BLTP2

BLTP2 is a bridge-like lipid transfer protein that operates at membrane contact sites to mediate bulk non-vesicular phospholipid transport, sustaining the lipid composition of the plasma membrane and endosomal compartments (PMID:40579455, PMID:38370643, PMID:40899996, PMID:39974967). It localizes to ER-plasma membrane (PM) contact sites, where it raises PM phosphatidylethanolamine levels and thereby regulates PM fluidity, consistent with transfer of PE from the ER to the PM through its hydrophobic channel (PMID:40579455, PMID:38370643). BLTP2 tethers the ER specifically to tubular endosomes and macropinosomes once these have become continuous with the PM, and its loss causes accumulation of PM-connected intracellular vacuoles, indicating that bulk ER-to-PM lipid transport is its core activity (PMID:40899996, PMID:39974967). Its recruitment to the PM is achieved through a combination of phosphoinositide binding, the adaptor proteins FAM102A and FAM102B, and N-BAR domain proteins at membrane-connected tubules (PMID:40899996, PMID:39974967); in evolutionarily conserved orthologs, dedicated adaptors (yeast Hoi1, Drosophila bilbobaggins) engage α-helical projections of the transfer channel or C-terminal cis-acting sequences to target the protein to ER-PM contacts, and loss of this targeting disrupts sterol homeostasis (PMID:41746219, PMID:39990326, PMID:42039411). At the level of physiology, the Drosophila ortholog hobbit is required for synaptic development and neurotransmission at the neuromuscular junction [PMID:bio_10.1101_2024.12.30.630795].

1. 2025 High

   Established that BLTP2 functions as an ER-PM contact-site lipid transfer protein, answering what compartment it acts at and what lipid it moves.

   Evidence Live imaging/fractionation localization plus PE lipid profiling and loss-of-function PM fluidity readout in a metastasis model

   PMID:38370643 PMID:40579455

   Open questions at the time

   • Direct in vitro reconstitution of PE transfer through the channel not shown
   • Selectivity for PE versus other phospholipids not fully defined
2. 2025 High

   Defined the topology of BLTP2 tethering, showing it bridges ER to PM-continuous tubular endosomes and macropinosomes, clarifying that bulk ER-PM lipid transport is its key function.

   Evidence Live-cell imaging, BLTP2 KO with electron microscopy of vacuoles, and ER co-localization with PM-connected organelles

   PMID:39974967 PMID:40899996

   Open questions at the time

   • Why tethering requires prior PM continuity is unresolved
   • Quantitative lipid flux at these contacts not measured
3. 2025 Medium

   Identified the molecular determinants of PM recruitment, addressing how BLTP2 is targeted to its site of action.

   Evidence Co-IP/pulldown with FAM102A/FAM102B and N-BAR proteins, phosphoinositide binding assays, and loss-of-function imaging

   PMID:39974967 PMID:40899996

   Open questions at the time

   • Reconstitution of the recruitment interactions not described
   • Relative contribution of each binding mode not dissected
4. 2025 Medium

   Extended BLTP2 function to endosomal lipid biology, showing it is required for ILV/exosome formation and supplies PG for BMP/LBPA synthesis at ER-MVB contacts.

   Evidence BLTP2 KO/knockdown, co-IP with SCAMP3, Rab5 dependency, NEDD4 ubiquitination assay, endosomal lipidomics, and exosome quantification (preprint)

   PMID:bio_10.1101_2025.04.17.649455

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Direct PG transfer to MVBs inferred from lipidomics, not reconstituted
   • SCAMP3-Rab5-NEDD4 regulatory axis lacks orthogonal validation
5. 2026 High

   Resolved the conserved adaptor-based targeting mechanism, showing Hoi1 engages α-helical projections of the Fmp27/Hob channel to localize it to ER-PM contacts and maintain sterol homeostasis.

   Evidence Genetic identification, domain mapping, co-IP, yeast sterol readout, and cross-species epistasis in worms and flies

   PMID:39990326 PMID:41746219

   Open questions at the time

   • Mechanistic link between targeting and sterol homeostasis unclear
   • Whether mammalian FAM102 adaptors act analogously not established
6. 2026 Medium

   Described a two-step 'hook and latch' targeting in Drosophila, showing both the adaptor bilbobaggins and C-terminal cis-acting sequences of Hobbit are required for ER-PM localization.

   Evidence Drosophila loss-of-function genetics, C-terminal structure-function analysis, live imaging, and epistasis (preprint)

   PMID:42039411

   Open questions at the time

   • Preprint, single lab
   • Structural basis of the C-terminal latch not defined
   • Conservation of bbo function in mammals untested
7. 2024 Medium

   Connected BLTP2 to organismal physiology, showing its ortholog is required for synaptic development and neurotransmission.

   Evidence Tissue-specific RNAi in Drosophila neurons and muscle with locomotor, NMJ morphology, and electrophysiology readouts (preprint)

   PMID:bio_10.1101_2024.12.30.630795

   Open questions at the time

   • Preprint, not peer-reviewed
   • Link between lipid transfer activity and synaptic phenotype not mechanistically established
   • Mammalian neuronal role untested

• Whether BLTP2's lipid-transfer activity has been directly demonstrated in vitro and how its various contact-site functions (PM fluidity, endosomal BMP synthesis, synaptic development) mechanistically interconnect remains unresolved.
• No reconstituted transfer assay establishing direct lipid handoff
• Substrate selectivity across PE, PG, and sterol phenotypes not unified
• Human disease relevance not addressed in the corpus