Species-specificity of the acrosome reaction in starfish

Abstract

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.