How Force Might Activate Talin's Vinculin Binding Sites: SMD Reveals a Structural Mechanism

Vesa P Hytönen,Viola Vogel,Manfred Schliwa

doi:10.1371/journal.pcbi.0040024

Abstract

Upon cell adhesion, talin physically couples the cytoskeleton via integrins to the extracellular matrix, and subsequent vinculin recruitment is enhanced by locally applied tensile force. Since the vinculin binding (VB) sites are buried in the talin rod under equilibrium conditions, the structural mechanism of how vinculin binding to talin is force-activated remains unknown. Taken together with experimental data, a biphasic vinculin binding model, as derived from steered molecular dynamics, provides high resolution structural insights how tensile mechanical force applied to the talin rod fragment (residues 486–889 constituting helices H1–H12) might activate the VB sites. Fragmentation of the rod into three helix subbundles is prerequisite to the sequential exposure of VB helices to water. Finally, unfolding of a VB helix into a completely stretched polypeptide might inhibit further binding of vinculin. The first events in fracturing the H1–H12 rods of talin1 and talin2 in subbundles are similar. The proposed force-activated α-helix swapping mechanism by which vinculin binding sites in talin rods are exposed works distinctly different from that of other force-activated bonds, including catch bonds.

Highlights

IntroductionThe talin head (TH), residues 1–432, has binding sites for integrin b-tails [3], PIP kinase c [4], focal adhesion kinase (FAK) [5], layilin [5] and actin [6] (Figure 1B)
Talin physically links integrins to the contractile cytoskeleton [1,2]
This is done by transmembrane proteins, including integrins, which externally bind to the extracellular matrix and on the cell interior to the contractile cytoskeleton via scaffolding proteins

Summary

Introduction

The talin head (TH), residues 1–432, has binding sites for integrin b-tails [3], PIP kinase c [4], focal adhesion kinase (FAK) [5], layilin [5] and actin [6] (Figure 1B). The talin rod contains up to eleven vinculin binding sites [10] (Figures 1B and S1), including five located within fragment H1–H12, residues 486–889, which is studied here (Figure 1). All of these five binding sites are buried inside helix bundles (native talin shows considerably lower affinity to vinculin compared to peptide fragments isolated from talin). In addition to the VBSs, the talin rod has binding sites for actin [11] and for integrins [12]

Methods

Results

Discussion

Conclusion