SPATA46

SPATA46 is a transmembrane nuclear envelope protein expressed in condensed spermatids that localizes to the subacrosomal region, where it is essential for maintaining nuclear envelope integrity, sperm head morphology, and sperm-egg fusion during spermiogenesis; knockout mice display discontinuous nuclear envelope, nuclear cavities, abnormal head shape, and fertilization failure (PMID:27488028). SPATA46 bridges the nuclear envelope to the developing acrosome by interacting with the inner acrosomal membrane protein SPACA1 and the perinuclear theca protein ACTRT1, thereby anchoring the acrosome to the nucleus (PMID:35616329).

1. 2016 High

   Whether the uncharacterized spermatid protein SPATA46 has a functional role in spermiogenesis was unknown; knockout and antibody-blocking experiments established it as a transmembrane nuclear envelope protein required for nuclear envelope integrity, sperm head shaping, and sperm-egg fusion.

   Evidence Spata46-knockout mouse model with immunolocalization, subcellular fractionation, antibody inhibition, and IVF assays

   PMID:27488028

   Open questions at the time

   • Molecular partners mediating SPATA46 function at the nuclear envelope were not identified
   • Mechanism by which SPATA46 loss disrupts sperm-egg fusion is unclear
   • Whether SPATA46 deficiency causes male infertility in humans was not tested
2. 2022 Medium

   How the acrosome is physically anchored to the spermatid nucleus was unresolved; co-immunoprecipitation in the context of Actrt1-knockout phenotyping revealed that SPATA46 interacts with ACTRT1 and SPACA1 to bridge the nuclear envelope to the inner acrosomal membrane.

   Evidence Co-immunoprecipitation and Actrt1-knockout mouse phenotypic analysis

   PMID:35616329

   Open questions at the time

   • Interaction validated by a single Co-IP without reciprocal pull-down or direct binding assay
   • Stoichiometry and structural basis of the SPATA46–ACTRT1–SPACA1 complex are unknown
   • Whether additional partners participate in acrosome–nucleus anchoring remains untested

• The precise mechanism by which SPATA46 loss leads to failure of sperm-egg fusion — independent of its structural role in head shaping — has not been dissected, nor have causative mutations been identified in human male infertility.
• No human genetic data linking SPATA46 mutations to infertility
• No structure of SPATA46 or its complexes
• Functional separation of SPATA46's role in nuclear envelope integrity versus sperm-egg fusion has not been achieved