Many signalling molecules have been shown to regulate integrin activation, yet the molecular links that connect agonist stimulation with the final steps that lead to integrin activation are not known. Han et al. now describe a pathway from agonist stimulation to integrin activation and establish that Rap1 induces the formation of an 'integrin activation complex' that binds to and activates integrin receptors.
Activation of the platelet integrin 
IIb
3 has been widely used to model activation of the whole integrin family. Platelet agonists stimulate the activation of integrin IIb3, but they fail to activate recombinant IIb3 expressed in Chinese hamster ovary (CHO) cells or several other nucleated cells.
Given the above discrepancies, Han et al. used a synthetic approach to reconstruct the integrin activation pathway in CHO cells. Binding of the adaptor protein, talin, is a final step in integrin activation, and platelets express abundant talin compared with most nucleated cells. This prompted the authors to propose that increased levels of talin in CHO cells enable IIb3 activation in response to a platelet agonist. Analysis of IIb3-expressing CHO cells that were transfected with talin and/or protein kinase C (PKC) — which has been implicated as a downstream mediator in the activation of IIb3 by agonists — showed that increased levels of talin and PKC promoted agonist-induced talin redistribution as well as activation of IIb3 integrin.
Although talin is a known substrate for PKC, PKC-mediated talin phosphorylation did not affect integrin activation. Instead, the authors found that Rap1 GTPase mediated the activity of PKC in agonist-induced integrin activation. So, is Rap1 the missing link between PKC and talin in integrin activation? Mutation analysis showed that talin binding to the -tails of integrins is required for the activation of 1-integrins and that this interaction is required for Rap1-mediated integrin activation. Han and colleagues also showed that active Rap1 GTPase stimulates the membrane localization of talin and enhances the interaction of talin with IIb3 integrin. But, how is Rap1 coupled to talin? Expression of Rap1 effectors in talin-expressing cells revealed that the Rap1 effector RIAM mediates the formation of a Rap1-induced integrin activation complex, which contains talin and RIAM, that binds to and activates the integrin.
The authors proposed that "agonist receptors promote ... the activation and/or translocation of active GTP bound Rap1 to the plasma membrane through activation of PKC or a Rap GEF. At the plasma membrane, activated Rap1 interacts with RIAM, leading to the recruitment of talin to form the integrin activation complex, thus unmasking the integrin binding site in talin leading to integrin activation." This pathway can now be used as a template to integrate different agonists and signalling pathways that control integrin activation.