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Conformational changes in talin on binding to anionic phospholipid membranes facilitate signaling by integrin transmembrane helices.

Kalli AC, Campbell ID, Sansom MS - PLoS Comput. Biol. (2013)

Bottom Line: The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop.Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane.A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Oxford, Oxford, United Kingdom.

ABSTRACT
Integrins are heterodimeric (αβ) cell surface receptors that are activated to a high affinity state by the formation of a complex involving the α/β integrin transmembrane helix dimer, the head domain of talin (a cytoplasmic protein that links integrins to actin), and the membrane. The talin head domain contains four sub-domains (F0, F1, F2 and F3) with a long cationic loop inserted in the F1 domain. Here, we model the binding and interactions of the complete talin head domain with a phospholipid bilayer, using multiscale molecular dynamics simulations. The role of the inserted F1 loop, which is missing from the crystal structure of the talin head, PDB:3IVF, is explored. The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop. Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane. Simulations of the complex of talin with the integrin α/β TM helix dimer in a membrane, show how this complex promotes a rearrangement, and eventual dissociation of, the integrin α and β transmembrane helices. A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.

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Related in: MedlinePlus

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.
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pcbi-1003316-g004: 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.

Mentions: Having established in the tal-sol-AT simulations (see above) that the F1 loop interacts with F0–F1 to form a positively charged surface that extends the positive patch on F2–F3, CG simulations with the loop in this location were performed to explore the nature of the interactions of the complete talin head domain with an anionic lipid bilayer. Note that in this simulation system a small helical region (h2 helix; see Fig. S1) was included within the F1 loop as indicated by NMR data [20]. During this modeling of the loop the remainder of the structure, with the exception of the region modeled as helical (res: 154–167), was restrained to maintain the talin conformation derived from the above simulations. These restraints were removed during the simulations. In the tal-h2F0-CG simulation (Table 1; Fig. 4 and Fig. 5), talin was observed to associate with the bilayer in four out of five simulation and in all four of these simulations talin bound to the bilayer initially via a basic loop (res: 318–330) in the F3 domain, and subsequently via the positively charged patch in the F2 domain (res: 255–285) (Fig. S3A). Contacts were defined by using a distance cut-off of 7 Å between the protein residues and the lipids. These two regions have been identified previously [11] to promote productive binding of the isolated F2–F3 fragment to an anionic lipid bilayer. The additional surface created by the F1 loop also interacted with the bilayer (Fig. 4C). Interestingly, in all simulations which resulted in a talin/bilayer complex, the talin head domain with the F1 loop adopted a V-shaped conformation due to rotation of the F0–F1 pair relative to the F2–F3 pair in the bilayer plane (Fig. 4A). The reorientation of the F2–F3 and F0–F1 domain pairs prior to the binding to the bilayer was also observed in other simulations in which a different starting conformation of talin was used (e.g. the talin crystal structure; data not shown). In contrast to the more dynamic variations in the angle between the F0–F1 and F2–F3 observed when talin was in solution (see above), the V-shaped conformer was stabilized by association with the bilayer (Fig. 4B). This conformation optimizes talin/lipid interactions and induces a more compact arrangement of domains, although this new arrangement is still different from the linear arrangement in the X-ray structure [17] and the canonical FERM domain packing of F0 to F3 [18]. During the AT-MD simulations (tal-h2F0-AT; see Table 1) that started from the final snapshot of the tal-h2F0-CG simulation, the V-shaped conformation of talin was retained, with talin interacting preferentially with the headgroups of the anionic POPG lipids (Fig. S6B). No restrictions in the position/flexibility of the loop or the domains were imposed in the AT-MD simulations. Simulations of the talin head domain with a neutral bilayer (containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC) lipids) resulted in no association of talin with the bilayer (Fig. S3B and Text S1). These results are in good agreement with the available experimental data [11], [12], [17] and augment our previous observation that electrostatic interactions are important in regulating the formation of a talin/membrane complex.


