Role of calcium in stabilizing the structure of hyalin, a major protein component of the sea urchin extraembryonic hyaline layer

The interactions of NaCl and CaCl2 with the sea urchin embryo coat protein hyalin were investigated. Endogenous protein tryptophan fluorescence was enhanced by almost 45% in the presence of 200mM NaCl while 1mM CaCl2 reversed this effect and brought the intensity of fluorescence back close to that of the native protein. Half-maximal concentrations of 53 and 0.32mM were determined for NaCl and Ca+2, respectively. Hyalin conformation, as measured by circular dichroic spectroscopy, was altered by NaCl and CaCl2 in a fashion parallel to the effects of these salts on tryptophan fluorescence. Sodium chloride disrupted hyalin secondary structure while CaCl2 affected the return of hyalin to its native conformation. The interactions of NaCl and CaCl2 with hyalin were not modulated by MgCl2. These results suggest a role for CaCl2 in stabilizing hyalin against the disruptive effects of the high concentration of NaCl present in sea water.