High molecular mass egg fucose sulfate polymer is required for opening both Ca2+ channels involved in triggering the sea urchin sperm acrosome reaction

A linear fucose sulfate polymer (FSP), > 10(6) daltons, is a major component of sea urchin egg jelly. FSP induces the sperm acrosome reaction (AR), an exocytotic process required for animal fertilization. Two Ca2+ channels activate during AR induction, the first opens 1 s after FSP addition, and the second opens 5 s after the first. Mild acid hydrolysis of FSP results in a linear decrease in polymer size. The ability of FSP to induce the AR and activate sperm Ca2+ channels decreases with increasing time of hydrolysis. Hydrolyzed FSP of similar to60 kDa blocks intact FSP from inducing the AR. At 44 mug/ml hydrolyzed FSP, Ca2+ entry into sperm is almost equal to that occurring in 3.8 mug/ml intact FSP; however the AR is not induced. The shape of the [Ca2+](i) increase curve and use of the Ca2+ channel blockers nifidipine and Ni2+ indicate that hydrolyzed FSP opens the second Ca2+ channel, but not the first, and thus does not induce the AR. The giant size of intact FSP is required to open both Ca2+ channels involved in triggering the AR.