BLTP3B

BLTP3B (SHIP164/UHRF1BP1L) is a bridge-like lipid transfer protein of the RBG superfamily that mediates bulk lipid transfer at membrane contact sites to drive endosomal sorting and retrograde trafficking (PMID:35499567, PMID:38565878). It contains a chorein motif characteristic of VPS13-family bridge-like lipid transfer proteins, possesses in vitro lipid transfer activity, and localizes to a subpopulation of CI-MPR-containing vesicle clusters in the early endocytic pathway, where its loss disrupts retrograde delivery of these organelles to the Golgi (PMID:35499567). At Golgi–early endosome contacts, BLTP3B forms a complex with the ATPase RhoBTB3 and the retromer subunit Vps26B to promote formation of Rab14-positive endosome buds; depletion produces enlarged Rab14+ endosomes lacking buds, a defect rescued by wild-type but not lipid transfer-defective protein, establishing that its lipid transfer activity is required for bud formation (PMID:38565878). BLTP3B also binds the Habc domain of Syntaxin 6 within a large complex that engages the GARP tethering complex, coupling it to endosomal tubulation and the retrograde transport of CI-MPR and transferrin receptor (PMID:20163565). Beyond these contact-site sorting roles, no further mechanistic detail has been characterized in the available corpus.

1. 2010 Medium

   Established the first molecular context for BLTP3B by placing it in the endosomal retrograde sorting machinery via physical links to Syntaxin 6 and the GARP tethering complex.

   Evidence Yeast two-hybrid/interaction screen, co-IP, and overexpression/knockdown with CI-MPR and transferrin receptor transport readouts in human cells

   PMID:20163565

   Open questions at the time

   • Did not define a molecular activity for the protein
   • Mechanism by which the Syntaxin 6/GARP complex recruits or activates BLTP3B unresolved
   • Single lab
2. 2022 High

   Defined BLTP3B as a bridge-like lipid transfer protein, converting its sorting role into a biochemical mechanism by demonstrating chorein-motif structure and in vitro lipid transfer linked to CI-MPR retrograde traffic.

   Evidence Structural/sequence analysis of chorein motif, in vitro lipid transfer assays, immunofluorescence, and siRNA loss-of-function with CI-MPR retrograde trafficking readout

   PMID:35499567

   Open questions at the time

   • Specific lipid species transferred in cells not defined
   • Contact sites bridged not directly imaged at this stage
3. 2022 Low

   Placed BLTP3B in the RBG superfamily phylogeny and predicted a distinctive C-terminal coiled-coil and surface residues for partner binding.

   Evidence Sequence analysis and AlphaFold structural prediction with phylogenetic analysis

   PMID:36571082

   Open questions at the time

   • Computational prediction only, no experimental validation of the coiled-coil or surface interaction sites
   • Functional role of C-terminal divergence untested
4. 2024 High

   Resolved the contact-site machinery, showing BLTP3B acts in a RhoBTB3–Vps26B complex at Golgi–early endosome contacts to drive Rab14+ endosome bud formation in a lipid-transfer-dependent manner.

   Evidence Co-IP complex identification, siRNA knockdown with EE bud microscopy, rescue with wild-type vs lipid transfer-defective mutants, and Rab14 effector assays

   PMID:38565878

   Open questions at the time

   • Stoichiometry and assembly order of the BLTP3B–RhoBTB3–Vps26B complex not defined
   • How Rab14 activity controls BLTP3B endosome association mechanistically unknown
5. 2024 Low

   Independently corroborated the Rab14-endosome localization by detecting BLTP3B in the RAB14 proximal proteome.

   Evidence APEX2 proximity labeling proteomics (preprint)

   PMID:bio_10.1101_2024.11.05.621850

   Open questions at the time

   • Proximity labeling does not establish direct interaction
   • Preprint, single method
6. 2017 Low

   Implicated BLTP3B in mitochondrial dynamics in a Parkinson's disease neuronal context, raising an unresolved link between its endosomal function and organelle homeostasis.

   Evidence RNAi screen in human neuronal cell culture with mitochondrial dynamics readout

   PMID:28137300

   Open questions at the time

   • No mechanistic pathway connecting BLTP3B lipid transfer to mitochondrial dynamics
   • Part of a large-scale screen, not a focused study
   • Not independently validated

• The identity of the lipids transferred in vivo and the directionality of transfer across the Golgi–endosome bridge remain undefined.
• Native lipid cargo and transfer direction not established
• Connection between endosomal sorting role and reported mitochondrial phenotype unexplained