Figure 1. AS69 binds to monomeric $\alpha$-synuclein, inducing local folding of the region comprising residues 37–54 into a $\beta$-hairpin conformation.

(a) Structural model of the AS69:$\alpha$-synuclein complex based on NMR (pdb entry 4BXL) (Mirecka et al., 2014), generated with PyMOL (The PyMOL Molecular Graphics System, 1.2; Schrödinger, LLC.). AS69 (grey) is a disulfide-linked homodimer. $\alpha$-Synuclein (orange) locally adopts $\beta$-hairpin conformation, while the remainder of the molecule, including the hydrophobic NAC segment (green), remains intrinsically disordered (Mirecka et al., 2014). Positions at which disease-related mutations have been identified are given in magenta. (b,c) The affinity of AS69 to $\alpha$-synuclein at pH 7.4 (b) and pH 5.0 (c) analyzed by isothermal titration calorimetry (ITC) experiments. Titration of 420 μM $\alpha$-synuclein into 47 μM AS69 in 20 mM sodium phosphate, 50 mM NaCl, pH 7.4 (b), or 320 μM $\alpha$-synuclein into 32 μM AS69 in 20 mM sodium acetate, pH 5.0 (c), at 30 °C. The upper panels show the baseline-corrected instrumental response. The lower panels show the integrated data (filled squares) and the fit to a 1:1 binding model (continuous line).