ULTRASTRUCTURAL-LOCALIZATION OF ACROSOME REACTION-INDUCING SUBSTANCE (ARIS) ON SPERM OF THE STARFISH ASTERIAS-AMURENSIS

Using colloidal gold tagged ligands we have identified the ultrastruct ural site of ARIS binding to intact and acrosome-reacted starfish sper m. In intact sperm, colloidal gold conjugated ARIS was specifically lo calized to a single domain (0.1-0.3 mu m in diameter) on the plasma me mbrane. This site was located on the anteriorlateral aspect of the spe rm head, that is, just peripheral to the region occupied by the acroso mal vesicle and periacrosomal components, When sperm were labeled with colloidal gold conjugated ARIS, washed to remove unbound label, and t hen induced to undergo the acrosome reaction, the labeled patch remain ed associated with the plasma membrane and was positioned just lateral to the acrosomal process. However, when sperm were suspended in label ed ARIS and induced to undergo the acrosome reaction, label was observ ed along the entire anterior aspect of the sperm head with the excepti on of the acrosomal process. Labeling along the entire anterior aspect of the sperm head in this case was deemed to be nonspecific and due t o binding of colloidal gold tagged molecules to components formerly lo cated within the acrosomal vesicle, as the same pattern was obtained u sing colloidal gold tagged bovine serum albumin. Quantitative and qual itative aspects of ARIS binding observed here by electron microscopy a re in agreement with measured binding characteristics previously repor ted (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 bi nding represents a specific plasma membrane domain occupied by ARIS re ceptors. (C) 1995 Wiley-Liss, Inc.