Crystallization and data collection. Unliganded human Sts-1HP

Initial crystallization screening of human Sts-1_HP was conducted at 18 C, using the hanging-drop vapor diffusion method, based on the conditions under which the mouse Sts-1_HP is known to crystallize¹⁶. Rod-shaped crystals were obtained overnight when 1 μL of Sts-1_HP was mixed with 1 μL reservoir solution containing 0.1M HEPES pH 7.0, 5% ethylene glycol, 0.3 M magnesium chloride and 13% PEG 8000. The crystal size and quality was improved by seeding. Prior to be being flash frozen in liquid nitrogen, the crystals were soaked in a solution containing the crystallization buffer with a final PEG 8000 concentration of 30%. Human Sts-1_HP with sulfate: Initial crystallization screening of human Sts-1_HP was conducted at 18 C, using the hanging-drop vapor diffusion method, based on the conditions under which the mouse Sts-1_HP is known to crystallize¹⁵. Lozenge-shaped crystals were obtained overnight when Sts-1_HP was mixed with the reservoir solution in a ratio of 1:1.5. The reservoir solution was consisted of 0.1 M sodium acetate pH 5.5, 0.2 M ammonium sulfate, 23% PEG 2000 MME and 2 mM DTT. These crystals were cryoprotected by soaking in the crystallization solution with added PEG 2000 MME to 30%, before being flash frozen in liquid nitrogen. Human Sts-2_HP: Sparse matrix screening was conducted for Sts-2_HP using the hanging-drop vapor diffusion method at 291 K. Cubic-shaped crystals were obtained after two days in a condition with equal volumes of Sts-2_HP and a solution consisting of 0.1 M HEPES pH, 7.0, 22% PEG 4000, 0.2 M potassium acetate and 0.2 M lithium sulfate. Crystals of similar morphology were also obtained using a reservoir solution consisting of 0.1M Tris pH 8.0, 30% PEG 4000, 0.2 M lithium sulfate and 15 mM strontium chloride. Crystals were first soaked in a solution containing the crystallization solution with a PEG 4000 final concentration of 30% before being flash frozen in liquid nitrogen. In all three cases, data was collected at 100 K at the Advanced Photon Source on beamlines 24 ID-C and 24 ID-E. Data collection statistics are provided in Table 2.

Data collection and refinement statistics