SPACA1

SPACA1 is a testis-specific transmembrane protein of the inner acrosomal membrane that anchors the developing acrosome to the sperm nucleus and is essential for normal sperm head morphogenesis during spermiogenesis (PMID:11870081, PMID:22949614). Knockout mice fail to form the nuclear plate and to expand the acrosome, producing globozoospermia-like, abnormally shaped sperm heads and male infertility, and a loss-of-function nonsense variant (p.Trp18*) causes globozoospermia in humans (PMID:22949614, PMID:34172998). SPACA1 carries out its anchoring role as a core membrane partner in a multi-protein 'inner acrosomal membrane–perinuclear theca–nuclear envelope' bridging system, interacting directly with the acroplaxome/perinuclear theca proteins ACTL7A, ACTRT1, calicin (CCIN), and cylicin-1, and with additional acrosome-associated factors MFSD6L, CCDC28A, and the CFAP65 cytoplasmic network; disruption of these partners weakens acrosome–nucleus attachment and deforms the sperm head (PMID:34172998, PMID:35616329, PMID:35793634, PMID:38573307, PMID:34231842, PMID:38909778, PMID:38597936). The protein is N-glycosylated, serine- and tyrosine-phosphorylated, and membrane raft-associated, and it migrates over the acrosomal surface during epididymal maturation (PMID:11870081, PMID:36821514, PMID:32507260). During the calcium-dependent acrosome reaction SPACA1 redistributes to the postacrosomal region and undergoes serine-protease-mediated proteolytic processing coincident with loss of acrosomal integrity, marking authentic acrosomal exocytosis (PMID:36821514, PMID:27449406).

1. 2002 High

   Established SPACA1 (SAMP32) as a testis-specific inner acrosomal membrane protein and implicated it directly in sperm–egg interaction, framing it as a gamete-fusion-relevant acrosomal factor.

   Evidence Triton X-114 partitioning, mass spectrometry, cDNA cloning, immunoelectron microscopy, and anti-recombinant antibody inhibition of sperm–egg binding/fusion in human sperm

   PMID:11870081

   Open questions at the time

   • Antibody inhibition does not define the molecular interaction partner on the egg side
   • Phosphorylation at serine 256 not linked to a specific kinase or function
2. 2012 High

   Defined the in vivo developmental requirement for SPACA1, showing it is needed for nuclear plate formation and acrosomal expansion rather than only for fusion.

   Evidence Spaca1 knockout mouse with histology, electron microscopy, and fertility assays showing globozoospermia-like heads

   PMID:22949614

   Open questions at the time

   • Molecular partners mediating nuclear plate formation not identified in this study
   • Mechanism linking acrosomal expansion failure to head deformity unresolved
3. 2021 High

   Connected SPACA1 to human disease and identified its first direct molecular partner, establishing the acrosome–acroplaxome anchoring mechanism.

   Evidence Human exome sequencing (p.Trp18*), reciprocal co-IP and yeast two-hybrid confirming SPACA1–ACTL7A interaction, plus proteomics and electron microscopy

   PMID:34172998

   Open questions at the time

   • Structural basis of the SPACA1–ACTL7A interaction unknown
   • Does not establish stoichiometry or full composition of the anchoring complex
4. 2021 Medium

   Placed SPACA1 within a larger cytoplasmic protein network governing acrosome biogenesis and proteostasis.

   Evidence Cfap65-knockout mice with endogenous immunoprecipitation, immunostaining, and proteomics identifying a CFAP65–MNS1–RSPH1–TPPP2–ZPBP1–SPACA1 network

   PMID:34231842

   Open questions at the time

   • Directness of SPACA1–CFAP65 contact within the network not resolved
   • Single lab; no reciprocal validation
5. 2022 Medium

   Defined the perinuclear theca side of the acrosome–nucleus bridge, showing SPACA1 participates in ACTRT1- and calicin-mediated connections.

