Biochemical analysis of the interaction of calcium with toposome: a major protein component of the sea urchin egg and embryo.

We have investigated the biochemical and functional properties of toposome, a major protein component of sea urchin eggs and embryos. Atomic force microscopy was utilized to demonstrate that a Ca2+-driven change in secondary structure facilitated toposome binding to a lipid bilayer. Thermal denaturation studies showed that toposome was dependent upon calcium in a manner paralleling the effect of this cation on secondary and tertiary structure. The calcium-induced, secondary, and tertiary structural changes had no effect on the chymotryptic cleavage pattern. However, the digestion pattern of toposome bound to phosphatidyl serine liposomes did vary as a function of calcium concentration. We also investigated the interaction of this protein with various metal ions. Calcium, Mg2+, Ba2+, Cd2+, Mn2+, and Fe3+ all bound to toposome. In addition, Cd2+ and Mn2+ displaced Ca2+, prebound to toposome, while Mg2+, Ba2+, and Fe3+ had no effect. Collectively, these results further enhance our understanding of the role of Ca2+ in modulating the biological activity of toposome.