Abnormal Sea Urchin Fertilization Envelope Assembly in Low Sodium Seawater

The structuralization of the sea urchin fertilization envelope (FE), a model for extracellular macromolecular assembly, was found to require sodium ions, the predominant cation of seawater. Eggs from Strongylocentrotus purpuratus activated in sea waters with sodium chloride substitutes (choline or Tris chloride) elevated incomplete FEs. In addition, the conversion of the microvillar casts of the FE from blunt (I-form) to angular (T-form) did not occur. The permeability of the abnormal FEs was also compromised, as approximately eight times more protein than normal was released into the ambient seawater. There were also significant increases in the escape of two cortical granule (CG) enzymes, {beta}- 1,3-glucanase and ovoperoxidase. Furthermore, FEs elevated in choline chloride (ChCl) seawater appeared to be deficient in the incorporation of ovoperoxidase, an enzyme that is normally bound to the FE and that cross-links structural proteins in the nascent FE. The morphology of FEs elevated in potassium chloride-substituted seawater was similar to those in normal sodium seawater. Thus, it appears that sodium, or at least a similar ion, is necessary for the proper functioning of ovoperoxidase and structural proteins in the elevation and normal assembly of the sea urchin FE.