BPIFB3

BPIFB3 is an endoplasmic reticulum-resident protein that functions as a negative regulator of a noncanonical autophagy pathway operating independently of the core autophagy initiation machinery (PMID:25491355). Silencing BPIFB3 enhances basal and virus-induced autophagy, and epistasis with core initiation components establishes that the BPIFB3-controlled pathway bypasses canonical autophagy initiation (PMID:25491355). Through this control of membrane turnover, BPIFB3 exerts opposing effects on distinct viruses: it restricts coxsackievirus B replication by suppressing autophagosome formation (PMID:25491355), yet positively supports dengue and Zika virus replication by restraining RETREG1 (FAM134B)-dependent reticulophagy, thereby preserving ER membranes required for assembly of flavivirus replication organelles (PMID:32102874). Its regulatory activity depends on the ER-localized interactors ARFGAP1 and TMED9, identified by proximity biotinylation and functionally required for both BPIFB3-regulated noncanonical autophagy and its control of enterovirus and flavivirus replication (PMID:33277377). BPIFB3 localizes exclusively to the ER and associates with other BPIFB family members (PMID:26962226). Beyond these findings, the biochemical mechanism by which BPIFB3 restrains autophagic membrane turnover has not been characterized in the available corpus.

1. 2014 High

   Established that BPIFB3 is an ER protein that negatively regulates autophagy through a pathway distinct from canonical autophagy initiation, answering where it acts and how it intersects with virus replication.

   Evidence RNAi, overexpression, LC3B vesicle microscopy, and double-silencing epistasis with core autophagy components during CVB infection

   PMID:25491355

   Open questions at the time

   • The molecular activity by which BPIFB3 suppresses noncanonical autophagy is undefined
   • No interacting partners were identified at this stage
2. 2016 Medium

   Confirmed exclusive ER localization independently and placed BPIFB3 in physical proximity to other BPIFB family members at the ER, defining its subcellular context.

   Evidence Confocal localization and co-localization/association assays with BPIFB family members including BPIFB6

   PMID:26962226

   Open questions at the time

   • Family-member association inferred from co-localization rather than direct binding assays
   • Functional consequence of BPIFB family association unknown
3. 2020 High

   Revealed that BPIFB3 has a virus-dependent dual role, promoting flavivirus replication by restraining RETREG1-dependent reticulophagy, answering how ER membrane turnover is coupled to viral replication organelle formation.

   Evidence BPIFB3 + RETREG1 double-knockdown epistasis with rescue, viral replication assays, and replication-organelle microscopy in DENV/ZIKV infection

   PMID:32102874

   Open questions at the time

   • The direct molecular link between BPIFB3 and the RETREG1 reticulophagy machinery is not defined
   • Whether BPIFB3 acts directly on RETREG1 or upstream remains unresolved
4. 2021 High

   Identified ARFGAP1 and TMED9 as ER interactors functionally required for BPIFB3 activity, providing the first molecular partners through which it regulates autophagy and viral replication.

   Evidence BioID proximity biotinylation with mass spectrometry, followed by RNAi of interactors with autophagy and viral replication readouts

   PMID:33277377

   Open questions at the time

   • Whether interactions are direct binary contacts versus proximity-based is not resolved by BioID alone
   • The biochemical step ARFGAP1/TMED9 perform with BPIFB3 is undefined

• The catalytic or biochemical mechanism by which BPIFB3, with ARFGAP1 and TMED9, restrains autophagic and reticulophagic ER membrane turnover remains unknown.
• No defined enzymatic or molecular activity for BPIFB3
• No structural model of BPIFB3 or its complexes
• Mechanism distinguishing autophagy suppression from reticulophagy restraint unresolved