Spectroscopic Characterization of a High-Affinity Calmodulin−Target Peptide Hybrid Molecule

We describe the properties of a hybrid protein comprising the full length of the Xenopus laevis calmodulin sequence, followed by a pentapeptide linker (GGGGS), and residues 3−26 of M13, the calmodulin binding region of skeletal muscle myosin light chain kinase. The properties of the hybrid protein are compared with those of the complex formed between Drosophila calmodulin and a peptide corresponding to residues 1−18 of the M13 sequence. The addition of calcium to the hybrid protein produces pronounced changes in the near- and far-UV CD spectra, in the fluorescence emission spectrum of the single tryptophan residue at position 4 in the M13 sequence, and in the accessibility of this tryptophan residue to acrylamide quenching. These changes are consistent with the tryptophan residue being immobilized in a hydrophobic environment and with the hybrid protein adopting a more α-helical structure when calcium is bound. The increased α-helicity derives from changes in both the calmodulin and peptide regions of the hybrid protein. Changes in the circular dichroism and fluorescence properties of the hybrid protein as a function of the calcium to hybrid protein ratio are consistent with the fact that these changes parallel the cooperative binding of all four calcium ions. The hybrid protein shows greatly increased affinity (>250-fold) for calcium compared with calmodulin itself. Macroscopic calcium binding constants (K 1−K 4) were determined from calcium titrations performed in the presence of the calcium chelator Quin 2. Values for log(K 1 K 2) and log(K 3 K 4) were determined to be 15.4 ± 0.2 and 15.59 ± 0.22 (20 °C). The corresponding values for Drosophila calmodulin alone are 11.65 ± 0.15 and 9.66 ± 0.25. Consistent with this increased affinity for calcium, stopped-flow kinetic studies suggest that the dissociation rate for the N-terminal calcium ions is reduced to at least 0.77 s-1, compared with ≈700 s-1 for Drosophila calmodulin in the absence of peptide. This hybrid protein illustrates the principle whereby the binding of a peptide sequence covalently attached to calmodulin can enhance the average calcium affinity by more than 2 orders of magnitude. Conversely, the target sequence in the hybrid protein undergoes a calcium-induced conformational change to bind to the calmodulin in a conformation very similar to that of the corresponding dissociable target sequence binding to calmodulin, but with a greatly enhanced affinity due to its physical proximity to the b