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* Department of Molecular Medicine, Most animal cells use a combination of actin-myosin-based contraction and actin polymerization-
based protrusion to control their shape and motility.
The small GTPase Rho triggers the formation of contractile stress fibers and focal adhesion complexes (Ridley, A.J., and A. Hall. 1992. Cell. 70:389-399) while a
close relative, Rac, induces lamellipodial protrusions
and focal complexes in the lamellipodium (Nobes,
C.D., and A. Hall. 1995. Cell. 81:53-62; Ridley, A.J.,
H.F. Paterson, C.L. Johnston, D. Diekmann, and A. Hall. 1992. Cell. 70:401-410); the Rho family of small
GTPases may thus play an important role in regulating
cell movement. Here we explore the roles of actin
polymerization and extracellular matrix in Rho- and
Rac-stimulated cytoskeletal changes. To examine the
underlying mechanisms through which these GTPases
control F-actin assembly, fluorescently labeled monomeric actin, Cy3-actin, was introduced into serum-starved Swiss 3T3 fibroblasts. Incorporation of Cy3-
actin into lamellipodial protrusions is concomitant with F-actin assembly after activation of Rac, but Cy3-actin
is not incorporated into stress fibers formed immediately after Rho activation. We conclude that Rac induces rapid actin polymerization in ruffles near the
plasma membrane, whereas Rho induces stress fiber assembly primarily by the bundling of actin filaments.
Activation of Rho or Rac also leads to the formation of
integrin adhesion complexes. Integrin clustering is not
required for the Rho-induced assembly of actin-myosin
filament bundles, or for vinculin association with actin
bundles, but is required for stress fiber formation. Integrin-dependent focal complex assembly is not required
for the Rac-induced formation of lamellipodia or membrane ruffles. It appears, therefore, that the assembly of
large integrin complexes is not required for most of the
actin reorganization or cell morphology changes induced by Rac or Rho activation in Swiss 3T3 fibroblasts.
Department of Biochemistry and Molecular Biology, Medical Research Council
Laboratory for Molecular Cell Biology, University College London, London WC1E 6BT, United Kingdom
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