Agonist induced increase in integrin affinity for ligands (activation) plays a pivotal role in T cell trafficking and functions. Activation requires Rap1 GTPase-mediated recruitment of talin1 to the integrins in the plasma membrane. Rap1-interacting adaptor molecule (RIAM) is a Rap1 effector that serves this function in T cells. In addition, Rap1 directly binds to talin1 to enable integrin activation in platelets. Here, we assessed the relative contributions of the Rap1-talin1 interaction and

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... AM and provide a complete accounting of the connections between Rap1 and talin1 that support integrin activation in conventional CD4+ (Tconv) and CD25HiFoxp3+CD4+ regulatory T (Treg) cells. Disruption of both Rap1 binding sites in talin1 (talin1 (R35E,R118E)) causes a partial defect in αLβ2, α4β1 and α4β7 integrin activation in both Tconv and Treg cells with resulting defects in T cell homing and functions. Over-expression of RIAM bypasses the integrin activation defect in Tconv cells expressing talin1 (R35E,R118E), indicating that RIAM can substitute for Rap1 binding to talin in integrin activation. Conversely, deletion of RIAM in talin1 (R35E,R118E) Tconv cells abrogates activation of αLβ2, α4β1 and α4β7. RIAM and lamellipodin (LPD) are mammalian members of the MRL protein family; LPD plays a more important role than RIAM in Treg cell integrin activation. Nevertheless, loss of RIAM profoundly exacerbates the defects in Treg cell function caused by the talin1 (R35E,R118E) mutation. Most importantly, deleting both MRL proteins combined with talin1 (R35E,R118E) phenocopies the complete lack of integrin activation observed in Rap1a/b null Treg cells. In sum, these data reveal the functionally significant connections between Rap1 and talin1 that enable αLβ2, α4β1 and α4β7 integrin activation in T cells.