Conformational changes in talin on binding to anionic phospholipid membranes facilitate signaling by integrin transmembrane helices.

Kalli AC, Campbell ID, Sansom MS - PLoS Comput. Biol. (2013)

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.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3814715&req=5

pcbi-1003316-g004: 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.
Mentions: Having established in the tal-sol-AT simulations (see above) that the F1 loop interacts with F0–F1 to form a positively charged surface that extends the positive patch on F2–F3, CG simulations with the loop in this location were performed to explore the nature of the interactions of the complete talin head domain with an anionic lipid bilayer. Note that in this simulation system a small helical region (h2 helix; see Fig. S1) was included within the F1 loop as indicated by NMR data [20]. During this modeling of the loop the remainder of the structure, with the exception of the region modeled as helical (res: 154–167), was restrained to maintain the talin conformation derived from the above simulations. These restraints were removed during the simulations. In the tal-h2F0-CG simulation (Table 1; Fig. 4 and Fig. 5), talin was observed to associate with the bilayer in four out of five simulation and in all four of these simulations talin bound to the bilayer initially via a basic loop (res: 318–330) in the F3 domain, and subsequently via the positively charged patch in the F2 domain (res: 255–285) (Fig. S3A). Contacts were defined by using a distance cut-off of 7 Å between the protein residues and the lipids. These two regions have been identified previously [11] to promote productive binding of the isolated F2–F3 fragment to an anionic lipid bilayer. The additional surface created by the F1 loop also interacted with the bilayer (Fig. 4C). Interestingly, in all simulations which resulted in a talin/bilayer complex, the talin head domain with the F1 loop adopted a V-shaped conformation due to rotation of the F0–F1 pair relative to the F2–F3 pair in the bilayer plane (Fig. 4A). The reorientation of the F2–F3 and F0–F1 domain pairs prior to the binding to the bilayer was also observed in other simulations in which a different starting conformation of talin was used (e.g. the talin crystal structure; data not shown). In contrast to the more dynamic variations in the angle between the F0–F1 and F2–F3 observed when talin was in solution (see above), the V-shaped conformer was stabilized by association with the bilayer (Fig. 4B). This conformation optimizes talin/lipid interactions and induces a more compact arrangement of domains, although this new arrangement is still different from the linear arrangement in the X-ray structure [17] and the canonical FERM domain packing of F0 to F3 [18]. During the AT-MD simulations (tal-h2F0-AT; see Table 1) that started from the final snapshot of the tal-h2F0-CG simulation, the V-shaped conformation of talin was retained, with talin interacting preferentially with the headgroups of the anionic POPG lipids (Fig. S6B). No restrictions in the position/flexibility of the loop or the domains were imposed in the AT-MD simulations. Simulations of the talin head domain with a neutral bilayer (containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC) lipids) resulted in no association of talin with the bilayer (Fig. S3B and Text S1). These results are in good agreement with the available experimental data [11], [12], [17] and augment our previous observation that electrostatic interactions are important in regulating the formation of a talin/membrane complex.

Bottom Line: The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop.Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane.A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Oxford, Oxford, United Kingdom.

ABSTRACT
Integrins are heterodimeric (αβ) cell surface receptors that are activated to a high affinity state by the formation of a complex involving the α/β integrin transmembrane helix dimer, the head domain of talin (a cytoplasmic protein that links integrins to actin), and the membrane. The talin head domain contains four sub-domains (F0, F1, F2 and F3) with a long cationic loop inserted in the F1 domain. Here, we model the binding and interactions of the complete talin head domain with a phospholipid bilayer, using multiscale molecular dynamics simulations. The role of the inserted F1 loop, which is missing from the crystal structure of the talin head, PDB:3IVF, is explored. The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop. Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane. Simulations of the complex of talin with the integrin α/β TM helix dimer in a membrane, show how this complex promotes a rearrangement, and eventual dissociation of, the integrin α and β transmembrane helices. A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.

Show MeSH
Related in: MedlinePlus