THE ASSEMBLY OF INTEGRIN ADHESION COMPLEXES REQUIRES BOTH EXTRACELLULAR-MATRIX AND INTRACELLULAR RHO RAC GTPASES/

Interaction of cells with extracellular matrix via integrin adhesion r eceptors plays an important role in a wide range of cellular functions , for example cell growth,movement, and differentiation. Upon interact ion with substrate, integrins cluster and associate with a variety of cytoplasmic proteins to form focal complexes and with the actin cytosk eleton. Although the intracellular signals induced by integrins are at present undefined, it is thought that they are mediated by proteins r ecruited to the focal complexes. It has been suggested, for example, t hat after recruitment to focal adhesions p125(FAK) can activate the ER K1/2 MAP kinase cascade. We have previously reported that members of t he rho family of small GTPases can trigger the assembly of focal compl exes when activated in cells. Using microinjection techniques, we have now examined the role of the extracellular matrix and of the two GTP- binding proteins, rac and rho, in the assembly of integrin complexes i n both mouse and human fibroblasts. We find that the interaction of in tegrins with extracellular matrix alone is not sufficient to induce in tegrin clustering and focal complex formation. Similarly, activation o f rho or rac by extracellular growth factors does not lead to focal co mplex formation in the absence of matrix. Focal complexes are only ass embled in the presence of both matrix and functionally active members of the rho family. In agreement with this, the interaction of integrin s with matrix in the absence of rho/rac activity is unable to activate the ERK1/2 kinases in Swiss 3T3 cells. In fact, ERK1/2 can be activat ed fully by growth factors in the absence of matrix and it seems unlik ely, therefore, that the adhesion dependence of fibroblast growth is m ediated through the ras/MAP kinase pathway. We conclude that extracell ular matrix is not sufficient to trigger focal complex assembly and su bsequent integrin-dependent signal transduction in the absence of func tionally active members of the rho family of GTPases.