TSSK3

TSSK3 is a testis-specific serine/threonine kinase essential for spermiogenesis. Its catalytic activity requires PDK1-mediated phosphorylation of a T-loop threonine (Thr168), and mutation of this residue abolishes kinase function (PMID:16336268). TSSK3 localizes to the sperm tail in elongating spermatids and is expressed in Leydig cells at puberty, but is absent from mature sperm (PMID:10781952, PMID:36306217, PMID:20729278). CRISPR/Cas9 knockout of TSSK3 causes complete male sterility with spermiogenesis arrest, and phosphoproteomics identifies GAPDHS, ACTL7A, ACTL9, and REEP6 as in vivo phosphorylation substrates whose modification is required for normal sperm morphogenesis (PMID:34623009, PMID:36306217).

1. 2000 Medium

   Identification of TSSK3 as a testis-specific kinase in Leydig cells that does not complex with TSSK1/2 established it as a functionally distinct family member with a unique cellular context.

   Evidence Differential screening and co-immunoprecipitation in mouse testis

   PMID:10781952

   Open questions at the time

   • Expression only characterized in mouse; human ortholog not yet studied
   • Kinase activity not demonstrated
   • Biological function unknown
2. 2001 Medium

   Demonstration that the human TSSK3 ortholog (STK22C) is also testis-restricted confirmed evolutionary conservation of its expression pattern and mapped the gene to chromosome 1p34-p35.

   Evidence Northern blot, RT-PCR, and FISH on human tissues

   PMID:11597141

   Open questions at the time

   • Catalytic activity still not tested
   • No functional data in any species
   • Protein localization within the testis not resolved for germ cells
3. 2005 High

   Biochemical reconstitution revealed that TSSK3 is an active kinase whose catalytic activity depends on PDK1-mediated phosphorylation of T-loop Thr168, answering how TSSK3 is activated and identifying its first substrate specificity motif (RRSSSY).

   Evidence In vitro kinase assays with recombinant proteins, T168A mutagenesis, PDK1 phosphorylation reconstitution

   PMID:16336268

   Open questions at the time

   • In vivo substrates unknown
   • PDK1-dependent activation not validated in testis tissue
   • Physiological role remains untested
4. 2010 Medium

   Absence of TSSK3 from mature sperm and germ cell compartments by immunolocalization distinguished it from other TSSK family members and implied a transient or somatic role during spermatogenesis.

   Evidence Immunolocalization with validated antibodies and Western blot on mouse and human sperm

   PMID:20729278

   Open questions at the time

   • Could not detect TSSK3 in germ cells, conflicting with later sperm tail localization data
   • Loss-of-function phenotype still unknown
   • Antibody sensitivity limits may have precluded detection in certain cell types
5. 2021 High

   CRISPR/Cas9 knockout demonstrated that TSSK3 is indispensable for spermiogenesis, resolving the long-standing question of whether this kinase has a non-redundant physiological role — KO males are completely sterile with spermiogenesis arrest.

   Evidence Three independent CRISPR/Cas9 KO mouse lines, fertility testing, histology, and sperm analysis

   PMID:34623009

   Open questions at the time

   • Molecular substrates mediating the phenotype unidentified
   • Whether kinase activity or a scaffolding function underlies the phenotype is unresolved
   • Stage of spermiogenesis arrest not precisely defined
6. 2022 High

   Phosphoproteomics of KO testes identified GAPDHS, ACTL7A, ACTL9, and REEP6 as in vivo TSSK3-dependent phospho-targets, linking kinase activity to specific effectors of sperm morphogenesis and tail formation.

   Evidence Phosphoproteomics on purified germ cells from KO vs. heterozygous mice, immunolocalization to sperm tail

   PMID:36306217

   Open questions at the time

   • Direct phosphorylation of these substrates by TSSK3 not confirmed by in vitro reconstitution
   • Specific phosphosites on each substrate not functionally validated by mutagenesis
   • Mechanism linking substrate phosphorylation to sperm tail assembly or morphogenesis not elucidated

• It remains unknown whether TSSK3 directly phosphorylates the identified in vivo substrates, what specific phosphosites are functionally critical, how PDK1 activation of TSSK3 is regulated in the testis, and whether TSSK3 loss underlies human male infertility.
• No in vitro reconstitution of TSSK3-substrate phosphorylation for GAPDHS, ACTL7A, ACTL9, or REEP6
• PDK1-TSSK3 axis not validated in vivo
• No human genetic studies linking TSSK3 mutations to infertility