EXPH5

EXPH5 (Slac2-b/exophilin-5) is a Rab27 effector that couples intracellular vesicle trafficking to exocytosis and cell adhesion. It binds specifically to GTP-bound Rab27A/B through its Slp homology domain (SHD) on secretory granules, and disruption of this interaction inhibits regulated exocytosis in secretory cells (PMID:15039459). EXPH5 mediates docking of multivesicular endosomes (MVEs) at the plasma membrane to promote exosome secretion, as its silencing causes perinuclear MVE redistribution and reduced exosome release (PMID:19966785). In keratinocytes, EXPH5 coordinates trafficking of CD63+ extracellular vesicles containing extracellular matrix proteins to control focal adhesion dynamics and cell–matrix adhesion, and loss-of-function mutations in EXPH5 cause inherited epidermolysis bullosa simplex with keratin cytoskeleton disruption and skin fragility (PMID:23176819, PMID:32890627).

1. 2004 High

   Establishing EXPH5 as a GTP-Rab27-specific effector answered how Rab27 couples to downstream vesicle trafficking machinery: Slac2-b binds GTP-Rab27A/B via its SHD and this interaction is required for stimulated exocytosis in secretory acinar cells.

   Evidence Co-immunoprecipitation, subcellular fractionation, and permeabilized cell secretion assay with recombinant SHD competition and blocking antibodies in parotid acinar cells

   PMID:15039459

   Open questions at the time

   • The downstream effector mechanism linking Rab27–EXPH5 binding to vesicle fusion was not identified
   • Whether EXPH5 functions in non-secretory cell types was unknown
2. 2006 Medium

   Demonstrating that the Slac2-b SHD quantitatively captures GTP-Rab27 in platelets confirmed the generality of the EXPH5–Rab27 interaction across secretory cell types and revealed that resting Rab27 is predominantly GTP-loaded.

   Evidence Recombinant SHD pull-down assay and thin-layer chromatography of Rab27-bound nucleotides in permeabilized platelets

   PMID:16880209

   Open questions at the time

   • EXPH5 was used as a biochemical tool rather than the primary subject; its endogenous role in platelets was not tested
   • No direct assessment of EXPH5 loss-of-function in platelet granule secretion
3. 2009 High

   Placing EXPH5 in the exosome secretion pathway resolved the question of which Rab27b effector mediates MVE docking at the plasma membrane: silencing EXPH5 phenocopied Rab27b loss, causing perinuclear MVE accumulation and reduced exosome release.

   Evidence RNAi screen in HeLa cells with exosome secretion quantification and MVE localization imaging

   PMID:19966785

   Open questions at the time

   • The molecular mechanism by which EXPH5 tethers MVEs to the plasma membrane was not determined
   • Whether EXPH5 acts alone or requires additional tethering/SNARE factors was unresolved
4. 2012 High

   Discovery that EXPH5 loss-of-function mutations cause epidermolysis bullosa simplex established the gene's non-redundant role in keratinocyte adhesion and linked its vesicle-trafficking function to human disease and cytoskeletal integrity.

   Evidence Whole-exome sequencing in patients, shRNA knockdown in keratinocytes, electron microscopy, immunofluorescence colocalization of Slac2-b with Rab27B and β4 integrin, and adhesion assays

   PMID:23176819

   Open questions at the time

   • How EXPH5 loss disrupts keratin intermediate filament organization was not mechanistically explained
   • Whether the skin fragility reflects defective integrin trafficking, ECM secretion, or both was unclear
5. 2020 High

   Demonstrating that EXPH5 controls CD63+ vesicle trafficking to deliver ECM cargo and regulate focal adhesion dynamics resolved the cellular mechanism underlying the adhesion defect in EXPH5-deficient keratinocytes.

   Evidence Patient-derived primary keratinocytes and shRNA knockdown with live imaging of CD63+ vesicles, extracellular vesicle proteomics, and focal adhesion dynamics analysis

   PMID:32890627

   Open questions at the time

   • The specific cargo receptor or sorting signal directing ECM proteins into EXPH5-dependent vesicles is unknown
   • Whether EXPH5 directly interacts with focal adhesion components or acts indirectly through vesicle delivery was not resolved
6. 2024 Medium

   Identification of EXPH5 as an ATM kinase substrate raised the possibility that DNA damage signaling modulates EXPH5-dependent vesicle trafficking, though the functional significance of this phosphorylation was not tested.

   Evidence Global phosphoproteomics with ATM/ATR substrate profiling in ATM-depleted human neuroblastoma cells

   PMID:39615799

   Open questions at the time

   • No functional validation of the specific EXPH5 phosphorylation event was performed
   • The phosphorylation site(s) on EXPH5 and their effect on Rab27 binding or vesicle trafficking are unknown
   • Whether ATM-dependent phosphorylation of EXPH5 is relevant in keratinocytes or other physiological contexts is untested

• Major unresolved questions include the structural basis of EXPH5–Rab27 interaction, the identity of downstream tethering/fusion partners at the plasma membrane, how EXPH5 loss leads to keratin cytoskeleton disruption, and whether ATM-mediated phosphorylation of EXPH5 is functionally significant.
• No crystal structure or cryo-EM structure of EXPH5 or its complexes exists
• The plasma membrane tethering or SNARE partners that cooperate with EXPH5 for vesicle fusion are unidentified
• The mechanism connecting EXPH5 vesicle trafficking to keratin intermediate filament organization is unknown