Surface lysine residues modulate the collisional transfer of fatty acid from adipocyte fatty acid binding protein to membranes

The carboxy terminus causes interfacial assembly of oleate hydratase on a membrane bilayer

The soluble flavoprotein oleate hydratase (OhyA) hydrates the 9-cis double bond of unsaturated fatty acids. OhyA substrates are embedded in membrane bilayers; OhyA must remove the fatty acid from the bilayer and enclose it in the active site. Here, we show that the positively charged helix-turn-helix motif in the carboxy terminus (CTD) is responsible for interacting with the negatively charged phosphatidylglycerol (PG) bilayer. Super-resolution microscopy of Staphylococcus aureus cells expressing green fluorescent protein fused to OhyA or the CTD sequence shows subcellular localization along the cellular boundary, indicating OhyA is membrane-associated and the CTD sequence is sufficient for membrane recruitment. Using cryo-electron microscopy, we solved the OhyA dimer structure and conducted 3D variability analysis of the reconstructions to assess CTD flexibility. Our surface plasmon resonance experiments corroborated that OhyA binds the PG bilayer with nanomolar affinity and we found the CTD sequence has intrinsic PG binding properties. We determined that the nuclear magnetic resonance structure of a peptide containing the CTD sequence resembles the OhyA crystal structure. We observed intermolecular NOE from PG liposome protons next to the phosphate group to the CTD peptide. The addition of paramagnetic MnCl2 indicated the CTD peptide binds the PG surface but does not insert into the bilayer. Molecular dynamics simulations, supported by site-directed mutagenesis experiments, identify key residues in the helix-turn-helix that drive membrane association. The data show that the OhyA CTD binds the phosphate layer of the PG surface to obtain bilayer-embedded unsaturated fatty acids.

Understanding FABP7 binding to fatty acid micelles and membranes

Fatty acid-binding proteins (FABPs) are chaperones that facilitate the transport of long-chain fatty acids within the cell and can provide cargo-dependent localization to specific cellular compartments. Understanding the nature of this transport is important because lipid signaling functions are associated with metabolic pathways impacting disease pathologies including cancer, autism, and schizophrenia. FABPs often associate with cell membranes to acquire and deliver their bound cargo as part of transport. We focus on brain FABP (FABP7), which demonstrates localization to the cytoplasm and nucleus, influencing transcription and fatty acid metabolism. We use a combined biophysical-computational approach to elucidate the interaction between FABP7 and model membranes. Specifically, we use multiple experiments to demonstrate that FABP7 can bind oleic acid and docosahexaenoic acid micelles. Data from NMR and multiscale molecular dynamics simulations reveal that the interaction with micelles is through FABP7's portal region residues. Simulations suggest that binding to membranes occurs through the same residues as micelles. Simulations also capture binding events where fatty acids dissociate from the membrane and enter FABP7's binding pocket. Overall, our data shed light on the interactions between FABP7 and OA or DHA micelles and provide insight into the transport of long-chain fatty acids.

Identification of a non-classical three-dimensional nuclear localization signal in the intestinal fatty acid binding protein

The intestinal fatty acid binding protein (FABP) is a small protein expressed along the small intestine that bind long-chain fatty acids and other hydrophobic ligands. Several lines of evidence suggest that, once in the nucleus, it interacts with nuclear receptors, activating them and thus transferring the bound ligand into the nucleus. Previous work by our group suggests that FABP2 would participate in the cytoplasm-nucleus translocation of fatty acids. Because the consensus NLS is absent in the sequence of FABP2, we propose that a 3D signal could be responsible for its nuclear translocation. The results obtained by transfection assays of recombinant wild type and mutated forms of Danio rerio Fabp2 in Caco-2 cell cultures, showed that lysine 17, arginine 29 and lysine 30 residues, which are located in the helix-turn-helix region, would constitute a functional non-classical three-dimensional NLS.

Metabolic functions of FABPs--mechanisms and therapeutic implications

Intracellular and extracellular interactions with proteins enables the functional and mechanistic diversity of lipids. Fatty acid-binding proteins (FABPs) were originally described as intracellular proteins that can affect lipid fluxes, metabolism and signalling within cells. As the functions of this protein family have been further elucidated, it has become evident that they are critical mediators of metabolism and inflammatory processes, both locally and systemically, and therefore are potential therapeutic targets for immunometabolic diseases. In particular, genetic deficiency and small molecule-mediated inhibition of FABP4 (also known as aP2) and FABP5 can potently improve glucose homeostasis and reduce atherosclerosis in mouse models. Further research has shown that in addition to their intracellular roles, some FABPs are found outside the cells, and FABP4 undergoes regulated, vesicular secretion. The circulating form of FABP4 has crucial hormonal functions in systemic metabolism. In this Review we discuss the roles and regulation of both intracellular and extracellular FABP actions, highlighting new insights that might direct drug discovery efforts and opportunities for management of chronic metabolic diseases.

