2 results
Species-specificity of the acrosome reaction in starfish
- Midori Matsumoto, Masako Ikeda, Motonori Hoshi
-
- Article
- Export citation
-
Animal eggs are generally encased in extracellular investments. These structures are not simply a protective barrier against infectious microbes, parasites and various small predators: in starfish, three components of the egg jelly, the outermost egg investment, are responsible for triggering the acrosome reaction. These components are a highly sulphated glycoprotein of an extremely large molecular size named acrosome reaction-inducing substance (ARIS), a steroid saponin named Co-ARIS, and asteroidal sperm-activating peptides (asterosaps) (Matsui et al., 1986a, b; Nishigaki et al., 1996). ARIS can induce the acrosome reaction in homologous spermatozoa with asterosaps or Co-ARIS in normal seawater. Specificity at the genus or order level was found for sperm activation by asterosaps, whereas the acrosome reaction by jelly components was species-specific. The main sugar saccharide chain of ARIS, composed of the pentasaccharide repeating units [Xyl-Gal-Fuc(SO3−)-Fuc(3−)-Fuc-], has been observed to induce the acrosome reaction in starfish sperm at high calcium concentrations (Koyota et al., 1997). Recently, we cloned cDNAs encoding asterosaps and elucidated their nucleotide sequences (Matsumoto et al., 1999). The mRNA encoding asterosaps was transcribed only in the oocytes but not in the follicle cells, and the length was 3.7 kb. The cDNA clones contained multiple isoforms of asterosaps. We assume that asterosap cursors are large prepolypeptide chains with an unusual ‘rosary-type’ structure composed of 10 successive similar stretches of 51–55 residues. Each stretch ends with a ‘spacer’ of 17–21 residues immediately followed by the sequence of one asterosap isoform. The amino terminal of this precursor has 19–21 successive glutamine-rich repeating units.
Acrosome reaction in starfish: signal molecules in the jelly coat and their receptors
- Motonori Hoshi, Takuya Nishigaki, Mayu Kawamura, Masako Ikeda, Jayantha Gunaratne, Shoichi Ueno, Manabu Ogiso, Hideaki Moriyama, Midori Matsumoto
-
- Article
- Export citation
-
Animal eggs are generally encased in one or more extra-cellular coats that protect the egg from biological, chemical and mechanical hazards. These coats contain some essential molecules for sperm to fertilise an appropriate egg, such as the specific ligand for sperm binding and the specific signal for induction of the acrosome reaction. In starfish, the outermost egg coat is a relatively thick gelatinous layer called the jelly coat. When starfish sperm encounter the jelly coat of homologous eggs, they undergo the acrosome reaction within a second or less (Dale et al., 1981; Ikadai & Hoshi, 1981; Sase et al., 1995). We have thus searched the jelly coat for the signal molecule(s) that triggers the acrosome reaction in the starfish, Asterias amurensis. It is known that three components in the jelly coat, namely acrosome reaction-inducing substance (ARIS), Co-ARIS and asterosap, act in concert on homologous spermatozoa to elicit the acrosome reaction immediately and efficiently (Hoshi et al., 1994,1999).
ARIS alone induces the acrosome reaction only in high calcium or high pH seawater. In normal seawater, besides ARIS, either Co-ARIS or asterosap is required for the induction. Without ARIS, no combination of Co-ARIS and asterosap can induce the acrosome reaction in normal, high calcium or high pH seawater. A mixture of ARIS and Co-ARIS increases the intracellular Ca2+ level, whereas asterosap increases the intra-cellular pH (Matsui et al., 1986a, b; Nishigaki et al., 1996). These events are prerequisites for the induction of the acrosome reaction. Indeed, the triad of ARIS, CoARIS and asterosap provides the best conditions for the induction of the acrosome reaction in normal sea-water (Hoshi et al., 1994, 1999).