Assembly of the sea urchin extraembryonic hyaline layer; Ca2+ and Mg2+ act independently and at different sites on the pathway leading to hyalin-gel formation.

We have studied the interactions of Ca2+ with the sea urchin extraembryonic coat protein hyalin. As reported previously, Ca2+ alone was ineffective in inducing hyalin-gel (large aggregate) formation. This reaction required the additional presence of Mg2+ and NaCl. However, the results of tryptic digestion and nondenaturing agarose gel electrophoresis experiments demonstrated that Ca2+ could induce hyalin self-association into small aggregates in the absence of Mg2+ and NaCl. Magnesium did not modulate the interactions of Ca2+ with hyalin. In addition, Mg2+ had minimal effects on the conformation of hyalin. These results have been incorporated into a model delineating the pathway leading to hyalin-gel formation.