Probing the interaction of brain fatty acid binding protein (B-FABP) with model membranes

Brain fatty acid-binding protein (B-FABP) interacts with biological membranes and delivers polyunsaturated fatty acids (FAs) via a collisional mechanism. The binding of FAs in the protein and the interaction with membranes involve a motif called “portal region”, formed by two small α-helices, A1 and A2, connected by a loop. We used a combination of site-directed mutagenesis and electron spin resonance to probe the changes in the protein and in the membrane model induced by their interaction. Spin labeled B-FABP mutants and lipidic spin probes incorporated into a membrane model confirmed that B-FABP interacts with micelles through the portal region and led to structural changes in the protein as well in the micelles. These changes were greater in the presence of LPG when compared to the LPC models. ESR spectra of B-FABP labeled mutants showed the presence of two groups of residues that responded to the presence of micelles in opposite ways. In the presence of lysophospholipids, group I of residues, whose side chains point outwards from the contact region between the helices, had their mobility decreased in an environment of lower polarity when compared to the same residues in solution. The second group, composed by residues with side chains situated at the interface between the α-helices, experienced an increase in mobility in the presence of the model membranes. These modifications in the ESR spectra of B-FABP mutants are compatible with a less ordered structure of the portal region inner residues (group II) that is likely to facilitate the delivery of FAs to target membranes. On the other hand, residues in group I and micelle components have their mobilities decreased probably as a result of the formation of a collisional complex. Our results bring new insights for the understanding of the gating and delivery mechanisms of FABPs.

Characterization of 4-HNE modified L-FABP reveals alterations in structural and functional dynamics

4-Hydroxynonenal (4-HNE) is a reactive α,β-unsaturated aldehyde produced during oxidative stress and subsequent lipid peroxidation of polyunsaturated fatty acids. The reactivity of 4-HNE towards DNA and nucleophilic amino acids has been well established. In this report, using proteomic approaches, liver fatty acid-binding protein (L-FABP) is identified as a target for modification by 4-HNE. This lipid binding protein mediates the uptake and trafficking of hydrophobic ligands throughout cellular compartments. Ethanol caused a significant decrease in L-FABP protein (P<0.001) and mRNA (P<0.05), as well as increased poly-ubiquitinated L-FABP (P<0.001). Sites of 4-HNE adduction on mouse recombinant L-FABP were mapped using MALDI-TOF/TOF mass spectrometry on apo (Lys57 and Cys69) and holo (Lys6, Lys31, His43, Lys46, Lys57 and Cys69) L-FABP. The impact of 4-HNE adduction was found to occur in a concentration-dependent manner; affinity for the fluorescent ligand, anilinonaphthalene-8-sulfonic acid, was reduced from 0.347 µM to Kd(1) = 0.395 µM and Kd(2) = 34.20 µM. Saturation analyses revealed that capacity for ligand is reduced by approximately 50% when adducted by 4-HNE. Thermal stability curves of apo L-FABP was also found to be significantly affected by 4-HNE adduction (ΔTm = 5.44°C, P<0.01). Computational-based molecular modeling simulations of adducted protein revealed minor conformational changes in global protein structure of apo and holo L-FABP while more apparent differences were observed within the internal binding pocket, revealing reduced area and structural integrity. New solvent accessible portals on the periphery of the protein were observed following 4-HNE modification in both the apo and holo state, suggesting an adaptive response to carbonylation. The results from this study detail the dynamic process associated with L-FABP modification by 4-HNE and provide insight as to how alterations in structural integrity and ligand binding may a contributing factor in the pathogenesis of ALD.

Human tissue transglutaminase is inhibited by pharmacologic and chemical acetylation

Human tissue transglutaminase (TGM2) is implicated in the pathogenesis of several neurodegenerative disorders including Alzheimer's, Parkinson's and expanded polyglutamine (polyQ) diseases. TGM2 promotes formation of soluble and insoluble high molecular weight aggregates by catalyzing a covalent linkage between peptide-bound Q residues in polyQ proteins and a peptide-bound Lys residue. Therapeutic approaches to modulate the activity of TGM2 are needed to proceed with studies to test the efficacy of TGM2 inhibition in disease processes. We investigated whether acetylation of Lys-residues by sulfosuccinimidyl acetate (SNA) or aspirin (ASA) would alter the crosslinking activity of TGM2. Acetylation by either SNA and/or ASA resulted in a loss of >90% of crosslinking activity. The Lys residues that were critical for inhibition were identified by mass spectrometry as Lys(444), Lys(468), and Lys(663). Hence, acetylation of Lys-residues may modulate the enzymatic function of TGM2 in vivo and offer a novel approach to treatment of TGM2 mediated disorders.

Effect of bilayer phospholipid composition and curvature on ligand transfer by the alpha-tocopherol transfer protein

We report here our preliminary investigations on the mechanism of alpha-TTP-mediated ligand transfer as assessed using fluorescence resonance energy transfer (FRET) assays. These assays monitor the movement of the model alpha-tocopherol fluorescent derivative ((R)-2,5,7,8-tetramethyl-chroman-2-[9-(7-nitro-benzo[1,2,5]oxadiazol-4-yl amino)-nonyl]-chroman-6-ol; NBD-Toc) from protein to acceptor vesicles containing the fluorescence quencher TRITC-PE. We have found that alpha-TTP utilizes a collisional mechanism of ligand transfer requiring direct protein-membrane contact, that rates of ligand transfer are greater to more highly curved lipid vesicles, and that such rates are insensitive to the presence of anionic phospholipids in the acceptor membrane. These results point to hydrophobic features of alpha-TTP dominating the binding energy between protein and membrane.

