Interplay between Rac and Rho in the control of substrate contact dynamics

Background: Substrate anchorage and cell locomotion entail the initiation a nd development of different classes of contact sites, which are associated with the different compartments of the actin cytoskeleton. The Rho-family G TPases are implicated in the signalling pathways that dictate contact initi ation, maturation and turnover, but their individual roles in these process es remain to be defined. Results: We monitored the dynamics of peripheral, Rac-induced focal complex es in living cells in response to perturbations of Rac and Rho activity and myosin contractility. We show that focal complexes formed in response to R ac differentiated into focal contacts upon upregulation of Rho, Focal compl exes were dissociated by inhibitors of myosin-II-dependent contractility bu t not by an inhibitor of Rho-kinase. The downregulation of Rac promoted the enlargement of focal contacts, whereas a block in the Rho pathway not only caused a dissolution of focal contacts but also stimulated membrane ruffli ng and formation of new focal complexes, which were associated with the adv ance of the cell front. Conclusions: Rac functions to signal the creation of new substrate contacts at the cell front, which are associated with the induction of ruffling lam ellipodia, whereas Rho serves in the maturation of existing contacts, with both contact types requiring contractility for their formation. The transit ion from a focal complex to a focal contact is associated with a switch to Rho-kinase dependence. Rac and Rho also influence the development of focal contacts and focal complexes, respectively, through mutually antagonistic p athways. (C) Elsevier Science Ltd ISSN 0960-9822.