A novel saccharide structure, Xyl 1----3 Gal 1----(SO3-)3,4 Fuc----, is present in acrosome reaction-inducing substance of the starfish, Asterias amurensis

Structure of the main saccharide chain in the acrosome reaction-inducing substance of the starfish, Asterias amurensis

The structure of the main saccharide chain of the acrosome reaction-inducing substance in the egg jelly coat of the starfish, Asterias amurensis, is composed of the following pentasaccharide repeating units (Structure I). A polymer consisting of 10-11 repeating units has been observed to induce the acrosome reaction in starfish sperm at high calcium concentrations. [STRUCTURE I:see text] The identities and linkage positions of constituent sugars were established using sugar, methylation, and sulfate analyses together with one- and two-dimensional nmr spectroscopy. The structure was supported by the data obtained for desulfation products and the Smith degradation of the polysaccharide.

Estimation by radiation inactivation of the minimum functional size of acrosome-reaction-inducing substance (ARIS) in the starfish, Asterias amurensis

In the starfish Asterias amurensis, the jelly coat of the eggs contains a glycoprotein essential for the induction of the acrosome reaction in homologous spermatozoa that is termed the acrosome-reaction-inducing substance (ARIS). ARIS is a highly sulphated and fucose-rich glycoprotein of extremely high molecular mass (> 10(4) kDa). ARIS was irradiated with high-energy electrons in order to estimate the minimum size required for its biological activity. The minimum functional unit or target size of ARIS was estimated to be c. 14 kDa by target size analysis. ARIS was significantly disintegrated by the irradiation, yet the total sugar content was not apparently reduced. The binding of 125I-labelled ARIS to spermatozoa competed with that of irradiated ARIS, although the affinity of ARIS was much reduced after irradiation.

Ultrastructural localization of acrosome reaction-inducing substance (ARIS) on sperm of the starfish Asterias amurensis

Using colloidal gold tagged ligands we have identified the ultrastructural site of ARIS binding to intact and acrosome-reacted starfish sperm. In intact sperm, colloidal gold conjugated ARIS was specifically localized to a single domain (0.1-0.3 micron in diameter) on the plasma membrane. This site was located on the anterior-lateral aspect of the sperm head, that is, just peripheral to the region occupied by the acrosomal vesicle and periacrosomal components. When sperm were labeled with colloidal gold conjugated ARIS, washed to remove unbound label, and then induced to undergo the acrosome reaction, the labeled patch remained associated with the plasma membrane and was positioned just lateral to the acrosomal process. However, when sperm were suspended in labeled ARIS and induced to undergo the acrosome reaction, label was observed along the entire anterior aspect of the sperm head with the exception of the acrosomal process. Labeling along the entire anterior aspect of the sperm head in this case was deemed to be nonspecific and due to binding of colloidal gold tagged molecules to components formerly located within the acrosomal vesicle, as the same pattern was obtained using colloidal gold tagged bovine serum albumin. Quantitative and qualitative aspects of ARIS binding observed here by electron microscopy are in agreement with measured binding characteristics previously reported (Ushiyama et al., 1993a: Zygote 1:121-127; Ushiyama et al., 1993b: J Reprod Dev 39:53-54), and indicate that the site of labeled ARIS binding represents a specific plasma membrane domain occupied by ARIS receptors.

Specific binding of acrosome-reaction-inducing substance to the head of starfish spermatozoa

In the starfish, spermatozoa undergo the acrosome reaction upon encountering the jelly coat of eggs. A highly sulphated glycoprotein in the jelly coat is called acrosome-reaction-inducing substance (ARIS) because it is the key signal molecule to trigger the acrosome reaction. The activity of ARIS is mainly attributed to its sulphate and saccharide residues. The extremely large molecular size and species-specific action of ARIS suggest the presence of a specific ARIS receptor on the sperm surface, but no experimental evidence for the receptor has been presented. We therefore measured specific binding of ARIS and its pronase digest (P-ARIS), which retains the full activity of ARIS, to homologous spermatozoa by using fluorescein-isothiocyanate-labelled ARIS and 125I-labelled P-ARIS, respectively. The spermatozoa had the ability to bind ARIS, as well as P-ARIS, specifically. The binding was species-specific and mostly localised to the head region of spermatozoa. Scatchard plot analysis indicated the presence of one class of ARIS receptor on the surface of acrosome-intact spermatozoa. Furthermore, the specific binding of P-ARIS to the anterior region of sperm heads was microscopically confirmed by using P-ARIS conjugated to polystyrene latex beads with intense fluorescence. It is concluded that starfish spermatozoa have a specific receptor for ARIS on the surface of the anterior region of heads.