For protein purification, 4 mL of a Ni-NTA resin (Thermo Fisher Scientific, USA) were used, pre-equilibrated with wash buffer (2 mM sodium phosphate, 12 mM sodium chloride, 10 mM imidazole, pH 8.0). The supernatant was added to the Ni-NTA-resin and incubated at 4 °C for 30 min. After incubation the resin was washed twice with 4 mL of wash buffer. The proteins were eluted by adding 5 mL of elution buffer (50 mM sodium phosphate, 300 mM sodium chloride, 250 mM imidazole, pH 8.0).