Regulatory and structural EF-hand motifs of neuronal calcium sensor-1: Mg<SUP>2+</SUP> modulates Ca<SUP>2+</SUP> binding, Ca<SUP>2+</SUP>- induced conformational changes, and equilibrium unfolding transitions

Neuronal calcium sensor-1 (NCS-1) is a major modulator of Ca2+ signaling with a known role in neurotransmitter release. NCS-1 has one cryptic (EF1) and three functional (EF2, EF3, and EF4) EF-hand motifs. However, it is not known which are the regulatory (Ca2+-specific) and structural (Ca2+- or Mg2+-binding) EF-hand motifs. To understand the specialized functions of NCS-1, identification of the ionic discrimination of the EF-hand sites is important. In this work, we determined the specificity of Ca2+ binding using NMR and EF-hand mutants. Ca2+ titration, as monitored by [15N,1H] heteronuclear single quantum coherence, suggests that Ca2+ binds to the EF2 and EF3 almost simultaneously, followed by EF4. Our NMR data suggest that Mg2+ binds to EF2 and EF3, thereby classifying them as structural sites, whereas EF4 is a Ca2+-specific or regulatory site. This was further corroborated using an EF2/EF3-disabled mutant, which binds only Ca2+ and not Mg2+. Ca2+ binding induces conformational rearrangements in the protein by reversing Mg2+-induced changes in Trp fluorescence and surface hydrophobicity. In a larger physiological perspective, exchanging or replacing Mg2+ with Ca2+ reduces the Ca2+-binding affinity of NCS-1 from 90 nM to 440 nM, which would be advantageous to the molecule by facilitating reversibility to the Ca2+-free state. Although the equilibrium unfolding transitions of apo-NCS-1 and Mg2+-bound NCS-1 are similar, the early unfolding transitions of Ca2+-bound NCS-1 are partially influenced in the presence of Mg2+. This study demonstrates the importance of Mg2+ as a modulator of calcium homeostasis and active-state behavior of NCS-1