Heteronuclear 3D NMR and isotopic labeling of calmodulin. Towards the complete assignment of the 1H NMR spectrum.

New methods are described that permit detailed analysis of the NMR spectra of calmodulin, an alpha-helical protein with a molecular weight of 16.7 kD. Two complementary approaches have been used: uniform labeling with 15N and labeling of specific amino acids with either 15N or 13C. It is demonstrated that uniform 15N labeling permits the recording of sensitive three-dimensional (3D) NMR spectra that show far better resolution than their conventional two-dimensional analogs. Selective 15N labeling of amino acids can be used for identifying the type of amino acid, providing information that is essential for the analysis of the 3D spectra. Simultaneous selective labeling with both 15N and 13C can provide a number of unique backbone assignments from which sequential assignment can be continued.