Subunit 1B

Figure 3. Bovine ARPC1B/p41-subunit singled out (green), with PIP3-binding domain (Trp-86) indicated as well as short helix interacting with Arp3 subunit (297-305).

Figure 3. Bovine ARPC1B/p41-subunit singled out (green), with PIP3-binding domain (Trp-86) indicated as well as short helix interacting with Arp3 subunit (297-305).

Of the non-Arp subunits, only the p41-subunit (ARPC1) has a sequence motif that is recognized: a WD40-domain, consisting of Trp-Asp repeats. Many proteins are folded in this 4-straded b-sheet structure, most notably the Gb-subunit of heterotrimeric G-proteins, however, sequence similarity or a similar phylogenetic profile are not necessarily shown. A set of interactions, mainly consisting of hydrophobic contacts and hydrogen bonds, causes folding in this distinct structure. Hydrophobic residues are packed within the cores of the sheets; aspartate residues seem to play the most important role in the folding of the WD40-protein family into their characteristic propeller-shape [4].

Within the ARPC1B-subunit, an insert of 35 residues between blades 6 and 7 is found, lacking in e.g. the G-b subunit, which presumably allows interaction with Arp3 within the complex, mainly through residues 297-305 that form a short helix; the connections of this short helical structure to the rest of the subunit could not be included in the model as they were highly disordered [1]. Additionally, a PIP3-binding site is located on the position 86 Tryptophan residue. Interestingly, the ARPC1B subunit has additionally been implicated in activation of Aurora A, involved in regulation of mitosis [6].

 

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