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J. Cell Biol.,
Volume 145, Number 5, May 31, 1999 1009-1026
Laboratory of Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706
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. 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.
Differential depolymerization experiments suggested
that the Arp2/3 complex also provided protection of
pointed ends from depolymerization. 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. These results suggest that ADF/cofilin, per se, is not sufficient for actin brush depolymerization and a regulatory step is required. Our evidence
supports a dendritic nucleation model (Mullins, R.D.,
J.A. Heuser, and T.D. Pollard. 1998. Proc. Natl. Acad.
Sci. USA. 95:6181-6186) 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.
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