Structural analysis of Mg2+ and Ca2+ binding to CaBP1, a neuron-specific regulator of calcium channels.

CaBP1 (calcium-binding protein 1) is a 19.4-kDa protein of the EF-hand superfamily that modulates the activity of Ca(2+) channels in the brain and retina. Here we present data from NMR, microcalorimetry, and other biophysical studies that characterize Ca(2+) binding, Mg(2+) binding, and structural properties of recombinant CaBP1 purified from Escherichia coli. Mg(2+) binds constitutively to CaBP1 at EF-1 with an apparent dissociation constant (K(d)) of 300 microm. Mg(2+) binding to CaBP1 is enthalpic (DeltaH = -3.725 kcal/mol) and promotes NMR spectral changes, indicative of a concerted Mg(2+)-induced conformational change. Ca(2+) binding to CaBP1 induces NMR spectral changes assigned to residues in EF-3 and EF-4, indicating localized Ca(2+)-induced conformational changes at these sites. Ca(2+) binds cooperatively to CaBP1 at EF-3 and EF-4 with an apparent K(d) of 2.5 microM and a Hill coefficient of 1.3. Ca(2+) binds to EF-1 with low affinity (K(d) >100 microM), and no Ca(2+) binding was detected at EF-2. In the absence of Mg(2+) and Ca(2+), CaBP1 forms a flexible molten globule-like structure. Mg(2+) and Ca(2+) induce distinct conformational changes resulting in protein dimerization and markedly increased folding stability. The unfolding temperatures are 53, 74, and 76 degrees C for apo-, Mg(2+)-bound, and Ca(2+)-bound CaBP1, respectively. Together, our results suggest that CaBP1 switches between structurally distinct Mg(2+)-bound and Ca(2+)-bound states in response to Ca(2+) signaling. Both conformational states may serve to modulate the activity of Ca(2+) channel targets.