Structural characterization by nuclear magnetic resonance spectroscopy of a genetically engineered high-affinity...

This paper reports the solution conformation of a peptide (P196-267) derived from the calmodulin-binding domain of Bordetella pertussis adenylate cyclase. P196-267 corresponding to the protein fragment situated between amino acid residues 196-267 was overproduced by a recombinant Escherichia coli strain. Its affinity for calmodulin is only one order of magnitude lower (Kd = 2.4 nM) than that of the whole bacterial enzyme (Kd = 0.2 nM). The proton resonances of the NMR spectra of P196-267 were assigned using homonuclear two-dimensional techniques (double-quantum-filtered J-correlated spectroscopy, total correlation spectroscopy, and nuclear Overhauser enhancement spectroscopy) and a standard assignment procedure. Analysis of the nuclear Overhauser effect connectivities and the secondary shift distribution of C alpha protons along the sequence allowed us to identify the elements of regular secondary structure. The peptide is flexible in solution, being in equilibrium between random coil and helical structures. Two segments of 11 amino acids (situated between V215 and A225) and 15 amino acids (situated between L233 and A247) populate in a significant proportion the helix conformational state. The two helices can be considerably stabilized in a mixed solvent, trifluoroethanol/water (30/70), suggesting that the corresponding fragment in the intact protein assumes a similar secondary conformation. No elements of tertiary structure organization were detected by the present experiments. The conformational properties of the isolated calmodulin target fragment are discussed in relation with the available NMR and X-ray data on various peptides complexed to calmodulin.