Figure 1.
Talin and the talin/integrin TM complex.
A. Structure of talin (F0–F3; PDB:3IVF), showing the F0 domain in purple, the F1 domain in green, the F1 insertion in orange, the F2 domain in cyan, and the F3 domain in yellow. The loop inserted in the F1 domain was generated using Modeller. B. Model of the talin/αβ complex. The β chimeric peptide was comprised of β3 residues 688–719/β1D residues 753–787. The part of the structure corresponding to the αIIb/β3 structure (PBD:2K9J) is shown in yellow, the F2–F3/β1D (PDB:3G9W) is in grey, and the talin head domain (PDB:3IVF) is in blue.
Figure 2.
Flowchart of the principal simulations.
Schematic representation of the inputs (white background boxes) and outputs (grey background boxes) of the simulations performed. See Table 1 for further information.
Figure 3.
Simulation of talin in water (tal-sol-AT; see Table 1).
A. Change in the angle of the F0–F1 pair relative to the F2–F3 pair from its initial position (i.e. the crystal structure) during a simulation in aqueous solution. The definition of the angle is shown in the inset. For this calculation two vectors were used: vector one was defined from the center of mass of the backbone particles of the F3 subdomain to the center of mass of the backbone particles of the F2 242–248/283–295 residue region. Similarly, vector two was defined from the center of mass of the backbone particles of the F0 subdomain to the center of mass of the backbone particles of F2 (residues 242–248/283–295). The angle displayed is the difference between the angle formed by these two vectors in the crystal structure and the angle formed by the two vectors in each snapshot of the simulation. B. Distance between the F1 loop (residue: L145) and the talin F0 domain (residue: G11) during the same simulation. The position of the loop is shown in the inset pictures at different time points.
Table 1.
Summary of the principal simulations.
Figure 4.
Simulation of the association of talin with a lipid bilayer (tal-h2F0-CG; seeTable 1).
A. Final snapshot from the tal-h2F0-CG simulation, illustrating how the F0–F1 pair (purple-green) has been displaced relative to the F2–F3 pair (cyan-yellow) resulting in a V-shaped conformation. B. Change in the angle between the F0–F1 and the F2–F3 domain pairs as a function of distance from the bilayer during the tal-h2F0-CG simulation. The diagram shows the probability of finding an angle between the F0–F1 and the F2–F3 pairs at three different regions away from the bilayer phosphate atoms. C, D. Normalized number of contacts between the talin and lipids mapped onto the final snapshot (C) of the tal-h2F0-CG simulations. Contacts are defined by using a distance cut-off of 7 Å between the protein residues and the lipids. Blue indicates a low number (i.e zero contacts) white indicates a medium number (i.e. 7500 contacts) and red a large number of contacts (i.e. 15000 contacts). The bilayer headgroups are shown as grey spheres and the lipid tails as green spheres. The sidechains of the key basic residues (i.e. ARG and LYS), which are in contact with the lipids, are also shown. The residues that made more than 90% of the contacts during the tal-h2F0-AT simulations are shown in Table S3.
Figure 5.
Simulation of the association of talin with a lipid bilayer (tal-h2F0-CG; see Table 1).
Distance between the centers of mass of talin and a lipid bilayer as a function of time for the simulation tal-h2F0-CG with an anionic (POPC/POPG) lipid bilayer. The different colored lines correspond to the five repeat simulations. The horizontal broken line indicates the distance observed when talin is associated with the bilayer surface.
Figure 6.
Coarse-grained simulations of a talin/integrin TM complex with a lipid bilayer (simulation αβ-talh2-CG; see Table 1).
A. The initial state of the αβ-talh2-CG simulation. B. Relative movement of the domains of the talin/αβ complex during the αβ-talh2-CG simulation. Displacements of all the Cα atoms of the F0 (purple), the F1 (green), the F2 (cyan), the F3 (yellow) and β subunit (red) during the simulation are shown as arrows. The arrows are mapped onto the initial structure and the length of each arrow represents the displacement/direction of the corresponding Cα atom. C. Movement of the integrin TM region observed in the αβ-talh2-CG simulation. The β subunit is shown in red and the α subunit in blue. The key residues in the IMC and OMC regions are also shown.
Figure 7.
Atomistic simulation of a talin/integrin TM complex with a lipid bilayer (simulation αβ-talh2o-AT; see Table 1).
A. The talin/integrin TM complex, indicating the four distances (d1 to d4) used to monitor the packing of the TM helices during the simulation. B. Interhelical distances (d1 to d4) as a function of time for the αβ-talh2o-AT simulation. Distance d1 shows the separation between the centers of mass of the backbone particles of residues 965–968 of integrin α and residues 694–697 of integrin β as a function of time. Similarly d2, d3 and d4 show the same distances for the α970–973/β700–703, α976-797/β705–708 and α984–987/β714–717 residue groups respectively. Note that all groups are located in the helical region of the integrin TM region. C. Proposed mechanism for the integrin inside-out activation by the bound talin head domain. Electrostatic interactions of talin with negatively charged lipid headgroups promotes reorientation of the talin head domain in a plane perpendicular to the bilayer normal. In turn this rotates the β integrin tail (∼30°) perpendicular to the membrane, disrupting the interactions in both the αIIb/β3 IMC and OMC TM regions. The weakened αIIb/β3 interactions, together with a ∼15° increase in the β TM helix tilt angle relative to the bilayer normal, results in a scissoring movement of the TM regions of the two helices with a modified IMC at the center of the scissors. The scissoring movement is followed by complete dissociation of the two integrin TM helices.
Figure 8.
The β-TM helix during simulation of a talin/integrin TM complex with a lipid bilayer (αβ-talh2-CG and αβ-tal2ho-AT simulations; seeTable 1).
A. Position of the β-TM helix at the beginning (t = 0 µs) and at the end (t = 5 µs) of the simulations. Residues W715, K716 and K725 are shown in orange, green and blue respectively. The talin head domain and the α integrin subunit are omitted for clarity, and the lipid bilayer is indicated via the phosphates. B. Positions along the bilayer normal (z) during the two simulations of residues W715, K716 and K725 (using the same color code as in A). The horizontal black lines indicate the positions of the phosphates. The inset shows the first 200 ns of the CG simulation. Note that the starting structure for the AT-MD simulations (indicated by a break in the data traces and the vertical broken line) was obtained by merging the trajectories from all five simulations of the αβ-talh2-CG simulation ensemble and clustering the TM region of the talin/αβ complex as described in our previous study (see [34]).