   Evidence Actrt1- and Ccin-knockout mice with co-IP, immunofluorescence, and electron microscopy; loss of ACTRT1 weakens the ACTL7A–SPACA1 interaction

   PMID:35616329 PMID:35793634

   Open questions at the time

   • Order of assembly among ACTL7A, ACTRT1, and calicin around SPACA1 unclear
   • Calicin interaction from single lab
6. 2022 Medium

   Linked SPACA1 to additional acrosome-shaping factors whose dysfunction reduces SPACA1 expression.

   Evidence Co-immunoprecipitation, exome sequencing, western blotting, and proteomics showing FSIP2 interacts with SPACA1 and that FSIP2 mutation downregulates SPACA1

   PMID:35654582

   Open questions at the time

   • Direct vs. complex-mediated FSIP2–SPACA1 interaction not distinguished
   • Mechanism of SPACA1 downregulation upon FSIP2 loss unknown
7. 2024 Medium

   Extended the anchoring model with the cylicin-1 sandwich and additional membrane/coiled-coil partners required for head shaping.

   Evidence Cylc1-, Mfsd6l-, and Ccdc28a-knockout mice with co-IP, immunofluorescence, electron microscopy, and human variant analysis defining SPACA1 interactions with cylicin-1/FAM209, MFSD6L, and CCDC28A

   PMID:38573307 PMID:38597936 PMID:38909778

   Open questions at the time

   • Whether these partners form one super-complex or distinct sub-complexes around SPACA1 is unresolved
   • MFSD6L and CCDC28A interactions from single labs
8. 2023 High

   Characterized the post-translational state and regulated proteolysis of SPACA1, linking its cleavage to loss of acrosomal integrity.

   Evidence Boar spermatozoa studies using mass spectrometry, PNGase F, serine protease inhibition (STI), and CASR antagonist (NPS2143) showing N-glycosylation, tyrosine phosphorylation, and serine-protease-mediated p32 generation

   PMID:36821514

   Open questions at the time

   • Identity of the responsible serine protease not determined
   • Functional consequence of glycosylation/phosphorylation for anchoring untested
9. 2016 Medium

   Established SPACA1 redistribution and processing as a marker of the authentic, calcium-dependent acrosome reaction.

   Evidence Double immunofluorescence (anti-SPACA1/FITC-PNA), western blotting, and Ca2+-dependent acrosome reaction induction with damaged-acrosome controls in boar sperm

   PMID:27449406

   Open questions at the time

   • Functional role of postacrosomal redistribution not defined
   • Protease generating the 15–28 kDa fragments not identified
10. 2020 Medium

    Defined SPACA1 as membrane raft-associated and dynamically relocalized during epididymal maturation.

    Evidence Sucrose gradient fractionation, immunocytochemistry, and western blotting in bull spermatozoa correlating SPACA1 movement with acrosomal tyrosine-phosphorylated proteins

    PMID:32507260

    Open questions at the time

    • Mechanism driving equatorial-to-anterior translocation unknown
    • Causal link to tyrosine phosphorylation not established
11. 2025 Medium

    Showed SPACA1 is de novo translated in the sperm head during capacitation, distinguishing fertile from reduced-fertility sperm and implicating mitochondrial translation.

    Evidence FUNCAT metabolic labeling, proximity ligation assay, chloramphenicol inhibition, and capacitation proteome analysis in bull spermatozoa

    PMID:40112915

    Open questions at the time

    • Mechanism and template of sperm-localized SPACA1 translation unresolved
    • Functional importance of capacitation-stage synthesis untested

• How the multiple SPACA1 partner interactions are spatially and temporally organized into a single anchoring apparatus, and what defines its assembly hierarchy and stoichiometry, remains unresolved.
• No structural model of the IAM–perinuclear theca–nuclear envelope complex
• Assembly order among ACTL7A, ACTRT1, calicin, cylicin-1, MFSD6L, CCDC28A, and CFAP65 unknown
• Direct vs. indirect nature of several partner interactions not all reciprocally validated