Females of the sea urchin Strongylocentrotus purpuratus differ in the structures of their egg jelly sulfated fucans

The egg jelly coats of sea urchins contain sulfated fucans which bind to asperm surface receptor glycoprotein to initiate the signal transductionevents resulting in the sperm acrosome reaction. The acrosome reaction isan ion channel regulated exocytosis which is an obligatory event for spermbinding to, and fusion with, the egg. Approximately 90% of individualfemales of the sea urchin Strongylocentrotus purpuratus spawned eggs havingonly one of two possible sulfated fucan electrophoretic isotypes, a slowmigrating (sulfated fucan I), or a fast migrating (sulfated fucan II)isotype. The remaining 10% of females spawned eggs having both sulfatedfucan isotypes. The two sulfated fucan isotypes were purified from eggjelly coats and their structures determined by NMR spectroscopy andmethylation analysis. Both sulfated fucans are linear polysaccharidescomposed of 1-->3-linked alpha-L-fucopyranosyl units. Sulfated fucan Iis entirely sulfated at the O -2 position but with a heterogeneoussulfation pattern at O -4 position. Sulfated fucan II is composed of aregular repeating sequence of 3 residues, as follows: [3-alpha-L-Fuc p -2,4(OSO3)-1-->3-alpha-L-Fuc p -4(OSO3)-1-->3-alpha-L-Fuc p -4(OSO3)-1]n. Both purified sulfated fucans have approximately equal potency ininducing the sperm acrosome reaction. The significance of two structurallydifferent sulfated fucans in the egg jelly coat of this species couldrelate to the finding that the sperm receptor protein which binds sulfatedfucan contains two carbohydrate recognition modules of the C-type lectinvariety which differ by 50% in their primary structure.