Physicochemical properties of a novel Mr-21 000 Ca2+-binding protein of bovine brain

The physicochemical properties of a novel Mr-21 000 Ca2+-binding protein isolated from bovine brain were investigated. The protein exhibited a partial specific volume of 0.724 ml/g, a degree of hydration of 0.47 g of water/g of protein and a mean residue weight of 119. Sedimentation equilibrium analysis revealed Mr = 22 600 in the absence of Ca2+; Ca2+ binding appeared to induce dimerization of the molecule. Size-exclusion chromatography indicated a compacting of the molecule on binding of Ca2+: the Stokes radius decreased from 2.75 nm in the absence of Ca2+ to 2.56 nm in its presence. Far-u.v.c.d. spectroscopy showed the apoprotein to be composed of 44% alpha-helix, 18% beta-pleated sheet and 38% random coil. Addition of either KCl (0.1 M) plus Mg2+ (1 mM), or Ca2+ (2 mM), changed the conformation to 49% alpha-helix, 18% beta-pleated sheet and 33% random coil. Near-u.v.c.d. and u.v. difference spectroscopy both indicated perturbations in the environments of all three types of aromatic amino acids on binding of Ca2+. Ca2+ binding also resulted in a 30% enhancement in the tryptophan fluorescence emission intensity. Ca2+ titration of the far-u.v.c.d. and fluorescence enhancement provided KD values of 9.91 microM and 4.68 microM respectively. Finally, the protein was shown to bind Zn2+ with KD = 1.44 microM (no Mg2+) and 1.82 microM (+ Mg2+). These observations strongly support the possibility that this novel Ca2+-binding protein resembles calmodulin and related Ca2+-binding proteins and undergoes a conformational change on binding of Ca2+ which reflects a physiological role in Ca2+-mediated regulation of brain function.