Review articleFatty acid binding protein isoforms: structure and function
Introduction
As early as 20 years ago it was reported that rat and bovine liver fatty acid binding protein L-FABP could be separated into three putative isoforms by isoelectric focusing (Ketterer et al., 1976). During the ensuing two decades, considerable effort was expended in identifying the primary structure of these operationally defined isoforms, especially for L-FABP and in extending these observations to other FABPs. Four factors have hampered efforts to distinguish the structure and physiological function of these isoforms.
An operational definition of an L-FABP isoform is that the purified protein be resolvable into multiple forms, usually by isoelectric focusing or ion exchange chromatography. An operationally identified isoform would potentially include contaminating proteins unrelated to FABPs as well as FABP forms differing in amino acid composition, post-translational modification, conformation, bound ligand, or even ampholyte interactions (Ketterer et al., 1976). There is some evidence that the pI observed for specific FABP isoforms may also shift and/or arise from interactions of the FABP with the isoelectric focusing ampholyte. For example, delipidized adipocyte A-FABP species were interconvertible by nonequilibrium pH gel electrophoresis (Matarese et al., 1989). Similarly, differences in pIs of delipidized rat L-FABP isoforms, noted after loading the isoforms at the basic end of the gel (Frolov et al., 1997), were abolished by isoelectric focusing after loading the isoforms at the acidic end of the gel2. In contrast to the operational definition, the more strict and exclusive biochemical definition of a true isoform is a protein variant produced either as a result of different genes or from differential processing of singe gene transcripts. The resultant protein variants are true isoforms differing by a single amino acid at a single position in the peptide chain. While data on single amino acid substitutions has been presented for some FABP isoforms, convincing evidence demonstrating that they arise due to different genes or to differential RNA processing has to date been available only for the human I-FABP (Baier et al., 1995, Baier et al., 1996).
There are many different fatty acid/fatty acyl CoA binding proteins found in mammalian cytosol: nearly 20 FABPs (at least 6 sequenced and shown to be unique) (Matarese et al., 1989, Paulussen and Veerkamp, 1990, Banaszak et al., 1994, Borchers and Spener, 1994, Gossett et al., 1996); sterol carrier protein-2 (SCP-2) (Schroeder et al., 1995, Frolov et al., 1996); and acyl CoA binding protein (ACBP) (Gossett et al., 1996). All three protein families are found in high amount in cells: 3–5; 0.1–0.4; and 0.1–0.9% of cytosolic proteins are FABPs, SCP-2 and ACBP, respectively (Gossett et al., 1996). Furthermore, all three protein families colocalize, but differ somewhat in intracellular distribution, in enterocytes (L-FABP, I-FABP, SCP-2, ACBP), hepatocytes (L-FABP, SCP-2, ACBP) and heart myocytes (H-FABP, SCP-2, ACBP) (Gossett et al., 1996). There is some controversy whether SCP-2 is exclusively peroxisomal or also found in cytoplasm and other intracellular organelles (Van der Krift et al., 1985, Keller et al., 1989, van Heusden et al., 1990, Reinhart et al., 1993, Ossendorp et al., 1994, Schroeder et al., 1998).
This was dramatically demonstrated when ACBP was discovered copurifying with liver L-FABP (Knudsen, 1990, Gossett et al., 1996). The problem is further exacerbated in tissues such as the intestinal enterocyte which contains at least six different types of fatty acid/fatty acyl CoA bindings proteins in the 10–15 kDa molecular weight range: L-FABP, I-FABP (Paulussen and Veerkamp, 1990, ILBP (Miller and Cistola, 1993), I15P (Kanda et al., 1991, Fujii et al., 1993), ACBP (Gossett et al., 1996, Gossett et al., 1997) and SCP-2 Ossendorp et al., 1990, He et al., 1991, Moncecchi et al., 1991, Yamamoto et al., 1991, Seedorf et al., 1993, Ohba et al., 1994). None of the other proteins fit the biochemical definition of an FABP isoform. Yet, if these proteins copurify with native FABPs, then isoelectric focusing and/or ion exchange chromatography may operationally resolve them into erroneously identified FABP isoforms. Consequently, it is difficult to evaluate the older literature regarding FABP isoforms. Great care must be taken to assure that the FABP preparations are devoid not only of impurities but also of other copurifying fatty acid/fatty acyl CoA binding proteins.
