STK32A

STK32A is an N-myristoylated serine/threonine kinase (PMID:26308446) that controls the directional polarity of sensory hair cells and the suppression of sensory and motor neuronal activity. In the inner ear, STK32A is transcriptionally repressed by EMX2, so that it is expressed only in hair cells on one side of the line of polarity reversal, where it aligns the intrinsic polarity of the stereociliary bundle with core PCP proteins by regulating the apical localization of the receptor GPR156 (PMID:37144879). Genetic epistasis establishes the pathway logic: EMX2 sets the boundary between the two hair-cell groups by repressing Stk32a, while STK32A blocks the bundle reorientation otherwise driven by GPR156 (PMID:41208475). In a separate neuronal context, STK32A acts downstream of neurotensin signaling and the serotonergic raphe—and downstream of hypothalamic Pth4/QRFP circuitry—to suppress sensory and motor neuron activity, phosphorylate neurofilament proteins, and constrain sleep, with loss of function increasing sleep and impairing sleep homeostasis [PMID:bio_10.1101_2025.09.09.675098, PMID:41406966]. STK32A also supports NF-κB p65 phosphorylation downstream of RUNX2 in non-small-cell lung cancer cells, where it promotes proliferation, migration, invasion, and EMT (PMID:32571328). Direct biochemical substrates and the catalytic basis of its kinase activity have not been characterized in the available corpus.

1. 2015 Medium

   Whether the STK32A gene product is post-translationally lipidated was unknown; establishing N-myristoylation defined a membrane-targeting modification on the kinase.

   Evidence Cell-free insect protein synthesis and metabolic labeling in transfected human cells with N-terminal fusion assay

   PMID:26308446

   Open questions at the time

   • No functional consequence of the myristoylation demonstrated for STK32A
   • Single lab, no localization readout tied to the modification
   • Catalytic activity and substrates not addressed
2. 2020 Medium

   STK32A's role in cancer was undefined; placing it downstream of RUNX2 and upstream of NF-κB p65 phosphorylation connected it to NSCLC tumor progression.

   Evidence Luciferase reporter, western blot, proliferation/migration/invasion assays, and xenografts in NSCLC cells

   PMID:32571328

   Open questions at the time

   • RUNX2-STK32A interaction shown by co-expression and rescue, not direct binding
   • Whether STK32A directly phosphorylates p65 or acts indirectly is unresolved
   • Single lab
3. 2023 High

   How hair cells on opposite sides of the line of polarity reversal acquire opposite orientations was unclear; STK32A was identified as the EMX2-repressed effector that aligns bundle polarity with PCP proteins via GPR156.

   Evidence Mouse Stk32a knockout and ectopic expression, bundle orientation imaging, and epistasis with Emx2 and Gpr156

   PMID:37144879

   Open questions at the time

   • Molecular mechanism by which STK32A controls GPR156 apical localization unknown
   • Direct kinase substrates in hair cells not identified
4. 2025 High

   The order of action among EMX2, STK32A, and GPR156 was not formally established; compound-mutant epistasis confirmed EMX2 represses Stk32a to set the group boundary and STK32A blocks GPR156-driven reorientation, in both maculae and cochlea.

   Evidence Genetic epistasis in Gpr156;Stk32a and Emx2;Stk32a double mutant mice with bundle orientation and PCP immunofluorescence

   PMID:41208475

   Open questions at the time

   • Biochemical link between STK32A and GPR156 still undefined
   • Phosphorylation events underlying the block on reorientation not mapped
5. 2025 Medium

   A neuronal role for STK32A was unknown; loss of function showed it suppresses sensory/motor activity downstream of neurotensin and the raphe and constrains sleep, acting in part through neurofilament phosphorylation.

   Evidence Zebrafish forward genetic screen, sleep/wake assays in fish and mice, phosphoproteomics, cell ablation, and optogenetics/calcium imaging (preprint)

   PMID:bio_10.1101_2025.09.09.675098

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Whether STK32A directly phosphorylates neurofilament proteins not established
   • Mechanistic link between neurotensin signaling and STK32A activation unknown
6. 2025 Medium

   The circuit context of STK32A's sleep function was incomplete; it was placed downstream of hypothalamic Pth4/QRFP signaling via prethalamic stk32a neurons expressing parathyroid hormone receptors.

   Evidence Genetic epistasis in zebrafish stk32a mutants with optogenetic Pth4 neuron stimulation and behavioral sleep assays

   PMID:41406966

   Open questions at the time

   • Signal transduction from PTH receptors to STK32A activity not defined
   • Single lab, single publication

• Whether the hair-cell polarity and neuronal-suppression roles share a common biochemical substrate set, and the direct catalytic targets of STK32A in any context, remain unknown.
• No direct kinase substrate biochemically validated
• No structural model of the kinase domain
• Mechanism coupling STK32A to GPR156 trafficking vs. neurofilament phosphorylation unresolved