EXPH5

EXPH5 (Slac2-b) is a Rab27 effector that couples GTP-bound Rab27A/B to membrane vesicle docking and regulated secretion across multiple cell types (PMID:19966785, PMID:15039459). Its Slp homology domain binds GTP-Rab27A/B with selectivity over other Rabs, and this domain functions both as a structural determinant of secretory granule exocytosis and as a sensor for the activated, GTP-loaded state of Rab27 (PMID:15039459, PMID:16880209). As a Rab27b effector, EXPH5 acts in the multivesicular endosome docking and exosome/extracellular vesicle secretion pathway, where its depletion phenocopies loss of Rab27b (PMID:19966785). In keratinocytes, EXPH5 directs trafficking of CD63+ vesicles to the plasma membrane and the secretion of extracellular vesicles carrying matrix proteins, thereby coupling vesicle delivery to focal adhesion turnover, keratin intermediate filament integrity, and cell–matrix adhesion; its loss reduces Rab27a expression and disrupts focal adhesion dynamics (PMID:23176819, PMID:32890627). A loss-of-function frameshift mutation in EXPH5 causes inherited skin fragility, with impaired keratinocyte adhesion in the lower epidermis (PMID:23176819). EXPH5 has additionally been recovered as an ATM-regulated phosphoprotein, linking it to a DNA-damage-responsive secretory axis (PMID:39615799).

1. 2004 High

   Establishing that Slac2-b is a bona fide Rab27 effector required showing its domain both binds the GTPase selectively and is functionally required for secretion, which this work demonstrated.

   Evidence Recombinant Slp homology domain binding specificity, Co-IP, and competition in streptolysin O-permeabilized parotid acinar cells with amylase release readout

   PMID:15039459

   Open questions at the time

   • Did not resolve EXPH5 domain architecture beyond the SHD
   • Mechanism of how SHD binding drives granule docking not defined
2. 2006 Medium

   The question of whether the SHD could report Rab27 activation state was addressed by repurposing it as a GTP-Rab27 capture reagent, establishing it as a functional sensor of activated Rab27.

   Evidence Pull-down with Slac2-b SHD coupled to nucleotide thin-layer chromatography and permeabilized platelet dense granule secretion assay

   PMID:16880209

   Open questions at the time

   • Used SHD as a tool rather than testing endogenous EXPH5 function in platelets
   • Single lab
3. 2009 High

   Whether EXPH5 contributes to exosome biogenesis was answered by placing it epistatically downstream of Rab27b in the MVE docking/exosome secretion pathway.

   Evidence RNAi knockdown in HeLa cells with exosome secretion assay and epistasis against Rab27b silencing

   PMID:19966785

   Open questions at the time

   • Molecular step EXPH5 controls within MVE docking not pinpointed
   • Cargo selectivity of EXPH5-dependent exosomes not defined
4. 2012 High

   The physiological consequence of EXPH5 loss in humans was established by linking a frameshift mutation to inherited skin fragility and connecting EXPH5 to keratinocyte adhesion and cytoskeletal integrity.

   Evidence Whole-exome sequencing of patients, shRNA knockdown phenocopy in normal keratinocytes, and immunofluorescence co-localization with Rab27B and β4 integrin

   PMID:23176819

   Open questions at the time

   • Mechanism connecting vesicle trafficking to keratin filament disruption not resolved
   • Relationship between perinuclear vesicle accumulation and adhesion loss unclear
5. 2020 High

   How EXPH5-dependent vesicle trafficking governs adhesion was clarified by linking CD63+ vesicle delivery and extracellular vesicle secretion of matrix proteins to focal adhesion dynamics.

   Evidence Live-cell imaging, shRNA knockdown, EV secretion assays, and focal adhesion dynamics in patient-derived and normal keratinocytes

   PMID:32890627

   Open questions at the time

   • Identity of the matrix cargo that drives adhesion not fully defined
   • Causal link between reduced Rab27a expression and EXPH5 loss not mechanistically explained
6. 2024 Medium

   An unexpected regulatory input was revealed by recovering EXPH5 as an ATM-responsive phosphoprotein, suggesting its secretory function is coupled to DNA-damage signaling.

   Evidence Global proteome and phosphoproteomics profiling of ATM/ATR substrates in ATM-depleted neuroblastoma cells

   PMID:39615799

   Open questions at the time

   • EXPH5 was one of many hits without gene-specific functional validation
   • Phosphosite function and effect on Rab27 binding unknown

• How EXPH5 phosphorylation, Rab27 binding, and cargo selection are integrated to control which vesicles dock and secrete in different cell types remains unresolved.
• No structural model of the EXPH5–Rab27 complex in the timeline
• Regulatory phosphorylation not functionally linked to secretion
• Tissue-specific cargo determinants undefined