(A) Sequence of the cytoplasmic regions of the β1A, β1D, and β3 integrin tails. Residues in β1D and β3 that differ from β1A are highlighted, and residues of particular significance are noted using β1 numbering. Secondary structure is based on the structure of the β1D/talin2 complex, with α helices denoted in blue and 3₁₀ helices in green. Membrane-proximal (MP) and membrane-distal (MD) binding sites are indicated.
(B) Sequence of the F2-F3 domains of talin1 and talin2. Residues in talin2 that differ from talin1 are highlighted, and residues of particular significance are noted using talin2 numbering. Secondary structure was determined as in panel A. The F2 domain is underlined in cyan and the F3 in yellow.
(C) The crystal structure of talin2 F2-F3 bound to the β1D integrin tail (3G9W) published previously (Anthis et al., 2009) is shown here for illustrative purposes, with significant β1D residues highlighted. See also Figure S3. All structure images were generated with MOLMOL (Koradi et al., 1996).

Chemical shift maps of β1A (A) and β3 (B) tails were generated as in Figure 2, but with mutants affecting either the membrane-proximal portion of the interaction (FF/AA) or the membrane-distal portion of the interaction (Y/A). Portions of these data have been published previously (Anthis et al., 2009), but are shown here for completeness. See also Figure S4.

ITC experiments were performed by titrating β1D tail (wt and layilin-like linker mutant) with increasing amounts of talin2 F3 domain. Panels B and C show two independent β1D wt experiments; the results were reproducible, with K_d values varying 8.9% between the experiments, ΔH varying 4.7%, and ΔS varying 6.9%.
(A) 5 μM β1D (D776/T777/Q778)V injected with 50 μM talin2 wt. Fitting the data to a 1:1 binding model gave K_d = 17.2 ± 5.0 nM, ΔH = −92.2 ± 1.6 kJ/mol, ΔS = −160 J/mol/K.
(B) 149 μM β1D wt injected with 1.87 mM talin2 wt. Kd = 21.3 ± 0.5 μM, ΔH = −69.66 ± 0.60 kJ/mol, ΔS = −144 J/mol/K.
(C) 139 μM β1D wt injected with 1.99 mM talin2 wt. Kd = 19.4 ± 0.2 μM, ΔH = −72.94 ± 0.23 kJ/mol, ΔS = −154 J/mol/K.

(A and B) Comparison of membrane-proximal regions of β1D (A) and β3 (B) bound to the talin2 or talin1 F3 domain. The image of β3 comes from the β3/PIPK1γ chimera/talin1 structure (PDB 2H7E) (Wegener et al., 2007). Highlighted residues differ between β3 and β1 integrins.
(C) Plot of the steady-state heteronuclear {¹H}-¹⁵N NOE effect versus residue number for the wt β1A, β1D, and β3 tails in the unbound state. Error bars were generated from spectral noise.
(D) As in panel C, but for β1A K768E/K770R and β3 E732K/R734K.
(E) Residue-level predictions from Agadir (Munoz and Serrano, 1994) of helical content for the wt β1A, β1D, and β3 tails. See also Figure S5.
(F) As in panel E, but for β1A K768E/K770R and β3 E732K/R734K.

Weighted chemical shift maps of perturbations observed in ¹H-¹⁵N HSQC spectra of the β1A (A), β1D (B), and β3 (C) tails (50 μM) upon addition of the talin1 or talin2 F3 domains (1 mM). Membrane-proximal (MP) and membrane-distal (MD) binding sites are indicated at the bottom of the figure. The shift map for β1D with talin2 has been cropped, and the full map can be seen in Figure S2. Most of the interactions studied here exist in the fast-intermediate chemical exchange regime, so resonances that experience a particularly large change in position upon binding are subject to extensive broadening and could not be tracked. These are denoted by the lighter grey bars in the chemical shift maps presented throughout this study. No other dynamic processes were observed in these studies that would contribute to broadening, so it can be assumed that these lighter bars correspond to residues with similar or larger magnitude shifts than the highest bars in these plots.

Sequence (A) and structure (B) alignments of four NPxY motifs bound to the talin1 or talin2 F3 domain. The β1D integrin tail (red, PDB 3G9W) (Anthis et al., 2009) is compared with β3 (magenta, PDB 1MK9) (Garcia-Alvarez et al., 2003), layilin (green, PDB 2K00) (Wegener et al., 2008), and PIPK1γ (blue, PDB 2H7E) (Wegener et al., 2007). In each case, the backbone of the talin F3 domain was aligned to the talin F3 backbone in the talin2/β1D structure. The bottom panels compare the membrane-distal region of the talin2/β1D complex (C) with that of the talin1/β3 complex (1MK9) (D), shown in the same orientation (based on the talin backbone) to highlight the different integrin arrangements. Talin is shown in yellow and the integrin tail in red. Intermolecular hydrogen bonds are highlighted in cyan. Residues that have been mutated in this study are shown in blue (integrin) or green (talin). Selected aspects of the talin2/β1D structure can be seen in greater detail in Figure 6. We note that the β1D/talin2 structure was solved by X-ray crystallography to a resolution of 2.2 Å; the β3/talin1 structure show here was solved to a resolution of 2.8 Å (and another, 1MK7, to 2.2 Å). Therefore, the broad structural differences illustrated here are within the range of what can be reliably observed from such data. The two non-integrin structures shown, on the other hand, were solved by NMR.

(A) The portion of the β1D/talin2 complex including β1D Y783, S785, and P786 and talin2 E375 and Y376. Talin2 is shown in yellow and β1D in red. Intermolecular hydrogen bonds are shown in cyan.
(B) The same view as panel A, but with talin2 shown in space-filling form to illustrate the interface between talin2 Y376 and β1D P786.
(C) Talin2 T358, shown forming a hydrogen bond from its side chain to the P355 backbone.
(D) Talin2 S392, shown forming a hydrogen bond from its side chain to the G388 backbone.
(E) The portion of the complex including β1D E779 and talin2 K360.