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© The Rockefeller University Press, 0021-9525/1997//913 $5.00
The Journal of Cell Biology, Volume 138, Number 4, , 1997 913-926

Article

Role of Actin Polymerization and Adhesion to Extracellular Matrix in Rac- and Rho-induced Cytoskeletal Reorganization



Laura M. Machesky* and Alan Hall{ddagger}

* Department of Molecular Medicine, {ddagger} Department of Biochemistry and Molecular Biology, Medical Research Council Laboratory for Molecular Cell Biology, University College London, London WC1E 6BT, United Kingdom

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.

Abbreviations used in this paper: F-actin, filamentous actin; SFM, serum-free medium; sphingosine-1-p, sphingosine-1-phosphate.

Please address all correspondence to Laura M. Machesky, Department of Molecular Medicine, Medical Research Council Laboratory for Molecular Cell Biology, University College London, Gower Street, London WC1E 6BT, United Kingdom. Tel.: (44) 171-380-7911. Fax: (44) 171-380-7805. e-mail: dmcblam{at}ucl.ac.uk


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