#sperm binding

Fertilization in mammals-l

Zona pellucida glycoprotein primarily binding sperm:

Species-specific sperm binding to human zona pellucida relies predominantly on recognition of intact ZP2 N-terminal domain within matrix context composed of ZP1-4 heteropolymers. Transgenic mouse models expressing human ZP genes show human sperm adhere only to zona containing human ZP2, sufficient alone to support binding, while human ZP3 insufficient. This contrasts mouse where ZP3 induces acrosome reaction. Following fertilization ovastacin cleaves ZP2 destroying binding epitope providing block. ZP1 crosslinks filaments structurally. Thus ZP2 provides primary ligand for human gamete attachment, monospermy regulation, species-restricted fertilization, recognition and post-fertilization polyspermy block.

Ref: Avella et al., J Cell Biol 2014: ZP2 as primary human sperm binding ligand in zona pellucida matrix.

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Fertilization in mammals-ll

Zona pellucida glycoprotein essential for human sperm binding:

Human zona pellucida contains four glycoproteins ZP1 to ZP4 forming elastic matrix surrounding oocyte. Gene-knockout humanized mouse assays demonstrate essential binding determinant resides in N-terminal region of ZP2 rather than ZP3 as in mouse model. Intact ZP2 supports tight adhesion of acrosome-reacted sperm; after fertilization ovastacin protease cleaves ZP2 abolishing binding, contributing to block to polyspermy. Thus human gamete recognition depends dominantly on supramolecular presentation of ZP2, whose cleavage status regulates post-fertilization incompatibility and ensures monospermic fertilization and diploidy maintenance for embryogenesis.

Ref: Avella et al., J Cell Biol 2014, Humanized zona: ZP2 N-terminus required for human sperm binding and fertility.

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Fertilization in mammals-ll

Protein essential for sperm-egg fusion:

Sperm-egg fusion depends on ADAM family proteins, particularly fertilin composed of ADAM1 and ADAM2 heterodimer concentrated on sperm membrane. After acrosome reaction equatorial segment exposes fertilin, which binds integrin alpha6beta1 on egg microvillar surface within CD9-enriched tetraspanin domains. This interaction stabilizes close apposition, promotes lipid bilayer merging and cytoplasmic continuity. While Izumo1-Juno provides primary adhesion, fertilin contributes fusogenic activity essential for completing membrane mixing and ensuring gamete coalescence during mammalian fertilization, subsequent zygote formation, activation cascade and monospermy establishment. This paradigm is central to understanding infertility and assisted fertilization.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Sperm-egg fusion – fertilin and ADAM proteins in gamete interaction.

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Fertilization in mammals-iv

ZP glycoprotein binding sperm (human):

Human zona pellucida contains four glycoproteins ZP1-ZP4 forming filamentous matrix. Primary binding of acrosome-intact sperm in humans involves ZP3 and ZP4, but enduring attachment of acrosome-reacted sperm requires secondary receptor ZP2, particularly its N-terminal domain. After fertilization ovastacin cleaves ZP2, destroying this binding site and preventing further sperm adhesion, contributing to zona block. Mouse ZP3 is traditionally called primary receptor, but human data highlight ZP2 as major ligand for reacted sperm. ZP1 is crosslinker structurally, ZP4 modulatory, so persistent secondary binding critical for species-specific gamete recognition centers on ZP2 integrity.

Ref: Gupta SK et al., Mol Hum Reprod 2012: Human ZP2 as secondary receptor for acrosome-reacted sperm and ZP2 cleavage block.

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Fertilization in mammals-iv

Deglycosylation of ZP3:

ZP3 sperm binding historically attributed to specific O-linked oligosaccharides, yet recent molecular studies show polypeptide backbone also participates. Chemically deglycosylated ZP3 or recombinant nonglycosylated C-terminal fragments retain ability to bind anterior head of capacitated sperm in vitro, though efficiency of acrosomal exocytosis induction decreases. This indicates protein-protein interactions contribute substantially and glycosylation modulates rather than absolutely determines binding. Species specificity arises from both peptide sequence divergence in putative sperm-binding region and glycan modifications, explaining why complete deglycosylation does not fully abolish sperm-egg recognition in many mammalian systems.

Ref: Clark GF et al., Mol Human Reprod 2010: ZP3 glycosylation and protein backbone roles in sperm-zona binding domain hypothesis.

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