ACRBP

ACRBP is an acrosomal matrix protein essential for male fertility that functions through two alternatively spliced isoforms with distinct roles in spermiogenesis and fertilization. ACRBP-V5 is required for acrosomal granule formation during early spermiogenesis, as demonstrated by knockout mice that exhibit fragmented acrosomes and severely reduced fertility, with transgenic rescue by ACRBP-V5 alone restoring acrosome biogenesis (PMID:27303034). The mature processed form, ACRBP-C (derived from ACRBP-W by PCSK4-dependent proteolytic cleavage of a ~60 kDa precursor), binds proacrosin to retain it in an inactive state within the acrosome and, upon acrosomal exocytosis, accelerates proacrosin autoactivation (PMID:8144514, PMID:23426433, PMID:22357636). ACRBP additionally undergoes tyrosine phosphorylation during capacitation and participates in sperm–zona pellucida binding and acrosome reaction priming (PMID:15955892, PMID:34086710).

1. 1994 High

   Identification of sp32 (ACRBP) as a proacrosin-binding protein that accelerates proacrosin autoactivation established the first molecular function for this acrosomal factor.

   Evidence Protein purification and in vitro binding/autoactivation assays using porcine sperm

   PMID:8144514

   Open questions at the time

   • Mechanism by which sp32 accelerates autoactivation unclear at molecular level
   • In vivo relevance of proacrosin autoactivation modulation not demonstrated
   • Post-translational processing from 61 kDa precursor not characterized
2. 2005 High

   Discovery that ACRBP is tyrosine-phosphorylated during capacitation linked this acrosomal protein to sperm signaling events that prime fertilization competence.

   Evidence 2D Western blot, mass spectrometry, reciprocal immunoprecipitation, and immunofluorescence in porcine sperm

   PMID:15955892

   Open questions at the time

   • Kinase responsible for ACRBP tyrosine phosphorylation not identified
   • Functional consequence of phosphorylation on proacrosin binding or activation not tested
   • Whether phosphorylation occurs in species other than pig not shown
3. 2012 Medium

   Demonstrating that PCSK4-null mice fail to process ACRBP from precursor to mature form and fail to activate proacrosin identified the proteolytic maturation pathway required for ACRBP function.

   Evidence 2D-DIGE, Western blot, and immunolocalization in PCSK4 knockout mouse sperm

   PMID:22357636

   Open questions at the time

   • Whether PCSK4 directly cleaves ACRBP or acts indirectly through another protease is unresolved
   • Exact cleavage site in ACRBP not mapped
   • Relationship between processing defect and acrosome morphology defects not mechanistically delineated
4. 2013 High

   Identification of two functional isoforms (ACRBP-V5 and ACRBP-C) that bind distinct proacrosin domains and have different subcellular distributions revealed that alternative splicing diversifies ACRBP function across spermiogenesis stages.

   Evidence Alternative splicing analysis, GST pull-down, in vitro proacrosin autoactivation assay, and immunolocalization in mouse spermatids and sperm

   PMID:23426433

   Open questions at the time

   • The functional significance of binding different proacrosin domains not mechanistically explained
   • Role of ACRBP-V5 in acrosomal granule formation was inferred from localization but not yet proven by loss-of-function
5. 2016 High

   Knockout and isoform-specific transgenic rescue proved that ACRBP-V5 is necessary and sufficient for acrosomal granule formation while ACRBP-W retains proacrosin in an inactive state, definitively assigning non-redundant functions to each isoform.

   Evidence ACRBP knockout mouse with ACRBP-V5 transgenic rescue, acrosome morphology analysis, fertility assay

   PMID:27303034

   Open questions at the time

   • Molecular mechanism by which ACRBP-V5 drives granule coalescence is unknown
   • Whether ACRBP-W rescue alone can restore fertility not tested
   • Structural basis for proacrosin retention by ACRBP-W not determined
6. 2018 Medium

   Demonstrating that ACRBP-null sperm reach the oviduct in reduced numbers and fail to access oocytes despite normal motility specified that the subfertility phenotype arises from acrosome reaction defects rather than transport failure.

   Evidence ACRBP knockout mouse with post-mating sperm counting and motility/morphology analysis in the female tract

   PMID:30606959

   Open questions at the time

   • Acrosome reaction kinetics not directly measured in vivo
   • Whether reduced oviductal numbers reflect impaired sperm reservoir binding not excluded
7. 2021 Medium

   Antibody-blocking experiments established that surface-localized ACRBP directly participates in sperm–zona pellucida binding and acrosome reaction priming, extending its function beyond an intracellular proacrosin regulator.

   Evidence Anti-ACRBP antibody inhibition in porcine IVF, sperm-ZP binding assay, acrosome reaction assay

   PMID:34086710

   Open questions at the time

   • ZP ligand for ACRBP not identified
   • Mechanism of ACRBP surface exposure not characterized
   • Whether ZP-binding role applies across species not tested

• Key unresolved questions include the structural basis of ACRBP–proacrosin interaction, the identity of the kinase mediating capacitation-dependent tyrosine phosphorylation, the mechanism by which ACRBP-V5 drives acrosomal granule coalescence, and whether surface ACRBP directly contacts zona pellucida glycoproteins.
• No structural model of ACRBP or its complex with proacrosin exists
• Tyrosine phosphorylation site(s) and responsible kinase unknown
• Mechanism of ACRBP-V5 function in granule biogenesis completely uncharacterized at molecular level