Modeling fatty acid delivery from intestinal fatty acid binding protein to a membrane

Intestinal fatty acid binding protein (IFABP) interacts with biological membranes and delivers fatty acid (FA) into them via a collisional mechanism. However, the membrane-bound structure of the protein and the pathway of FA transfer are not precisely known. We used molecular dynamics (MD) simulations with an implicit membrane model to determine the optimal orientation of apo- and holo-IFABP (bound with palmitate) on an anionic membrane. In this orientation, the helical portal region, delimited by the alphaII helix and the betaC-betaD and betaE-betaF turns, is oriented toward the membrane whereas the putative beta-strand portal, delimited by the betaB-betaC, betaF-betaG, betaH-betaI turns and the N terminus, is exposed to solvent. Starting from the MD structure of holo-IFABP in the optimal orientation relative to the membrane, we examined the release of palmitate via both pathways. Although the domains can widen enough to allow the passage of palmitate, fatty acid release through the helical portal region incurs smaller conformational changes and a lower energetic cost.

Evolutionary coupling of structural and functional sequence information in the intracellular lipid-binding protein family

We have mined the evolutionary record for the large family of intracellular lipid-binding proteins (iLBPs) by calculating the statistical coupling of residue variations in a multiple sequence alignment using methods developed by Ranganathan and coworkers (Lockless and Ranganathan, Science 1999:286;295-299). The 213 sequences analyzed have a wide range of ligand-binding functions as well as highly divergent phylogenetic origins, assuring broad sampling of sequence space. Emerging from this analysis were two major clusters of coupled residues, which when mapped onto the structure of a representative iLBP under study in our laboratory, cellular retinoic-acid binding protein I, are largely contiguous and provide useful points of comparison to available data for the folding of this protein. One cluster comprises a predominantly hydrophobic core away from the ligand-binding site and likely represents key structural information for the iLBP fold. The other cluster includes the portal region where ligand enters its binding site, regions of the ligand-binding cavity, and the region where the 10-stranded beta-barrel characteristic of this family closes (between strands 1' and 10). Linkages between these two clusters suggest that evolutionary pressures on this family constrain structural and functional sequence information in an interdependent fashion. The necessity of the structure to wrap around a hydrophobic ligand confounds the typical sequestration of hydrophobic side chains. Additionally, ligand entry and exit require these structures to have a capacity for specific conformational change during binding and release. We conclude that an essential and structurally apparent separation of local and global sequence information is conserved throughout the iLBP family.

Protein-membrane interaction and fatty acid transfer from intestinal fatty acid-binding protein to membranes. Support for a multistep process

Fatty acid transfer from intestinal fatty acid-binding protein (IFABP) to phospholipid membranes occurs during protein-membrane collisions. Electrostatic interactions involving the alpha-helical "portal" region of the protein have been shown to be of great importance. In the present study, the role of specific lysine residues in the alpha-helical region of IFABP was directly examined. A series of point mutants in rat IFABP was engineered in which the lysine positive charges in this domain were eliminated or reversed. Using a fluorescence resonance energy transfer assay, we analyzed the rates and mechanism of fatty acid transfer from wild type and mutant proteins to acceptor membranes. Most of the alpha-helical domain mutants showed slower absolute fatty acid transfer rates to zwitterionic membranes, with substitution of one of the lysines of the alpha2 helix, Lys27, resulting in a particularly dramatic decrease in the fatty acid transfer rate. Sensitivity to negatively charged phospholipid membranes was also reduced, with charge reversal mutants in the alpha2 helix the most affected. The results support the hypothesis that the portal region undergoes a conformational change during protein-membrane interaction, which leads to release of the bound fatty acid to the membrane and that the alpha2 segment is of particular importance in the establishment of charge-charge interactions between IFABP and membranes. Cross-linking experiments with a phospholipid-photoactivable reagent underscored the importance of charge-charge interactions, showing that the physical interaction between wild-type intestinal fatty acid-binding protein and phospholipid membranes is enhanced by electrostatic interactions. Protein-membrane interactions were also found to be enhanced by the presence of ligand, suggesting different collisional complex structures for holo- and apo-IFABP.

Invertebrate intracellular fatty acid binding proteins

Fatty acid binding proteins are multigenic cytosolic proteins largely distributed along the zoological scale. Their overall identity at primary and tertiary structure is conserved. They are involved in the uptake and transport of hydrophobic ligands to different cellular fates. The precise functions of each FABP type remain imperfectly understood, since sub-specialization of functions is suggested. Evolutionary studies have distinguished major subfamilies that could have been derived from a common ancestor close to vertebrate/invertebrate split. Since the isolation of the first invertebrate FABP from Schistocerca gregaria in 1990, the number of FABPs isolated from invertebrates has been increasing. Differences at the sequence level are appreciable and relationships with vertebrate FABPs are not clear, and lesser among invertebrate proteins, introducing some uncertainty to infer functional relatedness and phylogenetic relationships. The objective of this review is to summarize the information available on invertebrate FABPs to elucidate their mutual relationships, the relationship with their vertebrate counterparts and putative functions. Structure, gene structure, putative functions, expression studies and phylogenetic relationships with vertebrate counterparts are analyzed. Previous suggestions of the ancestral position concerning the heart-type of FABPs are reinforced by evidence from invertebrate models.

