ACBD5

ACBD5 is a peroxisomal tail-anchored membrane protein that couples very-long-chain fatty acid (VLCFA) metabolism to inter-organelle membrane contact, exposing an N-terminal acyl-CoA binding domain (ACBD) to the cytosol where it preferentially captures very-long-chain fatty acyl-CoAs and channels them into peroxisomes for β-oxidation; loss of ACBD5 or its ACBD causes cellular accumulation of VLCFAs and VLCFA-containing phospholipids without disrupting peroxisome biogenesis or protein import (PMID:27799409, PMID:27899449). Independently of this metabolic role, ACBD5 serves as the primary ER–peroxisome tether: through an FFAT-like motif it binds the MSP domain of the ER-resident proteins VAPA and VAPB, and loss of this interaction reduces peroxisome–ER contacts, increases peroxisome mobility, and impairs peroxisome growth, plasmalogen synthesis, and cholesterol homeostasis (PMID:28108526, PMID:28108524). Tether formation is dynamically controlled by GSK3β phosphorylation of the FFAT-like motif and its flanking regions, which modulates VAPB binding in a phosphatase-sensitive manner (PMID:35019937). ACBD5 also interacts with its paralog ACBD4 in a VAPB-independent fashion, with ACBD4 exerting a regulatory rather than tethering role over VLCFA β-oxidation (PMID:37414147). Functionally, ACBD5 deficiency drives neurological and retinal pathology: in mouse models, VLCFA accumulation deregulates actin dynamics and produces axonopathy, myelination defects, and cell-type-specific retinal lipid accumulation with synaptic degeneration, and AAV-mediated ACBD5 delivery ameliorates the neurological phenotypes (PMID:38066620, PMID:41324649). A radiation-induced ACBD5-RET gene fusion activates ERK/MAPK signaling and is oncogenic, but this reflects the fusion product rather than native ACBD5 function (PMID:25175022).

1. 2014 Medium

   Before native ACBD5 function was defined, a chromosomal rearrangement revealed the locus could contribute an oncogenic fusion, establishing ACBD5 as a fusion partner driving aberrant signaling.

   Evidence 5' RACE/RT-PCR identification of an ACBD5-RET fusion, transfection into NIH3T3, ERK phosphorylation Western blot, and xenograft tumor formation

   PMID:25175022

   Open questions at the time

   • Concerns the ACBD5-RET fusion protein, not native ACBD5 biology
   • Single case and single lab
   • Does not address normal cellular function of ACBD5
2. 2016 High

   It was unknown how ACBD5 contributes to lipid metabolism; demonstrating that it is a peroxisomal tail-anchored protein with a cytosol-facing ACBD that binds VLC-CoAs and is required for VLCFA β-oxidation established ACBD5 as a substrate-recruitment factor for peroxisomal fatty acid degradation.

   Evidence Subcellular fractionation, domain mutagenesis, in vitro acyl-CoA binding, CRISPR and patient-derived KO cells, lipidomics, and VLCFA biochemistry

   PMID:27799409 PMID:27899449

   Open questions at the time

   • Mechanism of how captured VLC-CoAs cross the peroxisomal membrane not resolved
   • Whether ACBD5 hands off substrate to a specific transporter unknown
   • No structural model of the ACBD–acyl-CoA interaction
3. 2017 High

   The molecular basis of ER–peroxisome contacts was unclear; identifying ACBD5 as the FFAT-motif partner of ER VAPA/VAPB established the ACBD5–VAP complex as the primary ER–peroxisome tether required for peroxisome growth and lipid homeostasis.

   Evidence Reciprocal Co-IP, FFAT-motif mapping, knockdown, live-cell peroxisome mobility imaging, electron microscopy of contact sites, and lipid biochemistry, replicated across two concurrent papers

   PMID:28108524 PMID:28108526

   Open questions at the time

   • How tethering mechanistically enables plasmalogen and cholesterol homeostasis not fully resolved
   • Whether lipid transfer occurs directly at the contact not shown
   • Relationship between the tethering and VLC-CoA capture functions not integrated
4. 2018 Medium

   Whether ACBD5 tethering relied solely on VAPB was untested; showing that ACBD5 overexpression redistributes neuronal peroxisomes independently of VAPB binding implied additional, undefined ACBD5 partners anchor peroxisomes in neurons.

   Evidence Confocal live-cell imaging in hippocampal neurons with wild-type and VAPB-binding-deficient ACBD5, quantifying peroxisome motility and distribution

   PMID:30589881

   Open questions at the time

   • The proposed additional peripheral tethering partners are not identified
   • Single cell-type, overexpression-based system
   • Physiological relevance of the redistribution not established
5. 2022 High

   How ER–peroxisome contacts are regulated was unknown; identifying GSK3β phosphorylation of the ACBD5 FFAT-like motif as a switch for VAPB binding established a signaling mechanism controlling contact-site dynamics.

   Evidence Mass spectrometry phospho-site mapping, phospho-mimetic/dead mutagenesis, GSK3β inhibition, proximity ligation assay, and Co-IP

   PMID:35019937

   Open questions at the time

   • Upstream signals activating GSK3β toward ACBD5 unknown
   • Whether phosphorylation also affects VLC-CoA capture not tested
   • Counteracting phosphatase not identified
6. 2023 Medium

   The role of the ACBD5 paralog ACBD4 was unclear; showing ACBD4 binds ACBD5 independently of VAPB and that its loss increases rather than decreases VLCFA β-oxidation established ACBD4 as a regulatory rather than tethering factor.

   Evidence Co-IP for ACBD4-ACBD5 interaction, KO of each gene in HEK293, lipidomics, and contact-site quantification

   PMID:37414147

   Open questions at the time

   • Mechanism by which ACBD4 restrains β-oxidation not defined
   • Stoichiometry of an ACBD4-ACBD5 complex unknown
   • Single-lab study
7. 2024 Medium

   The link between ACBD5 loss and neuropathology was undefined in vivo; an Acbd5-deficient mouse showed VLCFA-driven cytoskeletal deregulation, axonopathy, and myelination defects rescuable by AAV-delivered ACBD5, establishing causal disease mechanism and therapeutic proof-of-concept.

   Evidence CRISPR Gly357* mouse, lipidomics, proteomics, actin dynamics assay in VLCFA-treated neurons, and AAV gene-therapy rescue

   PMID:38066620

   Open questions at the time

   • Direct molecular link between VLCFA accumulation and actin deregulation not fully mechanistic
   • Single lab
   • Long-term durability of AAV rescue not addressed
8. 2025 Medium

   Why ACBD5 loss causes retinal disease was unclear; spatial lipidomics revealing cell-type-specific VLC-PUFA accumulation in inner retinal layers with ribbon-synapse degeneration established a localized lipid-homeostasis defect as the retinal pathogenic mechanism.

   Evidence Immunofluorescence, electron microscopy, full-field electroretinography, and MALDI MS imaging spatial lipidomics in ACBD5-deficient mouse retina

   PMID:41324649

   Open questions at the time

   • Causal link between lipid accumulation and synaptic degeneration is correlative
   • Why specific retinal layers are affected unexplained
   • Single lab

• How ACBD5's VLC-CoA capture and ER tethering functions are mechanistically integrated, and the identity of the non-VAP partners that anchor peroxisomes in neurons, remain open.
• No structure of ACBD5 or its complexes
• Mechanism of VLC-CoA membrane translocation downstream of capture unknown
• Additional neuronal tethering partners unidentified