The self-association reaction of hyalin, a major protein component of the sea-urchin extraembryonic hyaline layer, was examined. Concentrations of Ca2+ below 1 mM had little effect on the hyalin gelation reaction, but higher concentrations of the cation induced protein aggregation. Quantitative aggregate formation required a Ca2+ concentration in excess of 10 mM. This reaction was modulated by both NaCl and Mg2+. The effectiveness of Ca2+ in inducing hyalin gelation was markedly enhanced in the presence of 500 mM-NaCl, the concentration found in sea water. Similarly, 20 mM-Mg2+ also enhanced Ca2+-induced hyalin gelation. Neither NaCl nor Mg2+ alone induced hyalin gelation. Concentrations of Ca2+ as low as 1 mM effectively protected hyalin from tryptic digestion both in the presence and in the absence of 500 mM-NaCl. The latter result suggested that, although higher concentrations of Ca2+ were required to induce the hyalin gelation reaction, lower concentrations of the cation could mediate a protein-protein interaction in an NaCl-independent fashion. These results identify the parameters that modulate hyalin self-association, a reaction that is essential for hyaline-layer assembly around the developing sea-urchin embryo.

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