Fatty acid transfer from intestinal fatty acid binding protein to membranes: electrostatic and hydrophobic interactions

Intestinal fatty acid binding protein (IFABP) is thought to participate in the intracellular transport of fatty acids (FAs). Fatty acid transfer from IFABP to phospholipid membranes is proposed to occur during protein-membrane collisional interactions. In this study, we analyzed the participation of electrostatic and hydrophobic interactions in the collisional mechanism of FA transfer from IFABP to membranes. Using a fluorescence resonance energy transfer assay, we examined the rate and mechanism of transfer of anthroyloxy-fatty acid analogs a) from IFABP to phospholipid membranes of different composition; b) from chemically modified IFABPs, in which the acetylation of surface lysine residues eliminated positive surface charges; and c) as a function of ionic strength. The results show clearly that negative charges on the membrane surface and positive charges on the protein surface are important for establishing the "collisional complex", during which fatty acid transfer occurs. In addition, changes in the hydrophobicity of the protein surface, as well as the hydrophobic volume of the acceptor vesicles, also influenced the rate of fatty acid transfer. Thus, ionic interactions between IFABP and membranes appear to play a primary role in the process of fatty acid transfer to membranes, and hydrophobic interactions can also modulate the rates of ligand transfer.

Cytosolic fatty acid binding proteins catalyze two distinct steps in intracellular transport of their ligands

Cytosolic long-chain fatty acid binding proteins (FABPs) are found in tissues that metabolize fatty acids. Like most lipid binding proteins, their specific functions remain unclear. Two classes have been described. Membrane-active FABPs interact directly with membranes during exchange of fatty acids between the protein binding site and the membrane, while membrane-inactive FABPs bind only to fatty acids that are already in aqueous solution. Despite these binding proteins, most fatty acids in cell cytoplasm appear to be bound to membranes. This paper reviews data suggesting that FABPs catalyze transfer of fatty acids between intracellular membranes, often across considerable intracellular distances. This process occurs in three distinct steps: dissociation of the fatty acid from a 'donor' membrane, diffusion of the fatty acid across the intervening water layer, and binding to an 'acceptor' membrane. Membrane-active FABPs catalyze dissociation of the fatty acid from the donor membrane and binding to the acceptor membrane, while membrane-inactive FABPs catalyze diffusion of fatty acids across the aqueous cytosol. Thus, FABPs catalyze all three steps in intracellular transport. A simple quantitative model has been developed that predicts the rate of intracellular transport as a function of the concentration, affinity and diffusional mobility of the binding protein. Different FABPs may have evolved to match the specific transport requirements of the cell type within which they are found.

Interactions of chicken liver basic fatty acid-binding protein with lipid membranes

The interactions of chicken liver basic fatty acid-binding protein (Lb-FABP) with large unilamellar vesicles (LUVs) of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidylglycerol (POPG) were studied by binding assays, Fourier transform infrared (FT-IR) spectroscopy, monolayers at air-water interface, and low-angle X-ray diffraction. Lb-FABP binds to POPG LUVs at low ionic strength but not at 0.1 M NaCl. The infrared (IR) spectra of the POPG membrane-bound protein showed a decrease of the band corresponding to beta-structures as compared to the protein in solution. In addition, a cooperative decrease of the beta-edge band above 70 degrees C in solution was also evident, while the transition was less cooperative and took place at lower temperature for the POPG membrane-bound protein. Low- and wide-angle X-ray diffraction experiments with lipid multilayers indicate that binding of the protein produces a rearrangement of the membrane structure, increasing the interlamellar spacing and decreasing the compactness of the lipids.

Effect of charge reversal mutations on the ligand- and membrane-binding properties of liver fatty acid-binding protein

Liver fatty acid-binding protein (FABP) is able to bind to anionic phospholipid vesicles under conditions of low ionic strength. This binding results in the release of ligand, the fluorescent fatty acid analogue 11-dansylaminoundecanoic acid (DAUDA), with loss of fluorescence intensity (Davies, J. K., Thumser, A. E. A., and Wilton, D. C. (1999) Biochemistry 38, 16932-16940). Using a strategy of charge reversal mutagenesis, the potential role of specific cationic residues in promoting interfacial binding of FABP to anionic phospholipid vesicles has been investigated. Cationic residues chosen included those within the alpha-helical region (Lys-20, Lys-31, and Lys-33) and those that make a significant contribution to the positive surface potential of the protein (Lys-31, Lys-36, Lys-47, Lys-57, and Arg-126). Only three cationic residues make a significant contribution to interfacial binding, and these residues (Lys-31, Lys-36, and Lys-57) are all located within the ligand portal region, where the protein may be predicted to exhibit maximum disorder. The binding of tryptophan mutants, F3W, F18W, and C69W, to dioleoylphosphatidylglycerol vesicles, containing 5 mol% of the fluorescent phospholipid dansyldihexadecanoylphosphatidylethanolamine, was monitored by fluorescence resonance energy transfer (FRET). All three mutants showed enhanced dansyl fluorescence due to FRET on addition of phospholipid to protein; however, this fluorescence was considerably greater with the F3W mutant, consistent with the N-terminal region of the protein coming in close proximity to the phospholipid interface. These results were confirmed by succinimide quenching studies. Overall, the results indicate that the portal region of liver FABP and specifically Lys-31, Lys-36, and Lys-57 are involved in the interaction with the interface of anionic vesicles and that the N-terminal region of the protein undergoes a conformational change, resulting in DAUDA release.

