Structural effects of Mg2+ on the regulatory states of Neuronal Calcium Sensors operating in vertebrate phototransduction

Several lines of evidence suggest that free Mg2+ plays an important role in phototransduction, as the Neuronal Calcium Sensors (NCS) Recoverin and Guanylate Cyclase Activating Proteins 1 and 2 (GCAP1 and GCAP2) are also capable of binding Mg2+ via their EF-hand motifs. Previous studies showed that a Mg2+ -bound state is required for GCAP1 in order to activate GC and that Recoverin binds Mg2+ without triggering its physiological conformational change. No structural studies were performed so far about GCAP2, for which the effects that Mg2+ could exert were only hypothesized. Here we compared the effects of physiological [Mg2+] (1 mM) on the switch states of these three NCS in their myristoylated (myr) and non myristoylated (nonmyr) form over the extreme conditions of high and low [Ca2+], mimicking respectively dark and light states of the photoreceptor cell. We performed Circular Dichroism spectroscopy measurements to assess the differences in thermal stability, secondary and tertiary structure of all NCS in the aforementioned conditions. Intrinsic fluorescence spectroscopy titrations and Isothermal Titration Calorimetry were performed for monitoring the binding of Mg2+ to GCAP2. Molecular dynamics simulations (200 ns, all-atom force field) were performed to assess structural properties of GCAP1 in putatively activator, inhibitor and transitory states. Our results confirm that Mg2+ is unable to trigger the physiological conformational change of Recoverin (myristoyl switch) and that it decreases its thermal stability. Mg2+ induces a conformational change in GCAP2 both at high and low [Ca2+], however these variations are more substantial for apo-myrGCAP2. Apo-GCAP1 is responsive to Mg2+, acquiring a different tertiary structure from both apo and Ca2+-bound states, though this difference is lost when Ca2+ is saturating. GCAP1 seems to be stabilized by the presence of Mg2+ in solution, more notably its Ca2+-bound form. Molecular dynamics simulations point out that myrGCAP1 has a highly flexible loop (125-135) when at least one divalent cation is bound to EF-3. In line with experimental data, this is sufficient to stabilize the entire structure. Moreover all simulated transitory states show very similar dynamic properties, which differ from both apo and Ca2+- or Mg2+- loaded forms.