Structure, function and biosynthesis of sperm-activating peptides and fucose sulfate glycoconjugate in the extracellular coat of sea urchin eggs

A decapeptide (GFDLNGGGVG) isolated from the solubilized jelly layer of the sea urchin Hemicentrotus pulcherrimus stimulates the respiration and motility of H. pulcherrimus spermatozoa and, in addition, produces a number of biological effects on H. pulcherrimus spermatozoa including increases in cAMP and cGMP levels, activation of a Na+/H+ exchange system, and increases in intracellular pH (pHi) and [Ca2+] ([Ca2+]i). The peptide activates the metabolism of endogenous phosphatidylcholine and promotes the acrosome reaction as a specific co-factor of a major acrosome reaction-inducing substance, fucose sulfate glycoconjugate. The peptide also induces an electrophoretic mobility change in the guanylate cyclase of the sperm plasma membrane with concomitant dephosphorylation and inactivation of the enzyme. Seventy-four peptides producing similar biological effects, named sperm-activating peptide (SAP), have since been purified from the solubilized jelly layer of seventeen species of sea urchins distributed over five taxonomic orders. These peptides show essentially the same biological effects on sea urchin spermatozoa although their activity and structures are specific at the ordinal level. Equilibrium binding experiments using a radioiodinated SAP-I analogue [GGGY(125I)GFDLNGGGVG] to H. pulcherrimus spermatozoa suggests the presence of two classes of receptors (high affinity and low affinity) specific for SAP-I binding. Based on the Kd values and EC50's for SAP-I's biological activity, we presume that the high affinity receptor is associated with respiration-stimulating activity and elevations in pHi, while the low affinity receptor is coupled to elevations in cGMP and [Ca2+]i. The radioiodinated SAP-I analogue crosslinks to a 71 kDa protein which contains a single membrane-spanning domain at almost near C-terminus. A SAP-I precursor which is synthesized in the accessory cells contains five SAP-I and seven SAP-I-like decapeptides, each separated by a single lysine residue.

Fractionation of jelly substance of the sea urchin egg and biological activities to induce acrosome reaction and agglutination of spermatozoa

Jelly coat substance surrounding the egg of the sea urchin, Hemicentrotus pulcherrimus, was fractionated by gel filtration and three fractions designated A, B, and C were obtained which mainly consisted of fucose sulfate, sialic acid, and protein, respectively. The biological activities of the fractions were examined for induction of acrosome reaction (AR) and agglutination of spermatozoa. Only fraction A, a fucose-rich glycoprotein fraction, had activities for both AR and agglutination. Fraction A was found to lose activity for AR but to retain activity for agglutination after pronase digestion. Pronase-digested fraction A was further fractionated by the same gel filtration and three fractions designated P1, P2, and P3 were obtained, which contained mainly fucose sulfate, sialic acid, and proteinous material, respectively. These fractions had no activity for AR but activity for agglutination resided in fraction P1, a fucose sulfate fraction. Furthermore, beta-elimination of the jelly substance was carried out to separate protein and fucose sulfate polysaccharide and three fractions designated E1, E2, and E3 were obtained by gel filtration, of which the fucose-rich fraction (E1) exhibited activities for both AR and agglutination, and the sialoprotein fraction (E2) retained activity only for AR. However, the activity for AR of both fractions was destroyed by pronase digestion. These results suggest that activity to induce AR resides in the protein moiety of fucose-rich glycoprotein and activity for agglutination resides in the fucose sulfate polysaccharide moiety of the same glycoprotein of the jelly substance.