Studies on interaction of CaM with CaM-binding peptides M13 and RS20 in the presence of Al3+ ions

Ca-free CaM (apoCaM) contains two globular domains connected by a flexible central linker. Each domain contains two well-defined helix-loop-helix EF-motifs that are responsible for Ca2+ binding. Upon binding, the calcium ions organize and stabilize the four-domains structure inducing large conformational changes: in this active form CaM can bind to its numerous target regulatory proteins. Since most of them are large and multimeric proteins, the CaM-protein complexes are usually simulated with template peptides. A neurotoxic factor that alters the intracellular Ca2+ regulatory system is Al3+. The alteration of Ca2+ homeostasis and the Al3+-induced CaM conformational changes may constitute the molecular basis of aluminum toxicity in Alzheimer's disease. In the present work, fluorescence studies were aimed at the understanding of Al3+ role in CaM conformation and binding activity towards its physiological protein targets. To this purpose we studied, in the presence of Ca2+ and/or Al3+, the peptide fluorescence spectral changes induced by the formation of the complexes between CaM and two synthetic peptides: M13, corresponding to the sequence 577-602 of skMLCK, and RS20, corresponding to the sequence 796-815 of smMLCK.