Cells were re-suspended in lysis buffer (50 mM Trizma® base pH 8.0, 150 mM NaCl, 20 mM imidazole, 1 mM DTT) and lysed via passage through a microfluidizer (Microfluidics, LM20). Lysate was clarified via centrifugation at 40 000 g at 4°C for 30 min and the supernatant was loaded on a pre-packed Ni-IDA column (Macherey-Nagel) pre-equilibrated in lysis buffer. The protein bound to the column was washed with 40 column volumes of lysis buffer and eluted in 2 column volumes of elution buffer (50 mM Trizma® base pH 8.0, 150 mM NaCl, 250 mM imidazole, 1 mM DTT). The purified protein was dialyzed into storage buffer (50 mM Trizma® base pH 8.0, 150 mM NaCl, 1 mM DTT, 10 % glycerol) and frozen at -80°C.