Increasing data indicates that the amount of bound fatty acid, post-translational modification (phosphorylation, cysteinylation, glutathionylation, as well conformation alter the isoelectric focusing/ion exchange chromatographic behavior of an FABP. These alternate FABP forms are not true isoforms differing by a single amino acid.
Resolution of these issues is important to the understanding of the physiological function of both true isoforms and alternate FABP forms. For example, the majority of cellular unesterified fatty acid and fatty acyl CoA in mammalian tissues is associated with intracellular membranes (Gossett et al., 1996). However, the proportions found in the cytoplasm are dependent on the presence of fatty acid/fatty acyl CoA binding proteins (Knudsen, 1991, Faergeman and Knudsen, 1997, Jolly et al., 1997a). Within the cytoplasm, the unesterified fatty acids are not free, but instead more than 99% are bound to small cytosolic proteins, e.g. 14–16 kDa, FABPs (Glatz et al., 1993) and/or 13 kDa sterol carrier protein-2 (Schroeder et al., 1995, Stolowich et al., 1997). Likewise, cytoplasmic fatty acyl CoAs also appear largely bound to proteins, e.g. 10 kDa acyl CoA binding protein (ACBP) (Faergeman et al., 1996, Gossett et al., 1996), sterol carrier protein-2 (Frolov et al., 1996), or 14–16 kDa FABPs (Hubbell et al., 1994, Frolov and Schroeder, 1997, Frolov et al., 1997). What is the relative role of FABP isoforms and additional operationally defined forms in cellular fatty acid uptake, in cytosolic binding of fatty acids/fatty acyl CoAs, in intracellular trafficking/targeting of these ligands and in regulating fatty acid metabolism? Only recently, have these questions begun to be explored. A primary aim of this review is to detail some of these new findings.
Section snippets
Do all FABPs form operationally defined isoforms?
True FABP isoforms generally cannot be separated by gel permeation chromatography. Instead, in the majority of cases they have been operationally defined based on difference in charge, pI, and by resolution on isoelectric focusing or ion exchange chromatography. Over the past 20 years L-FABP (rat, bovine and human) has been separated into 2–4 putative isoforms by isoelectric focusing (Ketterer et al., 1976, Miyozawa and Hashimoto, 1979, Haunerland et al., 1984, Bass, 1985, Das et al., 1989,
Subclassification of operationally defined FABP isoforms
Not all of the operationally defined isoforms are true isoforms. In the following sections, the operationally defined FABP isoforms will be separated into four categories: (1) the true isoforms fitting the biochemical definition as protein forms differing by a single amino acid (Table 2); (2) the post-translationally modified forms (Table 3); (3) the conformational forms (Table 3); and (4) forms simply due to the amount of bound ligand, i.e. fatty acid (Table 1, Table 4). For the purposes of
Physiological function of FABP isoforms or alternate forms
With some exceptions shown below, little is known of the true function of FABP isoforms/alternate forms either in vitro or in vivo. Thus, this section of the review must be viewed with caution.
Conclusions and future directions
Although the early literature of operationally defined FABP isoforms is not conclusive with regard to whether these are biochemically defined isoforms (i.e. amino acid substitutions), increasing data indicate that several of the FABPs, especially H-FABP, are true isoforms. Most of the remaining operationally defined isoforms are not true isoforms, but instead appear due to bound ligands, post-translational modifications, or conformers. The latter brings up an interesting point—How would the
Acknowledgements
This work was supported by a grant from the USPHS NIH No. DK41402.
References (122)
- et al.
A polymorphism in the human intestinal fatty acid binding protein alters fatty acid transport across Caco-2 cells
J. Biol. Chem.
(1996) - et al.
