How Force Might Activate Talin's Vinculin Binding Sites: SMD Reveals a Structural Mechanism
Figure 6
Cartoon Model Shows How Vinculin Binding to Talin Might Be Force-Activated
VB helices are presented as colored cartoons, and other helices in H1–H12 (PDB 1XWX) are shown in light gray cartoon models. The gray shaded areas represent the talin regions for which no high resolution structures are available. The vinculin cartoon is based on the X-ray structure of autoinhibited full length vinculin (PDB 1TR2).
(A) No force applied: When talin is not stressed, vinculin has low affinity for talin. Talin forms a force-bearing linkage between the extracellular matrix (gray) bound integrins and actin filaments (shown in brown). It remains unclear whether talin is parallel or tilted with respect to the cell membrane, and whether it forms a dimer.
(B) Prerequisite for force-activation: When mechanical force is applied to the integrin-actin linkage, talin is stretched. Force causes the breakage of the talin rod into helix sub-bundles, which constitutes the major energy barrier. The subbundles H1–H5, H6–H8, and H9–H12 differ in their mechanical stabilities leading to a hierarchal sequence in which they break apart. The forced unfolding pathway of the talin rod might be altered by interaction with other molecules, including PIP2 and vinculin [30], or the potential the dimerization of talin.
(C) Activation of talin's VB helices: Continued talin extension causes sequential exposure of the VB helices to water and leads finally to a separation from their host bundles. While the buried surface area of the VB helices in unstrained talin is larger than if comlexed with vinculin, conformational strain gradually exposes their hydrophobic residues—once activated, they can form an energetically more favored complex with the unstrained vinculin head. In this schematic model, only one vinculin molecule is shown. Since there are multiple vinculin binding sites in talin, the number of exposed VB helices is dependent on the applied force and the force-exposure time of talin.
(D) Vinculin binding to talin is enabled by α-helix swapping: a water-exposed VB helix can minimize its free energy by associating with the α-helix bundle of the vinculin head thereby burying its VBSs from water. Experiments indicate that the interaction of the vinculin head with a talin VB helix releases the VH–VT interaction [22–26] and thus allows the VT (VT domain shown in red) to bind to actin. The linker connecting VT to the rest of the vinculin is constructed manually.
(E) Prolonged exposure to tensile mechanical force may cause complete unraveling of individual VB helices, and we propose that this leads to a deactivation of vinculin binding. However, binding of vinculin to a VH helix might stabilize the VB helix from force-induced unfolding.