PPL

Periplakin is a plakin-family cytolinker protein that localizes to desmosomes and the cornified envelope of stratified squamous epithelia, where it heterodimerizes with envoplakin to scaffold cornified envelope assembly and bridges keratin intermediate filaments to junctional complexes (PMID:9412476). Beyond epithelia, periplakin is a component of the EPPD complex (ezrin, periplakin, periaxin, desmoyokin) at cortex adhaerens junctions of lens fiber cells, contributing to a junctional plaque distinct from the cadherin–catenin complex (PMID:14625392). Periplakin also interacts with the cytoplasmic di-leucine motif of BTN3A1, and this interaction is required for phosphoantigen-dependent activation of Vγ9/Vδ2 γδ T cells, linking the cytoskeleton to innate immune surveillance (PMID:25637025). Periplakin is a target of autoantibodies in paraneoplastic pemphigus (PMID:10379702).

1. 1997 High

   Identification of periplakin as a plakin-family protein that co-immunoprecipitates with envoplakin and localizes to desmosomes in differentiated epidermis established it as a structural scaffold for cornified envelope assembly.

   Evidence cDNA cloning, co-immunoprecipitation, immunogold EM, and confocal microscopy in human keratinocytes

   PMID:9412476

   Open questions at the time

   • No knockout or loss-of-function data to assess essentiality for cornified envelope integrity
   • Stoichiometry and direct binding surfaces of the envoplakin–periplakin heterodimer unresolved
   • No structural model for periplakin domain architecture
2. 1999 Medium

   Recognition of periplakin as an autoantigen in paraneoplastic pemphigus linked it to an autoimmune blistering disease targeting desmosomal components.

   Evidence Immunoprecipitation and autoantibody profiling of PNP patient sera

   PMID:10379702

   Open questions at the time

   • Single immunological method without demonstration of pathogenic role for anti-periplakin antibodies specifically
   • Whether anti-periplakin autoantibodies are causative or bystander is unknown
3. 2003 High

   Discovery of periplakin within the EPPD complex at lens fiber cell junctions extended its role beyond keratinizing epithelia, revealing participation in a specialized junctional plaque distinct from the classical cadherin–catenin complex.

   Evidence Reciprocal co-immunoprecipitation and immunolocalization across bovine, porcine, and rat lens tissue

   PMID:14625392

   Open questions at the time

   • Functional consequence of periplakin loss on lens fiber cell architecture not tested
   • Molecular basis of periplakin interaction with periaxin and desmoyokin within the EPPD complex undefined
4. 2015 High

   Demonstration that periplakin binds the cytoplasmic di-leucine motif of BTN3A1 and that this interaction is required for phosphoantigen-dependent Vγ9/Vδ2 γδ T cell activation revealed an unexpected role for a structural cytolinker in innate immune sensing.

   Evidence Yeast two-hybrid screen, BTN3A1-knockdown and rescue with wild-type versus periplakin-binding-deficient BTN3A1, co-culture γδ T cell activation assays

   PMID:25637025

   Open questions at the time

   • Mechanism by which periplakin facilitates BTN3A1-mediated antigen presentation (e.g., clustering, trafficking, cytoskeletal anchoring) not elucidated
   • Whether periplakin knockdown alone abrogates γδ T cell activation not directly shown
   • Structural basis of the periplakin–BTN3A1 interaction unresolved

• A unifying model of how periplakin's cytolinker and immune functions are coordinated — and whether periplakin is essential in vivo for either epithelial integrity or γδ T cell immunity — remains to be established.
• No periplakin-knockout animal phenotype reported in this literature
• No high-resolution structure of periplakin or its complexes
• Potential redundancy with envoplakin or other plakins not addressed