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Definition
Sperm activating peptide (SAP) is a decapeptide (GFDLNGGGVG) isolated from the solubilized jelly layer of the sea urchin, Hemicentrotus pulcherrimus. It stimulates the respiration and motility of H. pulcherrimus spermatozoa and, in addition, produces a number of biological effects on the 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) 1.


Related Peptides
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. 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 1.


Discovery
SAPs are diffusible peptide components of the egg outer investment (or egg jelly) in echinoderms such as sea urchins and starfish. The activity of these peptides is easily detected in slightly acidified seawater (pH 6.2-6.8) in which suppressed sperm motility and respiration are restored by addition of ethanolic extracts of egg jelly. This recovery in motility and respiration depends on the ability of SAPs to increase sperm intracellular pH, and both NH4Cl and monensin, an ionophore which exchanges Na+/H+, can mimic the activity of SAPs. This property was used to screen components of the egg investments, and led to the sequencing of the first SAP which was purified from Hemicentrotus pulcherrimus by the group of Dr. Norio Suzuki. On the other hand, the group of Dr. David Garbers explored components of the egg jelly that transiently increase sperm cGMP in Strongylocentrotus purpuratus and purified speract, the identical molecule that Suzuki’s group had obtained. Since then, hundreds of SAPs have been identified by Suzuki’s group from numerous species of sea urchin. These peptides are classified into 5 (or 6) groups depending on sequence similarity and cross-reactivity. SAPs function as moderate, though not complete, species-specific molecules since species that belong to the same taxonomic order share the same class of SAPs 2.


Structural Characteristics
One sea urchin species can produce different SAP isoforms; one gene encodes multiple SAPs, which are connected in tandem with a spacer amino acid (or several amino acids) forming a large precursor protein2. SAP-I which was previously called speract (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly), SAP- II A, previously called resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly- Arg-Leu-NH2) and SAP-IIB, previously called alloresact (Lys-Leu-Cys-Pro-Gly-Gly-Asn-Cys-Val) have been purified from the egg jelly of Hemicentrotus pulcherrimus and Strongylocentrotus purpuratus, Arbacia punctulata, and Glyptocidaris crenufaris, respectively. There have been several studies focused on characterization of SAP receptors. A SAP-IIA analogue has been reported to be cross-linked to a 160 kDa protein in A. punctulata spermatozoa, which has been identified to be guanylate cyclase, while a SAP-I analogue has been found to cross-link to a 77 kDa protein in S. purpuratus. Chemically synthesized SAP analogues, GYGG-SAP-I (Gly-Tyr-Gly-Gly-Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly) and GYGG-SAP-IIB (Gly-Tyr-Gly-Gly-Lys-Leu-Cys-Pro-Gly-Gly-Asn-Cys-Val) exibit almost the same respiration-stimulating activity as the respective original peptide 3.


Mode of Action
Resact, a 14-mer peptide that belongs to the family of sperm-activating peptides was isolated from the egg jelly layer of A. punctulata. Thereafter, the receptor for resact was identified as a guanylyl cyclase. Resact binding to this receptor turns on the guanylyl cyclase activity of the latter and the resulting rise in cGMP apparently triggers a cascade of signal transduction events, one of which is elevation of the intracellular concentration of Ca2+ 4.   Equilibrium binding experiments using a radio-iodinated 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, it is presumed 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 1.


Functions
SAP activates the metabolism of endogenous phosphatidyl-choline 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 1. SAP from sea urchin eggs, induces various sperm responses including a transient increase in the intracellular Ca2+ concentration. Speract/SAP-induced increases in intracellular Ca2+ concentration play an actual role in regulation of the flagellar movement 5.


References

1.     Suzuki N (1995). Structure, function and biosynthesis of sperm-activating peptides and fucose sulfate glycoconjugate in the extracellular coat of sea urchin eggs. Zoolog Sci., 12(1):13-27.

2.     Darszon A, Guerrero A, Galindo BE, Nishigaki T, Wood CD (2008). Sperm-activating peptides in the regulation of ion fluxes, signal transduction and motility Int. J. Dev. Biol., 52(5-6): 595-606.

3.     Harumi T, Yamaguchi M,Suzuki N (1991). Receptors for Sperm-activating Peptides, SAP-I and SAP-IIB, on Spermatozoa of Sea Urchins, Hemicentrotus pulcherrimus and Glyptocidaris crenularis (Sea urchin spermatozoa/Sperm-activating peptides /Receptor/Cross-link) Develop. Growth & Differ., 33(11):67-73.

4.     Eisenbach M (2004). Towards Understanding the Molecular Mechanism of Sperm Chemotaxis. J Gen Physiol., 124(2):105-108.

5.     Shiba K, Ohmuro J, Mogami Y, Nishigaki T, Wood CD, Darszon A, Tatsu Y, Yumoto N, Baba SA (2005). Sperm-activating peptide induces asymmetric flagellar bending in sea urchin sperm. Zoolog Sci., 22(3):293-299.

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