Acyl-CoA-binding protein (ACBP) 1 is a ubiquitous intracellular lipid-binding protein whose physiological function remains to be determined (reviewed in Refs. 1-3). Until recently, ACBP was thought to be primarily cytosolic and unique among the soluble intracellular lipid proteins in exhibiting very high affinity (K d values of 0.5-10 nM) and exclusive specificity for long chain fatty acyl-CoAs (LCFA-CoAs) (1, 4 -7). Based on the fact that ACBP exclusively binds LCFA-CoAs, it may participate in several aspects of LCFA-CoA metabolism.First, ACBP may be involved in directly presenting LCFACoAs as substrates for lipid metabolic enzymes. A variety of studies in vitro suggest that ACBP extracts LCFA-CoAs from membranes (6) to increase the soluble LCFA-CoA pool available for intracellular transport (reviewed in Ref.2). The cytosolic ACBP⅐LCFA-CoA complexes then interact with and present LCFA-CoA to acyltransferase enzymes involved in phospholipid synthesis in the endoplasmic reticulum (8, 9), lysophosphatidic acid synthesis in mitochondria (10), cholesteryl ester synthesis in the endoplasmic reticulum (11), and oxidation in mitochondria (10).Second, ACBP may control the level of unbound LCFA-CoA available for interaction with regulatory sites on metabolic enzymes (e.g. acetyl-CoA carboxylase) and intracellular signaling proteins (e.g. protein kinase C) in the cytosol (reviewed in Refs. 12 and 13).Third, recent data demonstrating the presence of significant amounts of ACBP in the nuclei of transfected cells overexpressing ACBP suggest that ACBP may also be involved in direct or ligand (LCFA-CoA)-dependent regulation of nuclear proteins that activate transcription of genes involved in lipid and glucose metabolism (14,15). Several members of the nuclear receptor superfamily including the hepatocyte nuclear receptor 4␣ (HNF-4␣) (16 -18), thyroid hormone receptor (TR) (19), and peroxisome proliferator-activated receptor-␣ and -␦ (PPAR-␣ and -␦) (20, 21) interact with LCFA-CoAs. The relative order of affinities of these nuclear receptors for LCFA-CoAs is HNF-4␣ (K d of 1.5-4 nM) Ͼ Ͼ TR (K d of 120 nM) Ͼ Ͼ PPAR␣ (displaces Wy14643). On this basis, it appears that only HNF-4␣ binds LCFA-CoAs with affinities in the physiological range of LCFACoA levels in the nucleus, Ͻ10 nM (17, 18). HNF-4␣ is a nuclear receptor with major roles in hepatocyte differentiation during liver development and in regulating the transcription of nu-