ACBD4

ACBD4 is a tail-anchored peroxisomal membrane protein that interacts with the ER-resident protein VAPB via an FFAT motif to promote peroxisome–ER membrane contact sites (PMID:28463579, PMID:39271061). Unlike its paralog ACBD5, which serves as the primary peroxisome–ER tether, ACBD4 loss does not reduce peroxisome–ER connections or cause very long-chain fatty acid (VLCFA) accumulation; instead, ACBD4 knockout increases VLCFA β-oxidation rates, indicating a modulatory rather than essential tethering role (PMID:37414147). ACBD4 also physically interacts with ACBD5 independently of VAPB, and phosphorylation of residues at the ACBD4–VAPB interface regulates the tethering interaction (PMID:37414147, PMID:36952197).

1. 2017 High

   The discovery that ACBD4 localizes to the peroxisomal membrane via a tail anchor and interacts with ER-resident VAPB established it as a novel component of peroxisome–ER membrane contact sites.

   Evidence Reciprocal co-immunoprecipitation, live-cell imaging, and overexpression/loss-of-function experiments in mammalian cells

   PMID:28463579

   Open questions at the time

   • Whether ACBD4 is functionally redundant with ACBD5 at peroxisome–ER contacts was unresolved
   • The molecular determinants (e.g., FFAT motif) mediating VAPB binding were not yet mapped
2. 2019 High

   Orthogonal proximity-based assays confirmed that both ACBD4 and ACBD5 foster peroxisome–ER membrane associations and that altering their levels modulates inter-organelle lipid exchange.

   Evidence Proximity ligation assay and split-superfolder GFP reporter system with overexpression and knockdown in mammalian cells

   PMID:31198905

   Open questions at the time

   • The individual contribution of ACBD4 versus ACBD5 to tethering and lipid transfer was not separated
   • Direct measurement of specific lipid species transferred at contacts was lacking
3. 2023 High

   CRISPR knockout of ACBD4 revealed that, unlike ACBD5, ACBD4 is dispensable for maintaining peroxisome–ER contacts and VLCFA homeostasis, and instead acts as a negative regulator of peroxisomal β-oxidation; ACBD4 also interacts with ACBD5 independently of VAPB, redefining it as a modulator rather than a core tether.

   Evidence CRISPR knockout in HEK293 cells combined with lipidomics and co-immunoprecipitation

   PMID:37414147

   Open questions at the time

   • The mechanism by which ACBD4 suppresses β-oxidation rate is unknown
   • Whether the ACBD4–ACBD5 interaction modulates ACBD5 tethering activity has not been tested
   • Physiological consequences of ACBD4 loss in vivo in mammals are uncharacterized
4. 2023 Medium

   Phosphorylation of residues at the ACBD4/5–VAPB interface was identified as a regulatory switch for the tethering interaction, providing a post-translational mechanism controlling contact site dynamics.

   Evidence Co-immunoprecipitation with phospho-site mutagenesis and western blot

   PMID:36952197

   Open questions at the time

   • The kinase(s) responsible for phosphorylation were not identified
   • Functional impact of phosphorylation on contact site extent or lipid metabolism was not measured
   • Limited functional follow-up beyond a methodological context
5. 2024 Medium

   Phylogenetic analysis and Drosophila experiments demonstrated that the conserved FFAT motif is required for VAP-mediated peroxisome–ER tethering and that loss of the single ACBD4/5 ortholog causes peroxisome redistribution and reduced organismal lifespan.

   Evidence FFAT motif mutagenesis, RNAi depletion in Drosophila wing neurons, live-cell imaging

   PMID:39271061

   Open questions at the time

   • Drosophila ortholog data cannot fully disambiguate ACBD4- versus ACBD5-specific functions in mammals
   • Whether ACBD4's FFAT motif is regulated distinctly from that of ACBD5 in mammalian cells is untested

• The mechanism by which ACBD4 negatively regulates peroxisomal β-oxidation — whether through acyl-CoA binding, ACBD5 modulation, or contact site remodeling — remains undefined, as does the in vivo physiological role of ACBD4 in mammalian tissues.
• No acyl-CoA binding activity or substrate specificity has been directly demonstrated for ACBD4
• No mammalian in vivo knockout or disease model exists
• Structural basis of the ACBD4–ACBD5 interaction is unknown