Biophysical Analysis of Kindlin-3 Reveals an Elongated Conformation and Maps Integrin Binding to the Membrane-distal β-Subunit NPXY Motif

Kindlin-3, a 75-kDa protein, has been shown to be critical for hemostasis, immunity, and bone metabolism via its role in integrin activation. The Kindlin family is hallmarked by a FERM domain comprised of F1, F2, and F3 subdomains together with an N-terminal F0 domain and a pleckstrin homology domain inserted in the F2 domain. Recombinant Kindlin-3 was cloned, expressed, and purified, and its domain organization was studied by x-ray scattering and other techniques to reveal an extended conformation. This unusual elongated structure is similar to that found in the paralogue Talin head domain. Analytical ultracentrifugation experiments indicated that Kindlin-3 forms a ternary complex with the Talin and beta-integrin cytoplasmic tails. NMR showed that Kindlin-3 specifically recognizes the membrane-distal tail NPXY motif in both the beta(1A) and beta(1D) isoforms, although the interaction is stronger with beta(1A). An upstream Ser/Thr cluster in the tails also plays a critical role. Overall these data support current biological, clinical, and mutational data on Kindlin-3/beta-tail binding and provide novel insights into the overall conformation and interactions of Kindlin-3.