Role of the helical domain in fatty acid transfer from adipocyte and heart fatty acid-binding proteins to membranes: analysis of chimeric proteins

The adipocyte and heart fatty acid-binding proteins (A- and HFABP) are members of a lipid-binding protein family with a beta-barrel body capped by a small helix-turn-helix motif. Both proteins are hypothesized to transport fatty acid (FA) to phospholipid membranes through a collisional process. Previously, we suggested that the helical domain is particularly important for the electrostatic interactions involved in this transfer mechanism (Herr, F. M., Aronson, J., and Storch, J. (1996) Biochemistry 35, 1296-1303; and Liou, H.-L., and Storch, J. (2001) Biochemistry 40, 6475-6485). Despite their using qualitatively similar FA transfer mechanisms, differences in absolute transfer rates as well as regulation of transfer from AFABP versus HFABP, prompted us to consider the structural determinants that underlie these functional disparities. To determine the specific elements underlying the functional differences between AFABP and HFABP in FA transfer, two pairs of chimeric proteins were generated. The first and second pairs had the entire helical domain and the first alpha-helix exchanged between A- and HFABP, respectively. The transfer rates of anthroyloxy-labeled fatty acid from proteins to small unilamellar vesicles were compared with the wild type AFABP and HFABP. The results suggest that the alphaII-helix is important in determining the absolute FA transfer rates. Furthermore, the alphaI-helix appears to be particularly important in regulating protein sensitivity to the negative charge of membranes. The alphaI-helix of HFABP and the alphaII-helix of AFABP increased the sensitivity to anionic vesicles; the alphaI-helix of AFABP and alphaII-helix of HFABP decreased the sensitivity. The differential sensitivities to negative charge, as well as differential absolute rates of FA transfer, may help these two proteins to function uniquely in their respective cell types.

Transfer of fatty acids between intracellular membranes: roles of soluble binding proteins, distance, and time

Soluble fatty acid binding proteins (FABPs) are thought to facilitate exchange of fatty acids between intracellular membranes. Although many FABP variants have been described, they fall into two general classes. "Membrane-active" FABPs exchange fatty acids with membranes during transient collisions with the membrane surface, whereas "membrane-inactive" FABPs do not. We used modeling of fatty acid transport between two planar membranes to examine the hypothesis that these two classes catalyze different steps in intracellular fatty acid transport. In the absence of FABP, the steady-state flux of fatty acid from the donor to the acceptor membrane depends on membrane separation distance (d) approaching a maximum value (J(max)) as d approaches zero. J(max) is one-half the rate of dissociation of fatty acid from the donor membrane, indicating that newly dissociated fatty acid has a 50% chance of successfully reaching the acceptor membrane before rebinding to the donor membrane. For larger membrane separations, successful transfer becomes less likely as diffusional concentration gradients develop. The mean diffusional excursion of the fatty acid into the water phase (d(m)) defines this transition. For d<<d(m), dissociation from the membrane is rate limiting, whereas for d>>d(m), aqueous diffusion is rate limiting. All forms of FABP increase d(m) by reducing the rate of rebinding to the donor membrane, thus maintaining J(max) over larger membrane separations. Membrane-active FABPs further increase J(max) by catalyzing the rate of dissociation of fatty acids from the donor membrane, although frequent membrane interactions would be expected to reduce their diffusional mobility through a membrane-rich cytoplasm. Individual FABPs may have evolved to match the membrane separations and densities found in specific cell lines.

Molecular and immunological characterisation of a polymorphic cytosolic fatty acid binding protein from the human blood fluke of humans, Schistosoma japonicum

Most organisms obtain their fatty acids through their diet or by de novo synthesis, but human blood flukes belonging to the genus Schistosoma lack the oxygen-dependent pathways required for the synthesis of sterols and fatty acids so they are entirely dependent on their hosts for these and other complex lipids. Fatty acid binding proteins (FABPs) of the FABP/P2/CRABP/CRBP family of beta-barrel cytosolic lipid binding proteins (cLBP) appear to be particularly important to schistosomes in the uptake, transport and compartmentalisation of host-derived fatty acids and may provide important targets for immuno- and chemotherapy. Here we describe the isolation of a set of cDNAs prepared from the Asiatic schistosome, Schistosoma japonicum, which encode two groups of cLBPs based on sequence homology and unique cDNA restriction sites. Representative clones from the two groups, one encoding a complete Sj-FABP (F10), and the other encoding a deletion mutant (F25) were characterised at the nucleic acid level by Southern and Northern hybridisation analysis, and at the protein level by immunoblotting. The presence and size of introns in the genes encoding F10 and F25 were determined and, because of the interest in the Schistosoma mansoni FABP homologue (Sm14) as a putative vaccine candidate, the immunogenicity and protective efficacy of the two proteins were also evaluated. A particularly interesting finding was the degree of Sj-FABP amino acid sequence polymorphism found to occur within the S. japonicum worm population, which appears to be greater than that described from cLBPs from vertebrates or, indeed, any other group of organisms investigated to date.

