Arp2/3 Complex and ADF/Cofilin in Dendritic Organization and Treadmilling of Actin Filament Array in Lamellipodia. T. M. Svitkina and G.G. Borisy J. Cell Biol., 145(5): 1009-1026, 1999

The leading edge (~1 µm) of lamellipodia in Xenopus keratocytes and fibroblasts was shown to have an extensively branched organization of actin filaments which we term the dendritic brush. Pointed ends of individual filaments were located at Y-junctions at which the Arp2/3 complex was also localized, suggesting a role of the Arp2/3 complex in branch formation. Differential depolymerization experiments suggested that the Arp2/3 complex also provided protection of pointed ends from depolymerization. ADF/cofilin was excluded from the distal 0.4 µm of the lamellipodial network of keratocytes although, in fibroblasts, it was located within the depolymerization-resistant zone. These results suggest that ADF/cofilin per se is not sufficient for actin brush depolymerization, but that a regulatory step is required. Our evidence supports a dendritic nucleation model (Mullins et al., PNAS 95:6181-6186, 1998) for lamellipodial protrusion which involves treadmilling of a branched actin array instead of treadmilling of individual filaments. In this model, Arp2/3 complex and ADF/cofilin have antagonistic activities. 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.

	Cover Figure (215 K)
	Figure 1 (176 K) - Multiple branching of actin filaments in lamellipodia
	Figure 2 (202 K) - Improved visualization of actin filament branching in lamellipodia
	Figure 3 (344 K) - Localization of Arp2/3 complex in lamellipodia
	Figure 4 (218 K) - Localization of Arp2/3 complex at actin filament branching points
	Figure 5 (205 K) - Localization of cross-linking proteins in fibroblast cytoskeleton
	Figure 6 (209 K) - Structural differentiation of actin network in lamellipodium
	Figure 7 (105 K) - Differential response of lamellipodial actin network to latrunculin A
	Figure 8 (253 K) - Localization of XAC in Xenopus keratocytes
	Figure 9 (121 K) - Localization of XAC to posterior regions of depolymerization-resistant actin brush
	Figure 10 (233 K) - Localization of XAC in Xenopus fibroblasts
	Figure 11 (62 K) - Two treadmilling models for actin turnover in lamellipodia

	Sequence 1 (673 K) - Two types of crawling motion
	Sequence 2 (1.7 M) - Actin in lamellipodium
	Sequence 3 (626 K) - Latrunculin-treated keratocytes
	Sequence 4 (711 K) - Actin in latrunculin-treated keratocytes
	Sequence 5 (1.2 M) - ADF/cofilin in Xenopus keratocytes
	Sequence 6 (1 M) - ADF/cofilin in Xenopus fibroblasts