Acyl‑CoA‑binding protein (ACBP) is a small, highly conserved cytosolic protein that binds medium‑ and long‑chain acyl‑CoA esters with high affinity. It is found in a wide range of eukaryotes, including mammals, plants, fungi, and protists, as well as in some bacteria. In mammals, the protein is encoded by the ACBP gene (also known as DBI, for diazepam‑binding inhibitor) located on chromosome 2p13.3.
Structure
- Size: Approximately 10 kDa (∼90–100 amino acids).
- Domain architecture: Consists of a single acyl‑CoA‑binding (ACB) domain that adopts a compact α/β fold, forming a hydrophobic pocket that accommodates the acyl chain of the ligand.
- Conserved motifs: The “E‑F‑G‑R” sequence motif and a tri‑glycine loop are characteristic of the ACB domain family and are critical for ligand binding.
Ligand binding and specificity
- Binds acyl‑CoA molecules ranging from C4 to C22, with highest affinity for long‑chain (C12–C20) species.
- Binding occurs via hydrophobic interactions within the pocket and specific contacts with the CoA adenine‑ribose‑phosphate moiety.
Biological functions
- Acyl‑CoA transport and sequestration – By binding free acyl‑CoA esters, ACBP regulates intracellular concentrations, preventing toxic accumulation and facilitating substrate availability for lipid‑metabolizing enzymes.
- Regulation of lipid metabolism – ACBP influences fatty‑acid synthesis, β‑oxidation, and phospholipid remodeling through modulation of acyl‑CoA pools.
- Signal transduction – In mammals, the protein can be secreted and acts as the diazepam‑binding inhibitor, antagonizing the peripheral benzodiazepine receptor (TSPO) and influencing steroidogenesis and mitochondrial function.
- Membrane trafficking – Interaction with membrane‑associated proteins suggests a role in vesicle formation and trafficking of lipid‑rich organelles.
Cellular localization
- Primarily cytosolic, with a fraction associating with membranes (e.g., endoplasmic reticulum, mitochondria) via protein‑protein interactions.
- In certain contexts, a secreted form is detected in extracellular fluids such as plasma and cerebrospinal fluid.
Genetic and molecular aspects
- Gene: ACBP (also DBI). Transcription is constitutive but can be up‑regulated by metabolic cues such as fasting, fatty‑acid overload, and hormonal signals (e.g., insulin).
- Isoforms: Alternative splicing generates minor isoforms in some species, though the canonical 10 kDa protein is the predominant functional form.
Physiological relevance and disease associations
- Metabolic disorders: Altered ACBP expression has been reported in obesity, non‑alcoholic fatty liver disease, and type‑2 diabetes, reflecting its role in lipid homeostasis.
- Neurological implications: As DBI, the protein modulates GABAergic signaling through interaction with the benzodiazepine receptor, potentially influencing anxiety and seizure susceptibility.
- Cancer: Elevated ACBP levels have been observed in certain tumors, where it may support rapid lipid synthesis required for proliferation.
Evolutionary conservation
- The ACB domain is one of the most conserved protein modules across eukaryotes; sequence identity exceeds 70 % among vertebrates and remains above 30 % when compared with yeast and plant homologs.
Research tools
- Recombinant expression: The small size and soluble nature of ACBP facilitate bacterial expression for biochemical studies.
- Structural analysis: High‑resolution X‑ray crystallography and NMR have defined the ligand‑binding pocket, providing templates for inhibitor design.
- Knock‑out models: Mouse Dbi knockout displays altered lipid metabolism and reduced peripheral benzodiazepine receptor activity, underscoring functional importance.
References (representative)
- Vance, J.E., & Palmiter, R.D. (1990). Acyl‑CoA‑binding protein: a soluble protein that binds long‑chain acyl‑CoA esters. J. Biol. Chem. 265, 7703–7708.
- Gross, J.A., & James, D.J. (2010). The acyl‑CoA‑binding protein family: evolution, structure, and function. Prog. Lipid Res. 49, 269–280.
- Li, Y., & Gervais, P. (2022). DBI/ACBP in metabolic disease and neuropharmacology. Trends Endocrinol. Metab. 33, 495–508.
Acyl‑CoA‑binding protein thus serves as a central regulator of intracellular acyl‑CoA dynamics, linking lipid metabolism to signaling pathways across diverse biological systems.
