For the structure determination, a single 1.1-mM uniformly 13C and 15N-labeled sample was prepared in 20 mM phosphate buffer (pH 6.0), 100 mM NaCl, 1 mM dithiothreitol, 0.02% NaN 3 with the addition of D2O to 10% v/v.

NMR experiments were performed at 25°C on Bruker DRX 600 or Bruker AV 800 spectrometers operating at 600.280 and at 800.350 MHz proton frequency, respectively. 1H, 15N, and 13C chemical shifts were referenced relative to the frequency of the 2H lock resonance of water.