ACTRT1

ACTRT1 encodes actin-related protein ARP-T1, which functions as a structural scaffold in the sperm acroplaxome and as a transcriptional repressor of Hedgehog signaling in somatic cells. In spermatids, ACTRT1 forms a multimeric complex with ACTRT2, ACTL7A, and ACTL9 in the subacrosomal perinuclear theca and anchors the developing acrosome to the sperm nucleus through interactions with the inner acrosomal membrane protein SPACA1 and nuclear envelope proteins PARP11 and SPATA46; loss of ACTRT1 causes acrosome detachment, malformed sperm heads, and male infertility in mice and humans (PMID:35616329, PMID:34422805, PMID:38414365). In somatic cells, ARP-T1 directly binds the GLI1 promoter to repress Hedgehog pathway transcription, and loss-of-function mutations lead to aberrant GLI1 activation underlying Bazex-Dupré-Christol syndrome and basal cell carcinoma susceptibility (PMID:28869610). ACTRT1 also localizes to the basal body of primary cilia and regulates ciliary length, with knockdown or patient mutations resulting in shortened cilia (PMID:33972689).

1. 2002 Medium

   The identity and sperm-specific localization of ARP-T1 were unknown; protein sequencing and immunostaining established ACTRT1 as a novel actin-related protein of the perinuclear theca (calyx) of mammalian spermatids, defining its structural context.

   Evidence Partial amino acid sequencing, immunoblotting, and immunofluorescence of calyx protein fractions from mammalian sperm

   PMID:12243744

   Open questions at the time

   • No functional studies performed; role of ARP-T1 in sperm biology was unknown
   • Binding partners within the calyx were not identified
   • Expression and function in somatic cells were not explored
2. 2017 High

   The somatic function of ACTRT1 was uncharacterized; ChIP and functional rescue experiments demonstrated that ARP-T1 directly binds the GLI1 promoter to repress Hedgehog pathway transcription, linking ACTRT1 loss-of-function to Bazex-Dupré-Christol syndrome and basal cell carcinoma.

   Evidence ChIP for direct GLI1 promoter binding; loss-of-function mutations with GLI1 expression readout; in vitro and in vivo proliferation assays with ACTRT1 re-expression

   PMID:28869610

   Open questions at the time

   • Mechanism by which an actin-related protein binds DNA and represses transcription was not resolved
   • Structural basis for ARP-T1 interaction with the GLI1 promoter was not determined
   • Whether transcriptional repression extends to other Hedgehog target genes was not tested
3. 2021 Medium

   The subcellular localization and function of ACTRT1 in non-germ somatic cells beyond transcriptional repression were unclear; localization studies revealed ARP-T1 at the basal body of primary cilia and midbody during cytokinesis, and knockdown demonstrated a role in regulating ciliary length.

   Evidence Immunofluorescence for localization; mass spectrometry interactome; siRNA knockdown with ciliary length measurement; analysis of patient tissue from BDCS individuals

   PMID:33972689

   Open questions at the time

   • The interactome was identified by mass spectrometry without extensive reciprocal validation of individual interactions
   • Mechanism by which basal body-localized ARP-T1 controls ciliary length was not established
   • Relationship between ciliary length regulation and Hedgehog pathway repression was not resolved
4. 2021 Medium

   Whether ACTRT1 loss causes human male infertility was unknown; whole-exome sequencing identified whole-gene ACTRT1 deletions in men with acephalic spermatozoa syndrome, with acrosomal detachment and fertilization failure mirroring the Actrt1-KO mouse.

   Evidence Whole exome sequencing with Sanger confirmation; western blot and immunostaining for ACTL7A and PLCζ; comparison with Actrt1-KO mouse phenotype

   PMID:34422805

   Open questions at the time

   • Observed in a small number of probands from a single study
   • Whether partial loss-of-function mutations also cause infertility was not addressed
   • Molecular rescue in human sperm was not attempted
5. 2022 High

   The molecular composition and structural role of the ACTRT1 complex in spermatids were undefined; co-IP and knockout studies established that ACTRT1 forms a multimeric complex with ACTRT2, ACTL7A, and ACTL9 and bridges the acrosome to the nucleus via interactions with SPACA1, PARP11, and SPATA46.

   Evidence Co-immunoprecipitation for complex identification; Actrt1-knockout mouse with phenotypic, immunofluorescence, and western blot analyses

   PMID:35616329

   Open questions at the time

   • Stoichiometry and structure of the ACTRT1-containing complex are unknown
   • Whether ACTRT1 directly binds each partner or acts through ACTL7A was not fully distinguished
   • How the complex mechanically resists acrosomal detachment forces is not resolved
6. 2024 Medium

   Independent replication in human patients confirmed that ACTRT1 whole-gene deletion causes acrosomal ultrastructural defects with decreased ACTL7A and PLCζ expression, and demonstrated that ICSI with artificial oocyte activation can rescue fertilization failure.

   Evidence WES confirmed by WGS and PCR; electron microscopy of sperm ultrastructure; western blot and immunostaining

   PMID:38414365

   Open questions at the time

   • Number of reported families remains small
   • Genotype-phenotype correlation for hypomorphic alleles is not established
   • Whether PLCζ reduction is a direct or indirect consequence of ACTRT1 loss is unknown

• The structural basis for ARP-T1's dual roles — as a cytoskeletal scaffold in spermatid acroplaxomes and as a DNA-binding transcriptional repressor in somatic cells — remains unresolved, and no structure of the ACTRT1-containing complex or ARP-T1–DNA interface has been determined.
• No crystal or cryo-EM structure of ACTRT1 or its complexes exists
• The DNA-binding domain or motif in ARP-T1 is unidentified
• How basal body localization, ciliary length regulation, and GLI1 promoter repression are mechanistically coordinated is unknown