Kindlin-1 and -2 Directly Bind the C-terminal Region of β Integrin Cytoplasmic Tails and Exert Integrin-specific Activation Effects*

Integrin activation, the rapid conversion of integrin adhesion receptors from low to high affinity, occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin β subunits. Talin binding to integrin β tails provides one key activation signal, but additional factors are likely to cooperate with talin to regulate integrin activation. The integrin β tail-binding proteins kindlin-2 and kindlin-3 were recently identified as integrin co-activators. Here we report an analysis of kindlin-1 and kindlin-2 interactions with β1 and β3 integrin tails and describe the effect of kindlin expression on integrin activation. We demonstrate a direct interaction of kindlin-1 and -2 with recombinant integrin β tails in pulldown binding assays. Our mutational analysis shows that the second conserved NXXY motif (Tyr⁷⁹⁵), a preceding threonine-containing region (Thr⁷⁸⁸ and Thr⁷⁸⁹) of the integrin β1A tail, and a conserved tryptophan in the F3 subdomain of the kindlin FERM domain (kindlin-1 Trp⁶¹² and kindlin-2 Trp⁶¹⁵) are required for direct kindlin-integrin interactions. Similar interactions were observed for integrin β3 tails. Using fluorescence-activated cell sorting we further show that transient expression of kindlin-1 or -2 in Chinese hamster ovary cells inhibits the activation of endogenous α5β1 or stably expressed αIIbβ3 integrins. This inhibition is not dependent on direct kindlin-integrin interactions because mutant kindlins exhibiting impaired integrin binding activity effectively inhibit integrin activation. Consistent with previous reports, we find that when co-expressed with the talin head, kindlin-1 or -2 can activate αIIbβ3. This effect is dependent on an intact integrin-binding site in kindlin. Notably however, even when co-expressed with activating levels of talin head, neither kindlin-1 or -2 can cooperate with talin to activate β1 integrins; instead they strongly inhibit talin-mediated activation. We suggest that kindlins are adaptor proteins that regulate integrin activation, that kindlin expression levels determine their effects, and that kindlins may exert integrin-specific effects.