interact with.

Multiple sequence alignments were also used in order to compare sequences of Arp-subunits across a range of species: the remarkably high sequence similarity, even between invertebrates (Drosophila melanogaster) and vertebrates (e.g. Mus musculus, Homo sapiens) indicates that the Arp2 and Arp3-proteins are of high importance hence have a highly, or even fully, conserved sequence. Additionally, Arp2 and Arp3 respectively show a degree of similarity to actin, most notably sharing nucleotide-binding domains (originating from the HSP70 superfamily), which is crucial for activation of the complex.

Overall, the Arp2/3-complex can be considered a protein complex that is crucial in a wide range of species, as it is essential for regulation of the actin cytoskeleton. Its remarkably high sequence conservation and the similarity between actin and the two Arp-subunits have allowed a lot of functional characteristics to be revealed and most of the structure has been solved. However, many details about the less-conserved subunits and functionality of the protein complex with regards to the whole process of actin branching are yet to be revealed.


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