Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alpha llb beta 3. PMA activated alpha llb beta 3 only after expression of both PKC alpha (protein kinase C alpha) and talin at levels approximating those in platelets. inhibition of Rap1 GTPase reduced alpha llb beta 3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKC alpha. overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alpha llb beta 3 and bypassed the requirement for PKC alpha and Rap1. in addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alpha llb beta 3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alpha llb beta 3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.