Integrin-mediated Activation of Focal Adhesion Kinase Is Independent of Focal Adhesion Formation or Integrin Activation: STUDIES WITH ACTIVATED AND INHIBITORY β3CYTOPLASMIC DOMAIN MUTANTS

Abstract

Integrin alphaIIbbeta3 functions as the fibrinogen receptor on platelets and mediates platelet aggregation and clot retraction. Among the events that occur during either "inside-out" or "outside-in" signaling through alphaIIbbeta3 is the phosphorylation of focal adhesion kinase (pp125(FAK)) and the association of pp125(FAK) with cytoskeletal components. To examine the role of pp125(FAK) in these integrin-mediated events, pp125(FAK) phosphorylation and association with the cytoskeleton was determined in cells expressing two mutant forms of alphaIIbbeta3: alphaIIbbeta3(D723A/E726A), a constitutively active integrin in which the putative binding site for pp125(FAK) is altered, and alphaIIbbeta3(F727A/K729E/F730A), in which the putative binding site for alpha-actinin is altered. Both mutants were expressed on the cell surface and were able to bind ligand, either spontaneously or upon activation. Whereas cells expressing alphaIIbbeta3(D723A/E726A) were able to form focal adhesions and stress fibers upon adherence to fibrinogen, cells expressing alphaIIbbeta3(F727A/K729E/F730A) adhere to fibrinogen, but had reduced focal adhesions and stress fibers. pp125(FAK) is recruited to focal adhesions in adherent cells expressing alphaIIbbeta3(D723A/E726A) and is phosphorylated in adherent cells or in cells in suspension in the presence of fibrinogen. In adherent cells expressing alphaIIbbeta3(F727A/K729E/F730A), pp125(FAK) was phosphorylated despite reduced formation of focal adhesions and stress fibers. We conclude that activation of pp125(FAK) can be dissociated from two important events in integrin signaling, the assembly of focal adhesions in adherent cells and integrin activation following ligand occupation.