ARP3, an essential gene, encodes an actin-related protein (33% identical to yeast actin encoded by ACT1) that functions as part of a highly conserved actin-nucleation complex (3; for review see 4). This complex, termed the Arp2/3 complex, localizes to regions of actin-based motility, such as the actin comet tails of Listeria (5) and the leading edges of motile amoebae (Acanthoamoeba) and fibroblast cells (6, 7, 8). In S. cerevisiae, the Arp2/3 complex is required for actin-dependent processes such as endocytosis and organelle inheritance (9; for review, see 10). Arp3p localizes to actin cortical patches (2). At the restrictive temperature, a conditional arp3 mutant allele inhibits actin patch motility and eventually disrupts actin patches, giving rise to aberrant actin structures (2).

The Arp2/3 complex is a seven-protein complex containing two actin-related proteins, Arp2p and Arp3p, and five non-actin related proteins, Arc15p, Arc18p, Arc19p, Arc35p, and Arc40p (2). The Arp2/3 complex nucleates the formation of branched actin filaments by binding to the side of an existing (mother) filament and nucleating the formation of a new (daughter) actin filament at a 70 degree angle (11). Arp2p and Arp3p serve as the first two subunits of the daughter filament, likely mimicking actin monomers due to their structural similarity to actin (12). However, the Arp2/3 complex does not play a role in the formation of actin cables (unbranched actin structures), which are instead nucleated by the formins Bni1p and Bnr1p (13, 14).

To achieve optimal actin nucleation activity, the Arp2/3 complex interacts with an activator protein, such as Las17p/Bee1p (of the SCAR/WASp family), myosin I, Abp1p, or Pan1p (for reviews, see 15, 16). In vitro studies have shown that the activator protein alters the conformation of the Arp2/3 complex, as to bringing Arp2p and Arp3p together in a "filamentous" arrangement to mimic an actin dimer (12).

Last updated: 2003-11-25