To assign the peaks from labelled amino acids, we measured [ 1 H, H]-NOESY. For HSQC spectra, the 1 H carrier frequency was positioned at 4.703 ppm, the N carrier frequency at 119 ppm, and the spectral widths were 12.0 ppm ( 1 H, F2) and 33.0 ppm ( 15 N, F1). All these experiments were processed and analyzed using CcpNmr Analysis software (version 3.0.3). For the assignment, the complete spin systems of the amino acid residues were first identified using their proton-proton J-couplings ([ 1 H, 1 H]-TOCSY) combined with their proton-nitrogen J-coupling ([ 15 N, 1 H]-HSQC). These spin systems were then located within the peptide sequence by means of through-space, sequential NOE connectivities between adjacent residues. Spin relaxation times were acquired with standard pulse sequences using 1664 points in the F3 domain, 300 points in the F2 domain, and 8 scans. Delay times were set to 20, 50, 100, 200, 300, 500, 700, and 900 ms for T 1 , and 34,