ZPBP

ZPBP (also called sp38/IAM38) is an intra-acrosomal structural protein essential for acrosome biogenesis during spermiogenesis and for secondary sperm–zona pellucida binding during fertilization. Synthesized as a precursor that undergoes post-translational processing including PDIA6-catalyzed disulfide bond formation, ZPBP traffics from proacrosomal granule dense cores to the extracellular face of both inner and outer acrosomal membranes, where it mediates acrosome compaction through a zipper-like mechanism in the equatorial segment (PMID:7729589, PMID:19204925, PMID:41654813). After the acrosome reaction, ZPBP is retained on the inner acrosomal membrane and participates in calcium-dependent secondary zona pellucida binding, competing with proacrosin for ZP glycoprotein interaction (PMID:8216293, PMID:16386726). Loss of ZPBP1 in mice causes acrosome fragmentation and globozoospermia-type sterility, and homozygous nonsense mutations in human ZPBP cause globozoospermia, establishing it as a male infertility gene (PMID:17664285, PMID:31985809).

1. 1993 High

   Identification of sp38 as a sperm protein that directly and calcium-dependently binds zona pellucida glycoproteins established the first molecular function for ZPBP—zona pellucida recognition—and revealed competition with proacrosin for the same binding site.

   Evidence Protein purification from porcine epididymal sperm with ZP binding and competitive inhibition assays

   PMID:8216293

   Open questions at the time

   • Binding stoichiometry and affinity constants not determined
   • Specific ZP glycoprotein partner identity not resolved
   • Relevance beyond porcine species not tested
2. 1995 High

   Cloning of sp38 revealed it is synthesized as a precursor and post-translationally processed, localizes intra-acrosomally, and identified an 11-residue peptide critical for ZP binding, providing the first structure–function mapping of its ZP-recognition domain.

   Evidence cDNA cloning, immunostaining, and synthetic peptide inhibition of radiolabeled sp38 binding in porcine sperm

   PMID:7729589

   Open questions at the time

   • Nature of post-translational processing enzymes unknown
   • No crystal structure of the ZP-binding domain
   • Function of removed N-terminal propeptide unclear
3. 2005 High

   Demonstration that ZPBP1 is the dominant inner acrosomal membrane coat protein retained after the acrosome reaction, and that anti-ZPBP antibodies block IVF, established its role in secondary (post-acrosome reaction) zona binding rather than initial sperm-egg recognition.

   Evidence Biochemical fractionation, immunoelectron microscopy, and antibody blocking of bovine IVF

   PMID:16386726

   Open questions at the time

   • Whether ZPBP1 is sufficient for secondary binding or requires co-factors not addressed
   • Mechanism of retention on the inner acrosomal membrane not defined
4. 2007 High

   Gene knockout of Zpbp1 proved it is essential for acrosome compaction during spermiogenesis, not merely for zona binding; Zpbp1-null males are sterile with round-headed sperm and fragmented acrosomes, while the paralog Zpbp2 plays a cooperative but distinct structural role.

   Evidence Zpbp1 and Zpbp2 knockout mouse models with fertility assays and electron microscopy

   PMID:17664285

   Open questions at the time

   • Molecular mechanism of acrosome compaction (e.g., protein–protein bridging) not resolved
   • Functional redundancy between paralogs not fully delineated
   • Signaling pathways controlling ZPBP1 expression during spermiogenesis unknown
5. 2009 Medium

   Immunoelectron microscopy tracking of ZPBP1 through spermiogenesis revealed its sequential trafficking from proacrosomal granule cores to the extracellular face of both acrosomal membranes, supporting a zipper-like compaction model at the equatorial segment.

   Evidence Immunoelectron microscopy with anti-IAM38 antibodies across spermiogenesis stages in bull testis

   PMID:19204925

   Open questions at the time

   • Sorting signals directing ZPBP1 to the extracellular acrosomal face not identified
   • Zipper mechanism inferred from localization, not directly tested biophysically
   • Single-lab observation
6. 2011 Medium

   Discovery of ZPBP1 mutations in infertile men with abnormal sperm morphology linked the gene to human male infertility, suggesting dominant-negative effects through altered protein folding.

