FEMALES OF THE SEA-URCHIN STRONGYLOCENTROTUS-PURPURATUS DIFFER IN THESTRUCTURES OF THEIR EGG JELLY SULFATED FUCANS

The egg jelly coats of sea urchins contain sulfated fucans which bind to a sperm surface receptor glycoprotein to initiate the signal transd uction events resulting in the sperm acrosome reaction. The acrosome r eaction is an ion channel regulated exocytosis which is an obligatory event for sperm binding to, and fusion with, the egg. Approximately 90 % of individual females of the sea urchin Strongylocentrotus purpuratu s spawned eggs having only one of two possible sulfated fucan electrop horetic isotypes, a slow migrating (sulfated fucan I), or a fast migra ting (sulfated fucan II) isotype, The remaining 10 % of females spawne d eggs having both sulfated fucan isotypes, The two sulfated fucan iso types were purified from egg jelly coats and their structures determin ed by NMR spectroscopy and methylation analysis. Both sulfated fucans are linear polysaccharides composed of 1-->3-linked alpha-L-fucopyrano syl units. Sulfated fucan I is entirely sulfated at the O-2 position b ut with a heterogeneous sulfation pattern at O-4 position. Sulfated fu can II is composed of a regular repeating sequence of 3 residues, as f ollows: a-L-Fucp-4(OSO3)-1-->3-alpha-L-Fucp-4(OSO3)-1](n). Both purifi ed sulfated fucans have approximately equal potency in inducing the sp erm acrosome reaction. The significance of two structurally different sulfated fucans in the egg jelly coat of this species could relate to the finding that the sperm receptor protein which binds sulfated fucan contains two carbohydrate recognition modules of the C-type lectin va riety which differ by 50% in their primary structure.