KRT9

KRT9 is a type I intermediate filament keratin expressed exclusively in the suprabasal keratinocytes of palmoplantar epidermis, where it heterodimerizes with KRT6C via coiled-coil rod domain interactions to form cytoskeletal filaments essential for mechanical resilience (PMID:37467889). Pathogenic missense mutations clustering in the helix initiation motif (coil 1A) of the rod domain cause epidermolytic palmoplantar keratoderma (EPPK), with R162W representing a recurrent mutational hotspot conserved across type I keratins (PMID:7512862, PMID:15564199). Beyond its structural role, KRT9 interacts with 14-3-3σ to sequester YAP1 in the cytoplasm of differentiating keratinocytes; loss of KRT9 leads to aberrant nuclear YAP1 accumulation and hyperkeratosis that is rescued by YAP1 inhibition in vivo. KRT9 also participates in innate antiviral defense by binding GBP5, facilitating GBP5-mediated transport of RSV-SH protein into microvesicles and thereby suppressing respiratory syncytial virus replication (PMID:39835811).

1. 1994 High

   Identification of KRT9 as the causative gene for EPPK established that coil 1A rod domain integrity is essential for palmoplantar keratin function, answering which gene underlies this site-restricted genodermatosis.

   Evidence Sequencing of EPPK patients in multiple unrelated families identified missense mutations (N160K, R162Q, R162W) in the coil 1A helix initiation motif; confirmed by linkage in an independent French pedigree

   PMID:7511021 PMID:7512862

   Open questions at the time

   • Heterodimer partner of KRT9 in vivo was unknown
   • No animal model to assess phenotype of total KRT9 loss
   • Mechanism by which rod domain mutations disrupt filament assembly not resolved at atomic level
2. 1998 High

   Systematic survey of EPPK kindreds confirmed that all pathogenic KRT9 mutations map exclusively to the helix initiation motif and that KRT9 expression is restricted to suprabasal palmoplantar keratinocytes, defining the genotype-phenotype constraint.

   Evidence Direct sequencing across multiple EPPK families identifying novel mutations M156T/M156V; epidemiological survey of expression pattern

   PMID:9856842

   Open questions at the time

   • Mutations outside the rod domain had not been evaluated for pathogenicity
   • No functional assay to test mutant filament assembly in vitro
3. 2004 Medium

   Confirmation of R162W as a recurrent mutational hotspot across geographically diverse families reinforced the functional criticality of this conserved arginine in the coil 1A domain.

   Evidence RT-PCR and sequencing in five unrelated EPPK families from multiple ethnic backgrounds

   PMID:15564199

   Open questions at the time

   • No in vitro assembly assay comparing wild-type vs R162W filaments
   • No structural explanation for why arginine at this position is indispensable
4. 2013 Medium

   Discovery that a mutation outside the rod domain (L11V) causes EPPK by activating a cryptic splice site revealed a splicing-mediated pathogenic mechanism, expanding the mutational landscape beyond missense changes in coil 1A.

   Evidence RT-PCR demonstrating aberrant splicing product with 162-amino-acid deletion in patient keratinocytes

   PMID:23397986

   Open questions at the time

   • Single family; frequency of splice-site-mediated EPPK unknown
   • Effect of truncated protein on filament network not directly visualized
5. 2016 Medium

   Allele-specific siRNA silencing of the R163W mutant demonstrated that selective knockdown of the dominant-negative allele is feasible, establishing proof-of-concept for a therapeutic strategy targeting EPPK at the mRNA level.

   Evidence Transfection of wild-type and R163W KRT9 into HaCaT cells with allele-specific siRNA, qPCR and western blot readout

   PMID:27864007

   Open questions at the time

   • Performed in an immortalized cell line, not primary palmoplantar keratinocytes
   • No in vivo delivery or phenotype rescue demonstrated
6. 2023 Medium

   Identification of KRT6C as the endogenous heterodimer partner of KRT9 resolved a long-standing question about which type II keratin pairs with KRT9 in palmoplantar epidermis.

   Evidence Co-immunoprecipitation coupled with mass spectrometry from native palmoplantar tissue; coiled-coil modeling of KRT6C/KRT9 2B domains

   PMID:37467889

   Open questions at the time

   • No reciprocal IP or in vitro reconstitution of heterodimer
   • Functional consequence of disrupting the KRT6C–KRT9 pair specifically is untested
   • Single lab; awaits independent validation
7. 2025 Medium

   KRT9 was shown to function in innate antiviral immunity by interacting with GBP5 to facilitate transport of RSV-SH protein into microvesicles, establishing a non-structural role for this keratin.

   Evidence Affinity mass spectrometry, co-IP, domain mapping, and siRNA knockdown with functional readout of RSV replication

   PMID:39835811

   Open questions at the time

   • KRT9 is palmoplantar-restricted, so physiological relevance of antiviral function in vivo needs clarification
   • Mechanism of microvesicle cargo loading by KRT9–GBP5 not defined
   • Single lab finding; not independently replicated
8. 2025 High

   KRT9 was found to regulate YAP1 cytoplasmic sequestration via 14-3-3σ interaction, directly linking intermediate filament identity to Hippo pathway control and explaining the hyperproliferative pathology in EPPK.

   Evidence Krt9 knockout mice, co-IP of K9 with 14-3-3σ, YAP1 immunofluorescence, in vivo rescue by YAP1 inhibition (preprint)

   Open questions at the time

   • Preprint; not yet peer-reviewed
   • Whether EPPK patient mutations uniformly disrupt 14-3-3σ binding quantitatively is not fully characterized
   • Downstream transcriptional targets of nuclear YAP1 in palmoplantar keratinocytes not defined

• Key open questions include the atomic-resolution structure of the KRT9–KRT6C heterodimer, the full spectrum of signaling functions mediated by the KRT9–14-3-3σ complex, and whether the antiviral role of KRT9 via GBP5 operates in tissues beyond its canonical palmoplantar expression domain.
• No crystal or cryo-EM structure of KRT9-containing intermediate filaments
• In vivo relevance of KRT9–GBP5 antiviral axis unresolved
• Whether YAP1 sequestration is the sole signaling function of KRT9 is unknown