Fertilization envelope formation induced by melittin in sea urchin eggs does not depend on Ca

In sea urchin eggs, 10 μg/mL melittin was found to induce fertilization envelope formation without any increase in [Ca2+ ]i (the intracellular free Ca2+ level). On the other hand, 10 μmol/L Br-A23187 and 100 μg/mL SDS induced fertilization envelope formation associated with [Ca2+ ]i increase. If EGTA was injected into eggs to make an intracellular concentration of 2 mmol/L, [Ca2+ ]i became quite low and was not altered by melittin, or by Br-A23187 and SDS. In eggs containing EGTA, fertilization envelope formation was induced by melittin even in Ca2+ -free artificial sea water, but not by Br-A23187 or SDS. Thus [Ca2+ ]i is essential for induction of a fertilization envelope in sea urchin eggs by Br-A23187 or SDS but not by melittin. Melittin probably activates some Ca2+ -independent reaction downstream of Ca2+ -dependent reactions in a sequential reaction system that finally results in fertilization envelope formation.