Lipid-binding proteins: A family of fatty acid and retinoid transport proteins
Adv. Protein Chem.
(1994) Function and regulation of hepatic and intestinal fatty acid binding proteins
Chem. Phys. Lipids
(1985)- et al.
Effect of vanadate on the cellular accumulation of pp15, an apparent product of insulin receptor tyrosine kinase action
J. Biol. Chem.
(1988) - et al.
Fatty acid binding proteins
- et al.
Composition and behavior of head membrane lipids of fresh and cryopreserved boar sperm
Cryobiology
(1994) - et al.
Studies on the mode of action of sterol carrier protein in the dehydrogenation of 5-cholest-7-en-3 beta-ol
J. Biol. Chem.
(1985) - et al.
Heart fatty acid binding protein is a novel regulator of cardiac myocyte hypertrophy
Biochem. Biophys. Res. Commun.
(1994) - et al.
Cloning of murine adipocyte lipid binding protein in Escherichia coli. Its purification, ligand binding properties and phosphorylation by the adipocyte insulin receptor
J. Biol. Chem.
(1989) - et al.
Fatty acid interaction with rat intestinal and liver fatty acid-binding proteins expressed in Escherichia coli: a comparative 13C NMR study
J. Biol. Chem.
(1989)
Human fetal liver fatty acid binding proteins. Role on glucose-6-phosphate dehydrogenase activity
Biochim. Biophys. Acta
Modification of the fatty acid binding profile of liver fatty acid binding protein (L-FABP)
J. Nutr. Biochem.
Amino acid exchange and covalent modification by cysteine and glutathione explain isoforms of fatty acid-binding protein occurring in bovine liver
J. Biol. Chem.
Purification and characterization of fatty acid-binding proteins from brown adipose tissue of the rat
Biochim. Biophys. Acta
Fatty acid binding proteins reduce 15-lipoxygenase induced oxygenation of linoleic acid and arachidonic acid
Biochim. Biophys. Acta
Sterol carrier protein-2, a new fatty acyl coenzyme A-binding protein
J. Biol. Chem.
Cloning of a cDNA encoding rat intestinal 15 kDa protein and its tissue distribution
Biochem. Biophys. Res. Commun.
Cytoplasmic fatty acid binding proteins: Significance for intracellular transport of fatty acids and putative role on signal transduction pathways
Prostaglandins Leukot. Essent. Fat. Acids
Fatty acids in cell signalling: modulation by lipid binding proteins
Prostaglandins Leukot. Essent. Fat. Acids
Fatty acid transfer from liver and intestinal fatty acid-binding proteins to membranes occurs by different mechanisms
J. Biol. Chem.
The complete amino acid sequence of the rabbit P2 protein
J. Biol. Chem.
Intermembrane transfer of 5 alpha-cholest-7-en-3 beta-ol. Facilitation by supernatant protein (SCP)
J. Biol. Chem.
Fatty acid binding protein: Stimulation of microsomal phosphatidic acid formation
Arch. Biochem. Biophys.
Free fatty acid transfer from rat liver fatty acid–binding protein to phospholipid vesicles. Effect of ligand and solution properties
J. Biol. Chem.
Mechanism of free fatty acid transfer from rat heart fatty acid-binding protein to phospholipid membranes. Evidence for a collisional process
J. Biol. Chem.
Acyl-CoA-binding and transport, an alternative function for diazepam binding inhibitor (DBI), which is identical with acyl-CoA-binding protein
Neuropharmacology
An in vitro reversible interconversion of rat liver fatty acid binding protein having different isoelectric points by virtue of the fatty acid content
Arch. Biochem. Biophys.