Sj-FABPc fatty-acid-binding protein of the human blood fluke Schistosoma japonicum: structural and functional characterization and unusual solvent exposure of a portal-proximal tryptophan residue

Sj-FABPc of the blood fluke of humans, Schistosoma japonicum, is a member of the FABP/P2/CRBP/CRABP family of beta-barrel cytosolic fatty-acid-binding and retinoid-binding proteins. Sj-FABPc has at least eight different variants encoded by a single-copy polymorphic gene. In fluorescence-based assays, recombinant Sj-FABPc was found to bind 11-(dansylamino)undecanoic acid (DAUDA), inducing a shift in peak fluorescence emission from 543 to 493 nm. A similar spectral change was observed in dansyl-amino-octanoic acid (in which the dansyl fluorophore is attached at the alpha-carbon rather than the omega-carbon of DAUDA), indicating that the ligand enters entirely into the binding site. Sj-FABPc also bound the naturally fluorescent cis-parinaric acid, as well as oleic acid and arachidonic acid, by competition, but not all-trans-retinol. Dissociation constants were, for cis-parinaric acid, K(d)=2.5+/-0.1 microM (mean+/-S.E.M.) and an apparent stoichiometry consistent with one binding site per molecule of Sj-FABPc and, for oleic acid, K(i) approximately 80 nM. A deletion mutant from which alpha-II was absent failed to bind ligand. Sj-FABPc modelled well to known structures of the protein family; an unusually solvent-exposed Trp side chain was evident adjacent to the presumptive portal through which ligand is thought to enter and leave. Intrinsic fluorescence analyses of Sj-FABPc and of the deletion mutant (from which Trp-27 is absent) confirmed the unusual disposition of this side chain. Virtually all members of the FABP/P2/CRBP/CRABP protein family have prominent hydrophobic side chains in this position, with the exception of liver FABP and ileal FABP, which instead have charged side chains. Liver FABP is known to be distinct from other members of the protein family in that it does not seem to contact membranes to collect and deposit its ligand. It is therefore postulated that the unusually positioned apolar side chains in Sj-FABPc and others in the family are important in interactions with membranes or other cellular components.

The fatty acid transport function of fatty acid-binding proteins

The intracellular fatty acid-binding proteins (FABPs) comprise a family of 14-15 kDa proteins which bind long-chain fatty acids. A role for FABPs in fatty acid transport has been hypothesized for several decades, and the accumulated indirect and correlative evidence is largely supportive of this proposed function. In recent years, a number of experimental approaches which more directly examine the transport function of FABPs have been taken. These include molecular level in vitro modeling of fatty acid transfer mechanisms, whole cell studies of fatty acid uptake and intracellular transfer following genetic manipulation of FABP type and amount, and an examination of cells and tissues from animals engineered to lack expression of specific FABPs. Collectively, data from these studies have provided strong support for defining the FABPs as fatty acid transport proteins. Further studies are necessary to elucidate the fundamental mechanisms by which cellular fatty acid trafficking is modulated by the FABPs.

The muscle fatty acid binding protein of spadefoot toad (Scaphiopus couchii)

Fatty acid binding protein was purified from skeletal muscle of the spadefoot toad (Scaphiopus couchii), an estivating species. While estivating, this animal relies on the fatty acid oxidation for energy. Hence we were interested in the behaviour of fatty acid binding protein under conditions of elevated urea (up to 200 mM) and potassium chloride such as exist during estivation. Also we examined whether there were interactions between glycolytic intermediates and the binding ability of the protein. The amount of bound fatty acid (a fluorescence assay using cis-parinarate) was not affected (P < 0.05) by glucose, fructose 6-phosphate or phosphoenolpyruvate at physiological concentrations. By contrast, glucose 6-phosphate increased the amount of bound cis-parinarate but the apparent dissociation constant was not different from the control. Fructose 1,6-bisphosphate but not fructose 2,6-phosphate decreased cis-parinarate binding by 40%, commensurate with doubling the apparent dissociation constant (1.15-2.62 microM). Urea, guanidinium and trimethylamine N-oxide at 200 mM increased cis-parinarate binding 60% over controls. Urea (1 M) and KCl (200 mM) did not affect cis-parinarate binding compared to controls. The interaction of this fatty acid transporter with fructose 1,6-bisphosphate is discussed in terms of reciprocal interaction with phosphofructokinase since fatty acid is also an inhibitor of phosphofructokinase.

The adipocyte fatty acid-binding protein binds to membranes by electrostatic interactions

The adipocyte fatty acid-binding protein (AFABP) is believed to transfer unesterified fatty acids (FA) to phospholipid membranes via a collisional mechanism that involves ionic interactions between lysine residues on the protein surface and phospholipid headgroups. This hypothesis is derived largely from kinetic analysis of FA transfer from AFABP to membranes. In this study, we examined directly the binding of AFABP to large unilamellar vesicles (LUV) of differing phospholipid compositions. AFABP bound LUV containing either cardiolipin or phosphatidic acid, and the amount of protein bound depended upon the mol % anionic phospholipid. The K(a) for CL or PA in LUV containing 25 mol % of these anionic phospholipids was approximately 2 x 10(3) M(-1). No detectable binding occurred when AFABP was mixed with zwitterionic membranes, nor when acetylated AFABP in which surface lysines had been chemically neutralized was mixed with anionic membranes. The binding of AFABP to acidic membranes depended upon the ionic strength of the incubation buffer: >/=200 mM NaCl reduced protein-lipid complex formation in parallel with a decrease in the rate of FA transfer from AFABP to negatively charged membranes. It was further found that AFABP, but not acetylated AFABP, prevented cytochrome c, a well characterized peripheral membrane protein, from binding to membranes. These results directly demonstrate that AFABP binds to anionic phospholipid membranes and suggest that, although generally described as a cytosolic protein, AFABP may behave as a peripheral membrane protein to help target fatty acids to and/or from intracellular sites of utilization.

