Characterization of a calsequestrin-like protein from sea-urchin eggs

Following our studies on the identification of a calsequestrin-like protein (CSLP) from sea-urchin eggs [Oberdorf, Lebeche, Head and Kaminer (1988) J. Biol Chem. 2637-6806 6809], we have characterized its Ca²⁺-binding properties and identified it as a glycoprotein. The molecule binds 23 mol of Ca²⁺/mol of protein, as determined by equilibrium dialysis. This is in the range reported for cardiac calsequestrin but is about half the binding capacity of striated muscle calsequestrin. The affinities of the CSLP for Ca²⁺ are decreased by increasing KCl concentrations (20-250 mM) and the presence of Mg²⁺ (3mM) in the medium: the half-maximal binding values varied from 1.62 to 5.77 mM. Hill coefficients indicated mild co-operativity in the Ca²⁺ binding. Ca²⁺ 1-8 mM)-induced u.v. difference spectra and intrinsic fluorescence changes suggest a net exposure of aromatic residues to an aqueous environment. C.d. measurements showed minor Ca²⁺-induced changes in cc-helical and β-sheet content of less than 10 %. These spectral changes are distinctly different from those found in muscle calsequestrin. Immunoblotting studies showed that the CSLP is distinct from calreticulin, a low-affinity Ca²⁺-binding protein.