CHARACTERIZATION OF A MOUSE ZP3-DERIVED GLYCOPEPTIDE, GP55, THAT EXHIBITS SPERM RECEPTOR AND ACROSOME REACTION-INDUCING ACTIVITY IN-VITRO

During fertilization, free-swimming mouse sperm bind to mZP3 (similar to 83 000 M(r)), one of three zona pellucida glycoproteins, and once b ound undergo the acrosome reaction, a type of cellular exocytosis [Was sarman, P. M., & Litscher, E. S. (1995) Curr. Top. Dev. Biol. 30, 1-19 ]. Sperm recognize and bind to specific serine/threonine-linked oligos accharides located at the mZP3 combining site for sperm. Here, we exam ined certain characteristics of gp55, a similar to 55 000 M(r) glycope ptide derived from the carboxy-terminal half of mZP3 polypeptide to wh ich sperm bind [Rosiere, T. K., & Wassarman, P. M. (1992) Dev. Biol. 1 54, 309-317]. gp55 is heterogeneous with respect to M(r) (similar to 4 7 000-62 000 M(r)) and has a relatively low pi (similar to 4.3-4.5) co mpared to the polypeptide portion of the glycopeptide (pI similar to 6 .5). gp55 inhibits binding of sperm to eggs (i.e., exhibits sperm rece ptor activity) and induces sperm to undergo the acrosome reaction in v itro at about the same concentrations required for intact mZP3 (simila r to 50-200 nM). Each of three different size-fractions of gp55, separ ated by SDS-PAGE, also exhibits bioactivity in vitro. Removal of aspar agine-linked (N-linked) oligosaccharides from gp55, by extensive diges tion with N-glycanase, reduces its M(r) to similar to 21 000 and incre ases its pI to similar to 5.3, but does not significantly affect its a bility to inhibit binding of sperm to eggs or to induce sperm to under go the acrosome reaction. Similarly, digestion of gp55 with either end o-beta-galactosidase or neuraminidase alters its M(r) and/or pI, but d oes not significantly affect either of its bioactivities. These observ ations an consistent with the proposal that neither N-linked oligosacc harides nor sialic acid is an essential element of the mZP3 combining site for sperm. They also indicate that a relatively small mZP3 glycop eptide is able to induce sperm to undergo the acrosome reaction (i.e., cellular exocytosis) in vitro.