Differential mechanisms of retinoid transfer from cellular retinol binding proteins types I and II to phospholipid membranes

Cellular retinol-binding proteins types I and II (CRBP-I and CRBP-II) are known to differentially facilitate retinoid metabolism by several membrane-associated enzymes. The mechanism of ligand transfer to phospholipid small unilamellar vesicles was compared in order to determine whether differences in ligand trafficking properties could underlie these functional differences. Unidirectional transfer of retinol from the CRBPs to membranes was monitored by following the increase in intrinsic protein fluorescence that occurs upon ligand dissociation. The results showed that ligand transfer of retinol from CRBP-I was >5-fold faster than transfer from CRBP-II. For both proteins, transfer of the other naturally occurring retinoid, retinaldehyde, was 4-5-fold faster than transfer of retinol. Rates of ligand transfer from CRBP-I to small unilamellar vesicles increased with increasing concentration of acceptor membrane and with the incorporation of the anionic lipids cardiolipin or phosphatidylserine into membranes. In contrast, transfer from CRBP-II was unaffected by either membrane concentration or composition. Preincubation of anionic vesicles with CRBP-I was able to prevent cytochrome c, a peripheral membrane protein, from binding, whereas CRBP-II was ineffective. In addition, monolayer exclusion experiments demonstrated differences in the rate and magnitude of the CRBP interactions with phospholipid membranes. These results suggest that the mechanisms of ligand transfer from CRBP-I and CRBP-II to membranes are markedly different as follows: transfer from CRBP-I may involve and require effective collisional interactions with membranes, whereas a diffusional process primarily mediates transfer from CRBP-II. These differences may help account for their distinct functional roles in the modulation of intracellular retinoid metabolism.

Surface properties of adipocyte lipid-binding protein: Response to lipid binding, and comparison with homologous proteins

Adipocyte lipid-binding protein (ALBP) is one of a family of intracellular lipid-binding proteins (iLBPs) that bind fatty acids, retinoids, and other hydrophobic ligands. The different members of this family exhibit a highly conserved three-dimensional structure; and where structures have been determined both with (holo) and without (apo) bound lipid, observed conformational changes are extremely small (Banaszak, et al., 1994, Adv. Prot. Chem. 45, 89; Bernlohr, et al., 1997, Annu. Rev. Nutr. 17, 277). We have examined the electrostatic, hydrophobic, and water accessible surfaces of ALBP in the apo form and of holo forms with a variety of bound ligands. These calculations reveal a number of previously unrecognized changes between apo and holo ALBP, including: 1) an increase in the overall protein surface area when ligand binds, 2) expansion of the binding cavity when ligand is bound, 3) clustering of individual residue exposure increases in the area surrounding the proposed ligand entry portal, and 4) ligand-binding dependent variation in the topology of the electrostatic potential in the area surrounding the ligand entry portal. These focused analyses of the crystallographic structures thus reveal a number of subtle but consistent conformational and surface changes that might serve as markers for differential targeting of protein-lipid complexes within the cell. Most changes are consistent from ligand to ligand, however there are some ligand-specific changes. Comparable calculations with intestinal fatty-acid-binding protein and other vertebrate iLBPs show differences in the electrostatic topology, hydrophobic topology, and in localized changes in solvent exposure near the ligand entry portal. These results provide a basis toward understanding the functional and mechanistic differences among these highly structurally homologous proteins. Further, they suggest that iLBPs from different tissues exhibit one of two predominant end-state structural distributions of the ligand entry portal.

Fatty acid binding to rat liver fatty acid-binding protein is modulated by early glycolytic intermediates

Fatty acid binding to rat liver fatty acid binding protein in the presence of glycolytic metabolites and at different pH (optimal 7.2) and ionic strength was studied. Binding decreased logarithmically with ionic strength. Glucose and glucose-6-phosphate increased fatty acid binding significantly with K0.5 within physiological ranges while glucose-1-phosphate and phosphate ion caused no effect.

Simulations of fatty acid-binding proteins suggest sites important for function. I. Stearic acid

Molecular dynamics simulations of two structurally similar fatty acid-binding proteins interacting with stearic acid are described. The calculations relate to recent ligand binding measurements and suggest similarities and differences between the two systems. Charged and neutral forms of the fatty acid were examined. The charged forms led to rapid trajectory divergence, whereas the protonated forms remained stable over the length of their 1-ns production trajectories. The two protein systems showed similar sets of total interaction energies with the ligand. However, the strengths of individual amino acids interacting with the ligand differ. Furthermore, covariance analysis of the ligand with both protein and water suggests that the stearic acid in the adipocyte fatty acid-binding protein is coupled more strongly to the water than to the protein. The stearic acid in the muscle fatty acid-binding protein is seen to be coupled differentially along the length of the chain to the protein. These differences could help to rationalize the stronger binding affinity for stearic acid in the human muscle fatty acid-binding protein. An importance scale, based on both covariance and interaction energy with the ligand, is proposed to identify residues that may be important for binding function.

