Variable conformation and dynamics of calmodulin complexed with peptides derived from the autoinhibitory domains of target proteins

Calcium-saturated calmodulin (CaM) can bind and activate many target proteins through the direct association with the respective autoinhibitory domains. The CaM binding sequences within the autoinhibitory domains of these proteins have little sequence homology, and the mechanisms associated with CaM's ability to recognize and productively bind with these variable sequences is unclear. Common structural features of CaM bound to five peptides that are homologous to the autoinhibitory domains of smooth muscle myosin light chain kinase, CaM-dependent protein kinase II alpha, the plasma membrane Ca-ATPase, a MARCKS homolog, and glycogen phosphorylase kinase were assessed using frequency-domain fluorescence spectroscopy. In addition, the structural features of CaM complexed with the peptide melittin was also considered. We observe similar decreases in the average fluorescence lifetime and similar increases in the solvent accessibility of N-(1-pyrenyl)maleimide (PM) bound at Cys27 in calcium binding loop I in the amino terminal domain of CaM upon association with all six target peptides. Likewise, using fluorescence resonance energy transfer to measure the spatial separation between the opposing globular domains in CaM, we observe a similar spatial separation between the opposing globular domains of CaM bound to all six peptides. This indicates that CaM undergoes comparable structural changes upon association with all six target peptides. However, there are significant differences in the observed lifetime, solvent accessibility, correlation time associated with the segmented rotational motion of PM-CaM, and in the spatial separation between the opposing globular domains in CaM upon association with the individual target peptides, which indicates that CaM adopts a different tertiary structure that is dependent on the structural features of the bound target peptide. The correlation times associated with the overall hydrodynamic properties of CaM complexed with all six peptides are nearly identical (phi 2 approximately 10.6 +/- 0.4 ns) and are consistent with the known dimensions of CaM complexed to a peptide homologous to the CaM binding sequence of CaM-dependent protein kinase II alpha. Therefore, while these results are consistent with a common binding mechanism between CaM and all six target peptides, they indicate that the binding domains of CaM adopt different tertiary structures that allow them to bind with the variable sequences found in the autoinhibitory domains of target proteins with high affinity.