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Papers In Press, published online ahead of print March 11, 2002
Department of Chemical Engineering, The Johns Hopkins University, Baltimore, MD 21218
Corresponding Author: wirtz{at}jhu.edu
Rapid sol-gel transitions of the actin cytoskeleton are required for many key cellular processes, including cell spreading and cell locomotion. Actin monomers assemble into semiflexible polymers that rapidly intertwine into a network, a process which in vitro takes ~ 1 min for an actin concentration of 1 mg/ml. The same actin filament network, however, takes ~ 1h to exhibit a steady state elasticity. We hypothesize that the slow gelation of F-actin is due to the slow establishment of a homogeneous meshwork. Using a novel method, time-resolved multiple particle tracking, which monitors the range of thermally-excited displacements of microspheres imbedded in the network, we show that the increase in elasticity in a polymerizing solution of actin parallels the progressive decline of the network microheterogeneity. The rates of gelation and network homogenization slightly decrease with actin concentration and in the presence of the F-actin crosslinking proteins a-actinin and fascin, while the rate of actin polymerization increases dramatically with actin concentration. Our measurements show that the slow spatial homogenization of the actin filament network, not actin polymerization or the formation of polymer overlaps, is the rate-limiting step in the establishment of an elastic actin network and suggest that a new activity of F-actin binding proteins may be required for the rapid formation of a homogeneous stiff gel.
J. Biol. Chem, 10.1074/jbc.M110868200
Submitted on November 13, 2001
Revised on February 26, 2002
Accepted on March 8, 2002
Microheterogeneity controls the rate of gelation of actin filament networks
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