   Evidence cDNA sequencing of 381 teratozoospermic patients versus 240 controls

   PMID:21911476

   Open questions at the time

   • Dominant-negative mechanism not experimentally validated
   • Functional impact of individual missense variants not tested in vitro or in vivo
   • Relatively low mutation frequency (3.9%) limits generalizability
7. 2018 Medium

   Identification of ZPBP in multiprotein complexes with testisin (PRSS21), ZAN, and acrosin on the sperm surface revealed that zona pellucida binding involves an organized protein complex rather than isolated ligand–receptor pairs.

   Evidence Blue Native PAGE, co-immunoprecipitation, and mass spectrometry from stallion spermatozoa

   PMID:30549223

   Open questions at the time

   • Complex stoichiometry and assembly order not determined
   • Direct binary interactions within the complex not mapped
   • Single species (equine)
8. 2020 Medium

   A homozygous nonsense ZPBP mutation causing globozoospermia in a human patient provided definitive genetic proof that ZPBP is required for acrosome integrity in humans, mirroring the mouse knockout phenotype.

   Evidence Exome sequencing with familial segregation analysis and immunohistochemistry in human testis

   PMID:31985809

   Open questions at the time

   • Single family; additional families or rescue experiments would strengthen causality
   • Whether residual ZPBP2 function modifies severity not addressed
9. 2021 Medium

   Identification of ZPBP1 in a cytoplasmic CFAP65-containing network in spermatids expanded the known interactome beyond zona-binding partners to include acrosome biogenesis scaffold components.

   Evidence Endogenous co-immunoprecipitation and immunostaining in Cfap65 knockout and wild-type mouse testis

   PMID:34231842

   Open questions at the time

   • Whether CFAP65 directly binds ZPBP1 or association is indirect not resolved
   • Functional consequence of CFAP65 loss on ZPBP1 levels/localization not fully quantified
10. 2022 Medium

    Multiple acrosomal proteases (PRSS54, ACTL7A) were shown to interact with ZPBP and to be required for its proper localization, revealing that acrosome integrity depends on coordinated protein–protein interactions that position ZPBP correctly.

    Evidence Co-immunoprecipitation and knockout mouse models (Prss54-/-, Actl7a-/-) with immunofluorescence and ultrastructural analysis

    PMID:35863763 PMID:35921706

    Open questions at the time

    • Whether PRSS54 proteolytically processes ZPBP or acts as a non-catalytic scaffold unclear
    • Direct versus indirect nature of ACTL7A–ZPBP interaction not resolved
11. 2024 Medium

    SPEM2 was identified as a regulator of ZPBP processing and maturation in epididymal sperm, connecting ZPBP function to post-testicular sperm maturation events.

    Evidence Co-immunoprecipitation and Spem2 knockout mouse fertility assays with immunofluorescence

    PMID:38421455

    Open questions at the time

    • Precise processing step catalyzed or facilitated by SPEM2 not defined
    • Whether SPEM2 acts directly on ZPBP or indirectly through other proteases unknown
12. 2026 Medium

    ACTRT3 was identified as a perinuclear theca scaffold component that positions ZPBP on the sperm surface, and PDIA6 was shown to catalyze disulfide bond formation necessary for ZPBP synthesis, revealing both structural scaffolding and oxidative folding as upstream requirements for ZPBP function.

    Evidence Co-immunoprecipitation with Actrt3 and conditional Pdia6 knockout mouse models, proteomic quantitation, and electron microscopy

    PMID:41654813 PMID:41668650

    Open questions at the time

    • Specific disulfide bonds in ZPBP that require PDIA6 not mapped
    • Whether ACTRT3 is a direct ZPBP-binding partner or acts through intermediate scaffolds not fully resolved
    • Structural basis of perinuclear theca–acrosome connection unknown

• The atomic-level mechanism by which ZPBP mediates acrosome compaction and ZP binding remains unresolved: no high-resolution structure exists, the specific ZP glycoprotein partner in humans is undefined, and the assembly order and regulation of the acrosomal ZPBP-containing complex are unknown.
• No crystal or cryo-EM structure of ZPBP or its ZP-binding domain
• Identity of the human ZP glycoprotein bound by ZPBP not established
• Signaling pathways regulating ZPBP expression and trafficking during spermiogenesis undefined