The NPIY motif in the integrin b1 tail dictates the requirement for talin-1 in outside-in signaling

Introduction Integrins comprise a large family of a/b-heterodimeric receptors that mediate cell adhesion to components of the extracellular matrix. Integrins form transmembrane links with the actin cytoskeleton and activate cytoplasmic signaling cascades to promote cell adhesion, migration, proliferation, survival and differentiation (Gahmberg et al., 2009; Hynes, 2002). By regulating these cellular events, integrins contribute to many normal and pathological processes including tissue morphogenesis, wound healing and tumor metastasis (Gahmberg et al., 2009; Larsen et al., 2006). The activity of integrins is conformationally regulated (Askari et al., 2009). Interactions between the transmembrane and cytoplasmic domains (tails) of the aand b-subunits stabilize the inactive conformation of integrins (Hughes et al., 1996; Kim et al., 2003; Li et al., 2003; Luo et al., 2005; Partridge et al., 2005). Current evidence indicates that the ubiquitously expressed cytoskeletal protein talin-1 promotes integrin activation by binding to the conserved membrane-proximal NPxY motif present in b-tails, resulting in changes in the interactions between the aand bsubunits (Calderwood et al., 1999; Wegener et al., 2007). Talin-1 is composed of an N-terminal globular head domain and a large Cterminal rod domain (Critchley, 2004). The binding of the talin-1 head to the b-tail is thought to be the final step in integrin activation (Calderwood et al., 1999; Tadokoro et al., 2003). Inhibition of talin1 expression or substitution of tyrosine with alanine in the membrane-proximal NPxY is sufficient to inhibit integrin activation (O’Toole et al., 1995; Tadokoro et al., 2003). Talin-1 also provides a connection between the b-tail and the actin cytoskeleton (Critchley, 2004). The rod domain links integrins to the actin cytoskeleton by binding to F-actin, or through vinculin, which binds to both talin-1 and F-actin (Critchley, 2004). Other proteins can also connect integrins with the actin cytoskeleton, including a-actinin, kindlin, tensin and filamin (FLN) (Larjava et al., 2008; Legate and Fassler, 2009; Otey and Carpen, 2004). Talin1, tensin and FLN have overlapping binding sites on the b-tail. Although all three interactions are inhibited by a tyrosine to alanine substitution in the membrane-proximal NPxY motif, only talin-1 promotes integrin activation (Calderwood et al., 2003; Pfaff et al., 1998). Interestingly, interactions of FLN with the b-tail can inhibit integrin activation (Kiema et al., 2006). Thus, the mechanisms that regulate the binding of these proteins to the b-tail have important implications in controlling integrin function. The NPIY motif in the integrin b1 tail dictates the requirement for talin-1 in outside-in signaling