Crystallization, data collection, and processing.

All crystallization was performed using the hanging drop vapor diffusion technique in 48-well plates (Hampton Research). Each well contained 300 μl of reservoir solution, covered by a siliconized glass cover slip with 1.5 μl of reservoir solution mixed with 1.5 μl of protein:peptide mixed at a 1:2 molar ratio. The final crystallization solution contained 31% PEG 400, 0.1 M HEPES (pH 7.5), 0.2 M CaCl₂, and 2 mM dithiothreitol (DTT). Crystals were flash-cooled in liquid nitrogen and diffracted at the Australian synchrotron MX1 and MX2 beamlines (40, 41). Diffraction data were integrated using iMosfilm and merged and scaled using Aimless, and Phaser was used for initial phase determination and electron density maps (42,–44). Model building and refinement were performed using Coot and Phenix, respectively (45, 46).