Zona pellucida binding proteins Receptor aggregation initiates acrosome reaction

4. Zona pellucida binding proteins

A long list of putative zona pellucida receptors has been described up to date. However, only few are characterized regarding their carbohydrate specificity and structure of the carbohydrate-binding domain, e.g. rabbit sp17, mouse galactosyltrans- ferase, porcine spermadhesins and proracrosin. Spermadhesins represent a new class of lectins with specificity to galactose-containing structures, mannose or mannose-6-phos- phate, whereas, proacrosin binds to zp glycoproteins following a sulfate-recognition mechanism. Similarly, sp17 recognizes sulfated carbohydrate as they are presented in the zona pellucida and fucoidan and share consensus sequences with the class of C-type lectins. Porcine zonadhesin and mouse sp56 are proteins containing a molecular Ž structure with still unknown ligand-binding specificity. Zona receptor kinases mouse . Ž . p95 and human hu9 ZRK and human fertility antigen A-1 FA-1 are autophosphory- lated in response to the zona pellucida exhibiting an intrinsic signaling potential. Some proteins have been found by targeted mutagenesis not to be particularly relevant to Ž . gamete recognition e.g. mouse galactosyltransferase . They may rather function in Ž . earlier events such as capacitating and sperm–oviduct interactions spermadhesins . Others are obviously located in the wrong compartment of the sperm to participate in Ž . gamete recognition or primary binding e.g. proracrosin and PH-20 . These may rather be involved in the transient secondary binding of acrosome-reacted sperm during zona Ž penetration reviewed by Naz Rajesh, 1996; McLesky et al., 1998; Shur, 1998; Topfer- ¨ . Petersen, 1999 .

5. Receptor aggregation initiates acrosome reaction

The first evidence that the aggregation of zona pellucida receptor molecules within the plane of the sperm membrane triggers the signaling cascade resulting in acrosome Ž . reaction was demonstrated by Leyton and Saling 1989 . Zp glycoproteins and antibod- Ž . ies directed against zona receptor kinase ZRK , galactosyltransferase and some other sperm surface proteins are able to initiate the acrosome reaction whereas, a zp glycopeptide fraction retaining sperm binding ability and univalent Fab-fragments fail to Ž . induce the acrosome reaction McLesky et al., 1998; Shur, 1998 . Putative zona pellucida binding proteins have been found to traverse the sperm plasma membrane as ZRK, galactosyltransferase, human FA-1 and porcine zonadhesin whereas, porcine p47, spermadhesins and murine proteinase inhibitor-binding protein are peripherally associ- Ž ated to the sperm surface Aarons et al., 1991; Naz Rajesh, 1996; McLesky et al., 1998; . Shur, 1998; Topfer-Petersen, 1999 . Those surface-associated and transmembrane pro- ¨ teins may form the multimeric receptor upon binding to the zona pellucida thus initiating Ž the aggregation of the signaling molecules of the receptor complex ZRK, galactosyl- . transferase, zonadhesin or other still unknown components that then trigger the different Ž . pathways of acrosome reaction Florman et al., 1998 . The postulated multimeric sperm Ž . receptor Shur, 1998 may be composed by a varying set of proteins without loosing completely the ability to initiate recognition and to trigger the acrosome. The following model of the ionic events in ZP signal transduction is largely based on Ž . the information reviewed by Florman et al. 1998 . According to this model the activation of the sperm surface receptor by the association with mouse ZP3 initiates two separate pathways. One signaling sequence leads to the activation of a cation channel C producing inward currents depolarizing sperm membrane potential and opening low voltage-activated T-type Ca 2q channel. The other pathway initiates internal alkaliniza- tion by mechanisms likely reflecting the mediation of G proteins. This pH increase and transient Ca 2q current in response to ZP or membrane depolarization promote a sustained Ca 2q increase. The elevation of Ca 2q level, a rise of intracellular pH and the enhancement of membrane fusogenity are postulated to be the driving forces triggering Ž the cascade of acrosomal expcytosis Harrison and Roldan, 1990; Roldan and Harrison, . 1990; Kopf and Gerton, 1991; Aitken, 1997 .

6. Priming of signal transducing mechanisms during capacitation