![]() ![]() One recently characterized component, the Arp2/3 complex (for review see Machesky, 1997 Machesky and Way, 1998 Zigmond, 1998 Machesky and Gould, 1999), seemed to be a particularly good candidate for the role of Y-junction linker in the formation of an integrated actin filament array. In general, our findings on the structural organization of the lamellipodial network suggest that actin dynamics at the leading edge should be discussed in terms of the behavior of an integrated actin filament array, rather than of a collection of individual filaments. Such coupling would allow nascent filaments to push against the membrane immediately after formation and provide the structural basis for polymerization-driven protrusion. We suggested that the Y-junctions arose as a result of tightly coupled nucleation and cross-linking of actin filaments at the leading edge ( Svitkina et al., 1997). Pointed ends, where they could be detected, were involved in structural association with the sides of other filaments, resulting in Y-junctions. Barbed ends were numerous near the leading edge, but we were unable to identify free pointed ends within the lamellipodium. We recently studied the supramolecular organization of actin filaments in a model motility system, namely, lamellipodia of fish keratocytes ( Svitkina et al., 1997), and found a structural pattern differing significantly from the premise of the individual filament treadmilling model. Arp2/3 complex is responsible for integration of nascent actin filaments into the actin network at the cell front and stabilizing pointed ends from depolymerization, while ADF/cofilin promotes filament disassembly at the rear of the brush, presumably by pointed end depolymerization after dissociation of the Arp2/3 complex. In this model, Arp2/3 complex and ADF/cofilin have antagonistic activities. 95:6181–6186) for lamellipodial protrusion, which involves treadmilling of a branched actin array instead of treadmilling of individual filaments. Our evidence supports a dendritic nucleation model (Mullins, R.D., J.A. These results suggest that ADF/cofilin, per se, is not sufficient for actin brush depolymerization and a regulatory step is required. Actin depolymerizing factor (ADF)/cofilin was excluded from the distal 0.4 μm of the lamellipodial network of keratocytes and in fibroblasts it was located within the depolymerization-resistant zone. Differential depolymerization experiments suggested that the Arp2/3 complex also provided protection of pointed ends from depolymerization. Pointed ends of individual filaments were located at Y-junctions, where the Arp2/3 complex was also localized, suggesting a role of the Arp2/3 complex in branch formation. The leading edge (∼1 μm) of lamellipodia in Xenopus laevis keratocytes and fibroblasts was shown to have an extensively branched organization of actin filaments, which we term the dendritic brush. ![]()
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