HPS6

HPS6 is a subunit of the stable BLOC-2 complex (with HPS3 and HPS5) that functions in the biogenesis and maturation of lysosome-related organelles, existing as both a soluble pool and a peripheral membrane protein (PMID:15030569). Beyond its structural role in BLOC-2, HPS6 acts as a cargo adaptor that directly binds the dynactin p150Glued subunit to couple lysosomes to the dynein-dynactin retrograde motor, driving their centripetal transport to the perinuclear region; loss of HPS6 impairs lysosomal acidification, degradative capacity, and late endosome–lysosome fusion (PMID:25189619). In endothelial cells, HPS6 is required for Weibel-Palade body maturation and von Willebrand factor (VWF) tubulation, in part by binding the v-ATPase subunit ATP6V0D1 and delivering v-ATPase to the WPB limiting membrane to maintain the acidic luminal pH needed for VWF tubule formation (PMID:27889498, PMID:35252216). HPS6/BLOC-2 is not required for constitutive lysosomal enzyme secretion (PMID:15030569).

1. 2004 High

   Established HPS6 as a defined biochemical entity by showing it is a stable subunit of the BLOC-2 complex, framing all later function within the biogenesis of lysosome-related organelles.

   Evidence Reciprocal Co-IP from HeLa extracts plus size-exclusion chromatography and density gradient fractionation

   PMID:15030569

   Open questions at the time

   • Stoichiometry and subunit architecture of BLOC-2 not resolved
   • Membrane-recruitment mechanism for the peripheral pool not defined
2. 2004 Medium

   Tested whether BLOC-2 governs bulk lysosomal secretion and showed it does not, narrowing HPS6 function to organelle biogenesis rather than constitutive enzyme release.

   Evidence Beta-hexosaminidase secretion assay in HPS6-deficient fibroblasts (negative result)

   PMID:15030569

   Open questions at the time

   • A negative result does not exclude roles in regulated secretion
   • Single assay/single lab
3. 2014 High

   Defined a direct mechanistic role by identifying HPS6 as a cargo adaptor linking lysosomes to the dynein-dynactin motor, explaining how lysosome positioning is coupled to maturation and acidification.

   Evidence Co-IP of HPS6 with p150Glued plus siRNA knockdown in HeLa cells with live-cell imaging, acidification, degradation, and fusion assays

   PMID:25189619

   Open questions at the time

   • Whether this adaptor activity requires intact BLOC-2 is unclear
   • Binding interface on p150Glued not mapped
   • Knockdown only partially blocks transport, implying redundancy
4. 2016 Medium

   Extended HPS6 function to endothelial Weibel-Palade bodies, showing it is required for WPB maturation and VWF tubulation and thus regulated VWF release.

   Evidence HPS6-deficient mouse model; plasma VWF after DDAVP; EM and immunofluorescence of WPBs

   PMID:27889498

   Open questions at the time

   • Molecular mechanism behind the tubulation defect not defined in this study
   • Single lab
5. 2022 Medium

   Provided the molecular basis for the WPB phenotype by showing HPS6 binds ATP6V0D1 and delivers v-ATPase to the WPB membrane to maintain acidic luminal pH required for VWF tubulation.

   Evidence Co-IP of HPS6 with ATP6V0D1 plus parallel siRNA phenocopy of HPS6 and ATP6V0D1 in HUVECs with WPB/VWF imaging

   PMID:35252216

   Open questions at the time

   • Direct demonstration of luminal pH change upon HPS6 loss not shown
   • Whether trafficking is BLOC-2-dependent or adaptor-dependent unresolved
   • Single lab, two methods

• How HPS6's BLOC-2 membership, dynein-dynactin adaptor activity, and v-ATPase trafficking roles are mechanistically integrated remains unresolved.
• No structural model of HPS6 or its binding interfaces
• Unclear whether cargo-adaptor and v-ATPase functions require intact BLOC-2
• No defined recruitment signal for the peripheral membrane pool