Fig. 2. PRE profiles.

(A,B) PRE profiles for CaM with a spin-label at (A) S17C or (B) A128C. Experimental PRE profiles for apo or Ca²⁺-loaded CaM, in the presence or absence of MLCK peptide, are shown as circles (error bar, 1 s.d.). PREs too large (> 80 s⁻¹, roughly) to be accurately measured are plotted at the top of the charts. PRE profiles back-calculated from the structures of CaM-4Ca²⁺-MLCK (red), CaM-4Ca²⁺ (blue) and apo CaM (green) are shown as solid lines. Only for the CaM-4Ca²⁺-MLCK complex are the experimental interdomain PREs correctly predicted by the corresponding known structure. Note that slightly different PRE profiles can be back-calculated from the same structure depending on which experimental intradomain PRE data set is used to fit the spin-label position (and, in the case of CaM-4Ca²⁺-MLCK, whether the peptide is present—as in the top panel—which slightly restricts the conformational space available to the spin-label). In the case of A128C, the apo NMR structure predicts some small PREs at the N-terminus; these are likely artifacts of the specific conformation adopted by the deposited average regularized structure (PDB 1CFD) since no interdomain NOEs were observed in that study.²