ACTL9

ACTL9 is a testis-specific actin-related protein that localizes to the subacrosomal perinuclear theca of spermatids, where it forms a multimeric complex with ACTRT1, ACTRT2, and ACTL7A to anchor the acrosome to the nucleus (PMID:35616329). ACTL9 also interacts with CABS1, and disruption of this interaction leads to acrosome absence, abnormal PLCζ localization, and total fertilization failure (PMID:40407971). Beyond acrosomal attachment, ACTL9 is required for proper flagellar axonemal organization and mitochondrial sheath assembly, and homozygous loss-of-function mutations cause asthenoteratozoospermia and fertilization failure in both humans and knock-in mouse models (PMID:38963606). ACTL9 undergoes TSSK3-mediated phosphorylation in germ cells, indicating post-translational regulation of its function during spermiogenesis (PMID:36306217).

1. 2022 High

   Identifying ACTL9 as part of a subacrosomal multimeric complex with ACTRT1, ACTRT2, and ACTL7A established its primary molecular function: mediating the physical connection between the acrosome and the nucleus in spermatids.

   Evidence Co-immunoprecipitation, immunostaining, and genetic knockout mouse model in Development

   PMID:35616329

   Open questions at the time

   • Stoichiometry and assembly order of the ACTL9–ACTRT1–ACTRT2–ACTL7A complex are unknown
   • No structural model of the complex exists
   • Whether ACTL9 has intrinsic actin-like ATPase or filament-forming activity has not been tested
2. 2022 Medium

   Phosphoproteomic analysis of Tssk3-knockout germ cells revealed ACTL9 as a TSSK3 phosphorylation substrate, establishing a post-translational regulatory axis for the subacrosomal complex during spermiogenesis.

   Evidence Phosphoproteomics of purified germ cells from Tssk3-KO vs. heterozygous mice

   PMID:36306217

   Open questions at the time

   • The specific phosphorylation site(s) on ACTL9 and their functional consequences have not been determined
   • Whether TSSK3-mediated phosphorylation modulates ACTL9 complex assembly or stability is untested
   • No in vitro kinase assay confirming direct phosphorylation
3. 2024 Medium

   Discovery that homozygous ACTL9 mutations cause flagellar ultrastructural defects—irregular mitochondrial sheath and abnormal 9+2 axoneme—expanded ACTL9's role beyond acrosomal anchoring to flagellar assembly and sperm motility.

   Evidence Whole-exome sequencing, TEM/SEM of patient and knock-in mouse sperm, CASA motility analysis

   PMID:38963606

   Open questions at the time

   • Whether ACTL9 acts directly in flagellar assembly or the defect is secondary to perinuclear theca disruption is unclear
   • Single-lab study; independent replication in additional cohorts not yet reported
   • Molecular partners of ACTL9 in the flagellar compartment are unidentified
4. 2024 Medium

   Identification of CABS1 as an ACTL9-interacting partner, and demonstration that a CABS1 nonsense mutation disrupts this interaction and causes acrosome absence with fertilization failure, linked the ACTL9 interaction network to oocyte activation (via PLCζ mislocalization).

   Evidence Co-IP of wild-type vs. mutant CABS1 with ACTL9, immunofluorescence, western blot

   PMID:40407971

   Open questions at the time

   • Whether ACTL9 directly recruits PLCζ or the effect is indirect through CABS1 is unknown
   • Single study without reciprocal ACTL9 perturbation to confirm directionality
   • Binding domain mapping between ACTL9 and CABS1 has not been performed

• It remains unknown whether ACTL9 possesses intrinsic actin-like biochemical activities (ATPase, polymerization), how TSSK3 phosphorylation regulates complex dynamics, and whether ACTL9's flagellar role is direct or secondary to perinuclear theca disruption.
• No in vitro biochemical characterization of ACTL9
• No high-resolution structural data for ACTL9 or its complexes
• Causal relationship between ACTL9 loss and flagellar vs. acrosomal defects not dissected