Rapid modulation of ligand binding affinity ("activation") is a central property of the integrin cell adhesion receptors. Using a screen for suppressors of integrin activation, we identified the small GTP-binding protein, H-Ras, and its effector kinase, Raf-1, as negative regulators of integrin activation. H-Ras inhibited the activation of integrins with three distinct alpha and beta subunit cytoplasmic domains. Suppression was not associated with integrin phosphorylation and was independent of both mRNA transcription and protein synthesis. Furthermore, suppression correlated with activation of the ERK MAP kinase pathway. Thus, regulation of integrin affinity state is a novel, transcription-independent function of a Ras-linked MAP kinase pathway that may mediate a negative feedback loop in integrin function.

Fibronectin (Fn) matrices are vital to vertebrate development and wound healing and modulate tumorigenesis. We used a recombinant Fn-binding integrin alpha IIb beta3, to define rules for integrin-initiated Fn matrix formation. We report the following. First, multiple Fn-binding integrins can support matrix assembly; their activation state controls fibrillogenesis. Second, Fn binding to cells expressing an activated integrin is necessary but not sufficient for matrix assembly. Additional "postoccupancy" events involving the integrin beta, but not the alpha subunit, cytoplasmic domain are needed. Third, these postoccupancy events require an intact actin cytoskeleton. We propose a model for integrin involvement in Fn fibrillogenesis that reconciles previous paradoxes and suggests novel approaches to the therapeutic control of Fn matrix assembly.