Mastoparan/Mastoparan X altered binding behavior of La3+ to calmodulin in ternary complexes

Ca2+ binds to calmodulin (CaM) and triggers the interaction of CaM with its target proteins; CaM binding proteins (CaMBPs) can also regulate the metal binding to CaM. In the present paper, La3+ binding to CaM was studied in the presence of the CaM binding peptides, Mastoparan (Mas) and Mas X, using ultrafiltration and titration of fluorescence. Ca2+ binding was used as an analog to understand La3+ binding in intact CaM and isolated N/C-terminal CaM domain of metal-CaM binary system and metal-CaM–CaMBPs ternary system. Mas/Mas X increased binding affinity of La3+ to CaM by 0.5 ∼ 3 orders magnitude. The metal ions binding affinity to the C-terminal or the N-terminal CaM domain suggested that in the first phase of binding process both Ca2+ and La3+ bind to C-terminal of CaM in the presence of Mas/Mas X. In the presence of CaM binding peptides, La3+ binding preference was substantially altered from the metal-CaM binary system where La3+ slightly preferred binding to the N-terminal sites of CaM. Our results will be helpful in understanding La3+ interactions with CaM in the biological systems.