THE FUCOSE SULFATE POLYMER OF EGG JELLY BINDS TO SPERM REJ AND IS THEINDUCER OF THE SEA-URCHIN SPERM ACROSOME REACTION

The sea urchin sperm cell is an advantageous model for studying ligand -mediated exocytosis. Sperm can be obtained in vast quantities and ind uced to undergo exocytosis of the acrosomal vesicle with great synchro ny. During sea urchin fertilization, egg jelly (EJ) triggers the sperm acrosome reaction (AR) which is required for sperm binding and fusion with the egg. Uncertainty exists as to the exact biochemical nature o f the AR inducer. The following study was performed in an attempt to c larify the nature of the inducer. EJ from individual females (Strongyl ocentrotus purpuratus) was analyzed on SDS-PAGE gels. Each female had a unique composition of EJ macromolecules, but all females possessed t he previously described fucose sulfate polymer (FSP). Two electrophore tic isotypes of FSP were discovered; 87% of females had only one isoty pe and 13% had both. Both FSP isotypes bound to the REJ protein (recep tor for egg jelly) purified from sperm. The two FSP isotypes had almos t equal potency in inducing the AR. EJ was fractionated by DEAE chroma tography in 6 M urea/4% beta-mercaptoethanol. All AR-inducing activity coeluted with FSP. FSP, purified by trypsin digestion followed by dia lysis, was twice as active as the non-trypsin-digested control at indu cing the AR. EJ was digested with proteinase K, boiled in detergent an d beta-mercaptoethanol, and subjected to sucrose density gradient sedi mentation. The FSP and AR activity had superimposable sedimentation pa tterns. Purified FSP had no associated peptide component. Sperm from i ndividual males differed in the concentration dependency of purified F SP to induce the AR. The data indicate that the 138/82 kDa EJ glycopro teins, previously thought to act as AR inducers, do not appear to be i nvolved in triggering the AR. The data are consistent with the hypothe sis that FSP is the only inducer of the AR of this sea urchin species. (C) 1997 Academic Press.