HPS4

HPS4 is a core subunit of BLOC-3, a complex that regulates the biogenesis of lysosome-related organelles, and its loss underlies one form of Hermansky-Pudlak syndrome (PMID:11836498, PMID:12847290). HPS4 assembles with HPS1 into the stable ~175 kDa, predominantly cytosolic BLOC-3 complex through a divalent interaction in which a discrete HPS4 region (aa 340–528) engages both the N- and C-termini of HPS1 (PMID:12847290, PMID:12756248, PMID:12663659, PMID:23103514). Within this partnership HPS4 is required to stabilize HPS1 protein, since HPS4-deficient cells lack HPS1 (PMID:11836498, PMID:12663659), and certain HPS1 patient missense mutations at the HPS4 contact regions destabilize HPS1 (PMID:23103514). Functionally, BLOC-3 acts as a guanine nucleotide exchange factor for the Rab32 and Rab38 GTPases, and this Rab32/38-GEF activity is essential for tyrosinase trafficking and melanogenesis; HPS4 mutants lacking GEF activity fail to rescue pigmentation in HPS4-deficient melanocytes, whereas loss of Rab9 binding does not impair rescue (PMID:30837268). BLOC-3 also controls juxtanuclear positioning of late endosomes and lysosomes by a mechanism distinct from the AP-3 pathway, as HPS4-deficient cells retain normal LAMP-2 trafficking and Zn2+ storage (PMID:12847290, PMID:12756248).

1. 2002 Medium

   Established that HPS4 acts in the same organelle-biogenesis pathway as HPS1, the first link tying the two HPS proteins together functionally.

   Evidence Immunofluorescence co-localization in transfected melanoma cells and Western blot showing HPS1 loss in light-ear (HPS4-deficient) mouse tissues

   PMID:11836498

   Open questions at the time

   • Did not demonstrate a direct physical complex
   • Mechanism of HPS1 dependence on HPS4 unresolved
2. 2003 High

   Defined BLOC-3 as a stable physical HPS1·HPS4 complex, converting the genetic pathway link into a defined biochemical entity.

   Evidence Reciprocal co-immunoprecipitation and sedimentation-velocity ultracentrifugation with subcellular fractionation, replicated across contemporaneous studies

   PMID:12663659 PMID:12756248 PMID:12847290

   Open questions at the time

   • Did not define the molecular activity of the complex
   • Stoichiometry and membrane-recruitment determinants unresolved
3. 2003 Medium

   Showed HPS4 loss mislocalizes lysosomes and late endosomes away from the juxtanuclear region, assigning BLOC-3 a role in organelle positioning.

   Evidence Immunofluorescence microscopy of fibroblasts from HPS1- or HPS4-deficient patients and mice

   PMID:12847290

   Open questions at the time

   • Molecular mechanism of positioning control not defined
   • Effectors linking BLOC-3 to organelle movement unknown
4. 2003 Medium

   Distinguished BLOC-3 function from the AP-3 (HPS2/pearl) pathway, showing the two HPS complexes act by separate mechanisms.

   Evidence Immunofluorescence of LAMP-2 and fluorescent Zn2+ accumulation assays in HPS4-mutant versus pearl fibroblasts

   PMID:12756248

   Open questions at the time

   • Did not identify the cargo or effectors specific to BLOC-3
   • Functional overlap with other BLOC complexes not addressed
5. 2003 Medium

   Demonstrated HPS4 stabilizes HPS1 and mapped the interaction as indirect/self-associative, refining how the complex is held together.

   Evidence Co-immunoprecipitation, yeast two-hybrid, and size-exclusion chromatography of subcellular fractions in le-mutant cells

   PMID:12663659

   Open questions at the time

   • Discrepancy between co-IP association and lack of direct yeast two-hybrid interaction unexplained
   • Identity of additional subunits in larger fractions unresolved
6. 2012 Medium

   Mapped the divalent HPS1–HPS4 interaction interface and linked it to disease, explaining why some HPS1 patient mutations destabilize the complex.

   Evidence Co-immunoprecipitation of deletion/truncation mutants and domain-based pull-down interaction mapping

   PMID:23103514

   Open questions at the time

   • No structural model of the assembled complex
   • Single-lab mapping not corroborated by an independent structural method
7. 2019 High

   Identified the catalytic activity of BLOC-3/HPS4 as a Rab32/38 GEF and showed this activity, not Rab9 binding, is essential for melanogenesis.

   Evidence Site-directed mutagenesis of HPS4 with re-expression rescue, pigmentation/melanin and tyrosinase trafficking assays in melan-le cells

   PMID:30837268

   Open questions at the time

   • Structural basis of GEF catalysis not defined
   • How GEF activity connects to organelle positioning and to dense-granule biogenesis not resolved

• How BLOC-3 GEF activity is spatially targeted to nascent lysosome-related organelles and how it mechanistically controls organelle positioning remain unknown.
• Membrane recruitment determinants of cytosolic BLOC-3 undefined
• Downstream Rab32/38 effectors in melanosome and dense-granule biogenesis not identified