The Membrane Lipid Composition Regulates The Activity of Talin

Abstract

Integrins plays a central role in the dynamics of cell‐cell and cell to extra‐cellular matrix interactions and signaling. A key step in the activation of integrin receptors is the binding of the cytoplasmic domain of integrin subunits by talin. This interaction leads to separation of the integrin transmembrane domains and significant conformational changes in the extracellular domains, resulting in a dramatic increase in integrin's affinity for ligands. Talin is a 470 kDa adapter protein that consists of a N‐terminal head domain (THD) and a C‐terminal rod domain. THD is homologous to to the domain found in band 4.1/ezrin/radixin/moesin family of proteins (FERM domain) with uncanonical linear arrangement of F0–F3 domains. A second mechanism in the regulation of integrin activation is directly related to the fact that full‐length talin adopts an auto‐inhibited conformation. In the cytosol, the rod domain binds THD, preventing talin interactions with the membrane surface and the integrin cytoplasmic domain. The membrane bilayer also plays a critical role in the talin ‐integrin interaction, where anionic lipids are required for proper interaction, yet a detailed picture of the interplay between talin and the membrane has remained elusive. We describe a new fluorescence based assay for probing talin – membrane interactions with Nanodisc bilayers of controlled composition. We show that recruitment of THD to the membrane surface is governed by the absolute charge. Measurement of the donor‐acceptor distances reveals that anionic lipids promote a conformational change in the THD favoring a direct interaction of the F3 domain with the phospholipid bilayer. The magnitude of this conformational change is regulated by the identity of the phospholipid headgroup with phosphatidylinositides (e.g. PI4P and PIP2) promoting the largest conformational motions. In addition, the binding of THD to bilayers containing PIP2 is insensitive to the presence of the inhibitory segments of the talin rod domain, while the association of THD to other tested anionic lipid bilayers are strongly inhibited by purified talin rod domains. Furthermore, we have mapped the activation to the interaction of PIP2 with the F2–F3 domain of the THD. Thus, we show that PIP2 plays a central role in the converting talin to a conformation optimized for interactions with integrin cytoplasmic tails and regulation of the auto‐inhibited form of talin, which are essential for integrin inside‐out activation.Support or Funding InformationSupported by NIH grant GM101048.