For the elution of His6-tagged target proteins, a gradient over 60 ml to 100% buffer B (50 mM Tris•SO4, 300 mM Na2SO4, 500 mM imidazole, pH 7.9) was used to collect 1 ml fractions. For desalting, fractions with highest UV absorbance were first pooled and then loaded in portions of 15 ml onto an HiPrep 26/10 desalting columns (GE Healthcare), pre-equilibrated with TE buffer (10 mM Tris•SO4, 4 mM EDTA, pH 7.9, 10% (v/v) glycerol). Subsequently, proteins of interest were eluted in TE-buffer and 5 ml fractions. Fractions with highest UV absorbance were concentrated using Vivaspin Turbo 15 centrifugation units (Sartorius, Göttingen, Germany) with 10 kDa molecular weight cut-off. Protein concentrations were calculated by measuring absorbances at 280 nm using a NP80 nanophotometer (Implen, München, Germany) via Lambert-Beer law with molar extinction coefficients and molecular weights obtained from Protparam . For variants with extra cysteine substitutions, 5 mM β-mercapto ethanol were added to purification buffers and 5mM dithiotreitol to storage buffer.