Phosphorylation primes vinculin for activation

Abstract

Vinculin phosphorylation has been implicated as a potential mechanism for focal adhesion growth and maturation. Four vinculin residues-Y100, S1033, S1045, and Y1065-are phosphorylated by kinases during focal adhesion maturation. In this study, phosphorylation at each of these residues is simulated using molecular dynamics models. The simulations demonstrate that once each phosphorylated vinculin structure is at equilibrium, significant local conformational changes result that may impact either vinculin activation or vinculin binding to actin and PIP2. Simulation of vinculin activation after phosphorylation shows that the added phosphoryl groups can prime vinculin for activation. It remains to be seen if vinculin can be phosphorylated at S1033 in vivo, but these simulations highlight that in the event of a S1033 phophorylation vinculin will likely be primed for activation.

Figure 1

Structure of vinculin and its phosphorylation sites. A cartoon representation of molecular vinculin in the autoinhibited state, with phosphorylation sites circled and labeled. The domain 1 (D1) helix bundle is colored in green, with the remaining helix bundles (D2–4) in the head domain colored in gray. The proline-rich linker region is colored in yellow and the tail domain (Vt) helix bundle is colored in orange. Talin would bind vinculin at D1, whereas Vt is suggested to interact with actin filaments. Movement of D1 away from Vt allows for Vt to potentially interact with actin. Phosphorylation of vinculin at each of the labeled residues has been implicated in regulating the growth and maturation of focal adhesions (20,26). Two of these, pY100 and pS1033, are at the interface between Vt and D1 and likely impact vinculin activation. Two of these, pS1045 and pY1065, are near Vt binding sites for actin or PIP2 and likely impact Vt binding.

Figure 2

Phosphorylation of vinculin at residue Y100. (A) D1 of vinculin is shown in green, whereas Vt of vinculin is shown in orange. Phosphorylation of Y100 at the top of the D1 helix bundle domain introduces a new electrostatic interaction between K35 and pY100. This new interaction removes K35 from a chain of electrostatic interactions stabilizing the autoinhibited vinculin conformation, potentially reducing the strength of binding between Vt and D1. Other residues involved in the chain of electrostatic interactions include E31, R105, E106, and R1008. (B) As the vinculin with pY100 is simulated for 50 ns K35 moves and reorients itself toward vinculin. The red plot shows the increase in distance between K35 and E31 throughout the 50 ns simulation as a result of the interaction with pY100, and the green plot captures the decrease in distance between pY100 and K35 for nearly 15 ns (between 8 ns of simulation and 23 ns of simulation) due to their association.

Figure 3

Phosphorylation of vinculin at residue S1033. (A) Phosphorylation of S1033 causes a shift of R987 toward the new phosphoryl group. After this structural rearrangement the hydration of vinculin is increased through penetration of the D1-Vt interface. D1 is shown in green, Vt in orange, and D2–D4 in gray. (B) The separation of R987 from E186 (red plot) and the association of R987 with pS1033 (green plot) is captured throughout the 50 ns of simulation.

Figure 4

Phosphorylation of vinculin at residue S1045. (A) Phosphorylation of S1045 releases the loop region near D4. After phosphorylation pS1045 interacts with both R1057 and R1060 releasing Q851 and the loop region from association with R1057. The flexible loop region is shown in yellow, the Vt domain in orange, D1 in green, and D2–D4 in gray. (B) Four distances were tracked throughout the 50 ns simulation. The distance between R1057 and Q851 increases early in the simulation (red plot) and remains separated. The R1057 then associates with pS1045 (purple plot) along with R1060 (blue plot). The released Q851 has intermittent association with a nearby loop region residue D841 (green plot).

Figure 5

Phosphorylation of vinculin at residue Y1065. (A) Phosphorylation of Y1065 introduces a new interaction between Vt and the loop region near Vt. K881 forms an interaction with pY1065. Vt is shown in orange, D2–D4 in gray, and the loop region in yellow. (B) The new linkage between K881 and pY1065 is formed early and is maintained throughout the 50 ns simulation.

Figure 6

Effect of phosphorylation on vinculin activation. Four models of vinculin were simulated with an activating force applied to D1: unphosphorylated vinculin (red), vinculin with pY100 (green), vinculin with pS1033 (purple), and vinculin with both pS1033 and pY100 (blue). Activation of unphosphorylated vinculin and vinculin with pY100 required over 800 pN of force, whereas activation of vinculin with pS1033 and with both pS1033 and pY100 required 700 pN of force. The reduced level of force required results from significant local conformational changes introduced by pS1033 that prime vinculin for force-induced activation.

Figure 7

Trajectory of vinculin activation. Two critical events are necessary for vinculin to activate: (a) D1 regions near residue S1033 need to separate from Vt and (b) the chain of electrostatic interactions involving R1008, E106, R105, E31, and K35 needs to be broken. (A) Simulation of vinculin with pS1033 showed rearrangement of residues near pS1033 allowed for separation of D1 from Vt near pS1033 even before (b) occurs. pS1033 also increased the hydration of the interface between D1 and Vt (arrow). D1 is shown in green, Vt is shown in orange, and D2–D4 is shown in gray. (B) Simulation of vinculin with pY100 showed increased resilience to activation. Part of this increased stability of the autoinhibited conformation resulted from an interaction between pY100 and R1008 of Vt (arrow).