Capturing Spontaneous Membrane Insertion and Membrane-Induced Conformational Changes of Talin at an Atomic Resolution

Abstract

Integrins are a diverse set of proteins that play a central role in complex biological processes, such as tumor metastasis and thrombus formation. The integrin heterodimer is often expressed in a low-affinity, inactive state, relying on specific cytoplasmic or extracellular signals for its activation. The cytoskeletal-associated protein talin constitutes one of the major activation pathways of integrin through a membrane-mediated mechanism. While the involvement of activated, membrane-bound talin in this process is well established, atomic details of membrane binding of talin and of talin-dependent integrin activation have been lacking. Using our novel, highly mobile membrane mimetic simulation system, we have successfully captured complete insertion of the talin head domain (THD) in a phosphatidylserine membrane in three independent unbiased simulations, revealing key molecular events involved in the process. The THD is initially recruited to the membrane via the documented membrane orientation patch (MOP), consisting of a large number of positively charged residues. Electrostatic potential calculations revealed THD to be highly polarized, providing a potential mechanism explaining how the protein is aligns for optimal encounter with the membrane. We also observe a large, membrane-induced interdomain conformational change (>2.5 nm), which brings the F3 subdomain into contact with the anionic membrane via residues K325, N326, and K327. This result explains how F2 and F3 subdomains can simultaneously bind the membrane, a biochemically established aspect that could not be explained by the crystal structure. Moreover, we characterize a phenylalanine-rich region as the hydrophobic membrane anchor, consisting mainly of F261 and F283, which is released through the snorkeling motion of a few critical lysine residues within the membrane. Although such an anchor has been hypothesized to exist, none had been identified prior to this study.