1, ezrin, radixin and moesin) with three subdomains (F1, F2, F3), which binds integrin cytoplasmic tails (Fig. 1) and a large C-terminal rod domain that binds actin.66,67 The F3 subdomain contains a phosphotyrosine-binding

(PTB) domain that binds the integrin β subunit tail at the membrane-proximal NXXY site.67 Talin is enriched at the leading edge of chemokine-stimulated lymphocytes and in the immunological synapse together with LFA-1, vinculin this website and F-actin.68 Hence, talin acts as a bridge to link the extracellular matrix and the actin skeletal network. Kindlin is another essential player that binds differently to the integrin β subunit tail at the membrane-distal NXXY site and activates integrin (Fig. 1). Kindlin is named after the Kindler syndrome which is a kind of skin blistering disease caused by a kindlin-1 gene mutation.69 The kindlin family has three members, including kindlin-1 (Unc-112-related protein 1, URP1), kindlin-2 (Mig2) and

kindlin-3 (URP-2), which all have a conserved FERM domain composed of four subdomains. Among them, kindlin-3 is expressed exclusively in cells of haematopoietic origin. The FERM subdomain 2 in kindlin-3 is featured by a pleckstrin homology domain that is involved in membrane binding,70 and subdomain 3 in kindlin-3, which binds the Sirolimus cost distal motif of integrin β1, β2 and β3 tails.71–73 Mutations in kindlin-3 result in defective

DOK2 integrin activation in leucocytes and platelets and lead to leucocyte adhesion deficiency III.74 Kindlins are not sufficient to induce integrins to a high-affinity state, but they can promote the binding of talins to integrin tails. Talin is also not sufficient to increase integrin affinity without the aid of kindlin. Other actin-associated proteins have also been identified to interact with integrins. Paxillin is a cytoskeletal phosphotyrosine-containing protein and binds directly to the cytoplamic domain of integrin α4.75 The interaction is regulated in a protein kinase A-dependent manner. Phosphorylation of the α4 cytoplasmic domain at serine988 leads to release of paxillin from integrin.76 It mediates initial capture and rolling interactions during leucocyte migration on vascular cell adhesion molecule 1-expressing and mucosal addressin cell adhesion molecule-1-expressing vascular endothelium.77 Integrins play many essential roles in leucocytes and many key players in both ‘inside-out’ and ‘outside-in’ pathways have been well characterized since the middle 1980s. However, challenging questions remain. One major question is how different integrins coordinate with other surface receptors in different cell types to regulate cellular functions when responding to various agonists including antigens, chemokines, selectins and others.