Proteinase K: Proteinase K was acquired from GoldBio, dissolved at 50 mg/mL in 50 mM HEPES pH 7.0, and crystals were grown by vapor diffusion in sitting drop trays against 1.2 M ammonium sulfate and 0.1 M Tris HCl pH 8.0 to achieve a needle-shaped polymorph amenable to producing high-resolution (2.0 Angstrom) diffraction by microED.