Signaling pathways leading to integrin activation. (A) Integrin at resting state (E⁻H⁻), with homotrimeric kindlin-3 and autoinhibited talin-1 in the cytosol. (B) Integrin activation is initiated by 2 signaling pathways: the selectin signaling and chemokine signaling. Engagement of PSGL-1 by selectins triggers serial activation of kinases and recruitment of adaptors. Engagement of chemokine receptors on leukocytes by inflammatory signals leads activation of PLC and PI3K. PLC breaks PIP2 into IP3 and DAG. IP3 further activates IP3 receptors on the endothelium reticulum (ER) and Ca²⁺ release from the ER store. Rap1 GTPases such as CALDAG-GEFI are activated by Ca²⁺ and DAG, which stimulates Rap1 bound on the membrane by its C-terminal geranyl-geranyl moieties. PI3K kinases convert PIP2 to PIP3. (C) Rap1 can directly activate talin-1 or indirectly through recruitment of MRL (mig-10/RIAM/lamellipodin) proteins, forming a complex with talin-1. Kindlin-3 is recruited to the plasma membrane using its PH domain. The kindlin-3 PH domain prefers PIP3. Talin-1 and kindlin-3 recruitment to the membrane-proximal compartment and binding to integrin β2 tail are common final steps in triggering integrin activation to adopt the extended high affinity (E⁺H⁺) conformations. GPCR, G-protein coupled receptor; LPD, lamellipodin.