Inhibition of rat liver acetyl coenzyme A carboxylase by long chain acyl coenzyme A and fatty acid. Modulation by fatty acid-binding protein
J. Biol. Chem.
Intracellular fatty acid trafficking and the role of cytosolic lipid binding proteins
Prog. Lipid Res.
cDNA sequence and bacterial expression of mouse liver sterol carrier protein-2
J. Biol. Chem.
A comparative study of the conformational properties of Escherichia coli-derived rat intestinal and liver fatty acid binding proteins
Biochim. Biophys. Acta
Characterization of a cloned cDNA encoding rabbit myelin P2 protein
J. Biol. Chem.
Interaction of fatty acids with recombinant rat intestinal and liver fatty acid-binding proteins
Arch. Biochem. Biophys.
Polyene fatty acid interactions with recombinant intestinal and liver fatty acid binding proteins
J. Biol. Chem.
Fatty acid binding protein from rat heart is phosphorylated on Tur19 in response to insulin stimulation
J. Lipid Res.
Differential regulation and phosphorylation of the fatty acid-binding protein from rat mammary epithelial cells
Biochim. Biophys. Acta
Fatty acid binding protein. Isolation from rat liver, characterization and immunochemical quantification
J. Biol. Chem.
Characterization of a fatty acid-binding protein from rat heart
J. Biol. Chem.
The structure of the human sterol carrier protein X/sterol carrier protein 2 gene (SCP2)
Genomics
Regulation of long chain fatty acid activation in heart muscle
J. Biol. Chem.
Amino acid sequence of rat liver non-specific lipid transfer protein (sterol carrier protein-2) is present in a high molecular weight protein: Evidence from cDNA Analysis
Biochem. Biophys. Res. Commun.
Intracellular phospholipid transfer proteins
Curr. Topics Membr.
Equilibrium constants for the binding of fatty acids with fatty acid binding proteins from adipocyte, intestine, heart and liver measured with the flourscent probe ADIFAB
J. Biol. Chem.
Analysis of the ligand binding properties of recombinant bovine liver-type fatty acid binding protein
Biochim. Biophys. Acta
Rat intestinal fatty acid binding protein: a model system for analyzing the forces that can bind fatty acids to proteins
J. Biol. Chem.
Lipid regulation of CTP: phosphocholine cytidylyltransferase: electrostatic, hydrophobic and synergistic interactions of anionic phospholipids and diacylglycerol
Biochemistry
An amino acid substitution in the human intestinal fatty acid binding protein is associated with increased fatty acid binding, increased fat oxidation and insulin resistance
J. Clin. Invest.
Effect of fatty acid binding proteins on developing human placental malate dehydrogenase activity
Indian J. Exp. Biol.
Isoforms of fatty-acid-binding protein in bovine heart are coded by distinct mRNA
Eur. J. Biochem.
Cellular binding proteins for fatty acids and retinoids: similar or specialized functions?
Mol. Cell. Biochem.
Cited by (118)
The emerging role of fatty acid binding protein 5 (FABP5) in cancers
2023, Drug Discovery TodayLoss of fatty acid binding protein 3 ameliorates lipopolysaccharide-induced inflammation and endothelial dysfunction
2023, Journal of Biological ChemistryFABP5 as a novel molecular target in prostate cancer
2020, Drug Discovery TodayEffect of liver fatty acid binding protein (L-FABP) gene ablation on lipid metabolism in high glucose diet (HGD) pair-fed mice
2019, Biochimica et Biophysica Acta - Molecular and Cell Biology of LipidsCitation Excerpt :These findings were consistent with human studies with an isocaloric high glucose diet that also increased serum TG and VLDL, attributable to decreased serum TG clearance and/or decreased fatty acid oxidation [76]. It is important to note that earlier in vitro studies with isolated microsomes and recombinant L-FABP protein showed that L-FABP enhances fatty acyl-CoA incorporation into glycerides [14,15,72,73]. Furthermore, studies with transfected cells overexpressing L-FABP and with cultured primary hepatocytes from LKO mice indicated that L-FABP enhances and inhibits fatty acyl-CoA targeting towards fatty acid oxidation, respectively [9,18,26,77].
Scp-2/Scp-x ablation in Fabp1 null mice differentially impacts hepatic endocannabinoid level depending on dietary fat
2018, Archives of Biochemistry and Biophysics