Inhibition of lipid absorption as an approach to the treatment of obesity

A reduction in fat intake may be achieved by making educated choices to reduce total calorie intake, to consume a lower quantity of total fats, or to modify the ratio of saturated-to-polyunsaturated lipids. Leptin agonists or NPY or CCK antagonists may prove to be useful to diminish appetite and thereby reduce the total intake of food. But eating has such cultural, social, and hedonistic attributes that such a single-pronged approach is unlikely to be successful. The use of fat substitutes may prove to be popular to provide a wide range of snack food options, but these are likely to be of minimal use in weight reduction programs because of their distribution of additives in only a limited number of foods. The inhibitors of lipid digestion will be modestly successful in the short term; their long-term success will be influenced by gastrointestinal adverse effects and the need to consume fat-soluble vitamin supplements to prevent the development of fat-soluble vitamin deficiencies. The inhibition of lipid absorption is an attractive targeted approach for the treatment of obesity, since this would reduce the uptake of visible as well as invisible fats, which would potentially offer convenient dosing, and could also be a means to inhibit secondarily the uptake of carbohydrate calories.

Modelling intracellular fatty acid transport: possible mechanistic role of cytoplasmic fatty acid-binding protein

A computer model is presented in which the role of cytoplasmic fatty acid-binding protein (FABP) in the intracellular translocation of fatty acids (FA) from one membrane to an opposite membrane is studied. The model consists of a cubical space, in which FABP and FA are allowed to diffuse at random. The amount of FA released from the donor membrane and reaching an opposite acceptor membrane is calculated in a variety of conditions. The data provided by the various simulations suggest that FABP can play a significant role in intracellular FA transport only if FABP is able to take up FA directly from FA containing membranes and to directly deliver FA to an acceptor membrane, thus preventing the unfavourable thermodynamical situation in which FA must solubilize in an aqueous environment prior to binding to FABP.

Biochemical and crystallographic analyses of a portal mutant of the adipocyte lipid-binding protein

A number of crystallographic studies of the adipocyte lipid-binding protein have established that the fatty acid-binding site is within an internalized water-filled cavity. The same studies have also suggested the existence of a region physically distinct from the fatty acid-binding site which connects the cavity of the protein with the external solvent, hereafter referred to as the portal. In an effort to examine the portal region, we have used site-directed mutagenesis to introduce the mutations V32D/F57H into the murine ALBP cDNA. Mutant protein has been isolated, crystallized, and its stability and binding properties studied by biochemical methods. As assessed by guanidine-HCl denaturation, the mutant form exhibited a slight overall destabilization relative to the wild-type protein under both acid and alkaline conditions. Accessibility to the cavity in both the mutant and wild-type proteins was observed by stopped-flow analysis of the modification of a cavity residue, Cys117, by the sulfhydryl reactive agent 5, 5'-dithiobis(2-nitrobenzoic acid) at pH 8.5. Cys117 of V32D/F57H ALBP was modified 7-fold faster than the wild-type protein. The ligand binding properties of both the V32D/F57H mutant and wild-type proteins were analyzed using a fluorescent probe at pH 6.0 and 8.0. The apparent dissociation constants for 1-anilinonaphthalene-8-sulfonic acid were approximately 9-10-fold greater than the wild-type protein, independent of pH. In addition, there is a 6-fold increase in the Kd for oleic acid for the portal mutant relative to the wild-type at pH 8.0. To study the effect of pH on the double mutant, it was crystallized and analyzed in two distinct space groups at pH 4.5 and 6.4. While in general the differences in the overall main chain conformations are negligible, changes were observed in the crystallographic structures near the site of the mutations. At both pH values, the mutant side chains are positioned somewhat differently than in wild-type protein. To ensure that the mutations had not altered ionic conditions near the binding site, the crystallographic coordinates were used to monitor the electrostatic potentials from the head group site to the positions near the portal region. The differences in the electrostatic potentials were small in all regions, and did not explain the differences in ligand affinity. We present these results within the context of fatty acid binding and suggest lipid association is more complex than that described within a single equilibrium event.

Intracellular lipid-binding proteins and their genes

Intracellular lipid-binding proteins are a family of low-molecular-weight single-chain polypeptides that form 1:1 complexes with fatty acids, retinoids, or other hydrophobic ligands. These proteins are products of a large multigene family of unlinked loci distributed throughout the genome. Each lipid-binding protein exhibits a distinctive pattern of tissue distribution. Transcriptional control, regulated by a combination of peroxisome proliferator activated receptors and CCAAT/enhancer-binding proteins, allows for a variety of both cell and tissue-specific expression patterns. In some cells, fatty acids increase the expression of the lipid-binding protein genes. Fatty acids, or their metabolites, are activators of the peroxisome proliferator-activated receptor family of transcription factors. Therefore, as the concentration of lipid in the diet increases, the expression of lipid-binding proteins coordinately increases. As revealed by X-ray crystallography, the lipid-binding proteins fold into beta-barrels, forming a large internal water-filled cavity. Fatty acid ligands are bound within the cavity, occupying only about one-third of the accessible volume. The bound fatty acid is stabilized via a combination of enthalpic and entropic forces that govern ligand affinity and selectivity. Cytoplasmic lipid-binding proteins are the intracellular receptors for hydrophobic ligands, delivering them to the appropriate site for use as metabolic fuels and regulatory agents.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.