Lipid-binding proteins: a family of fatty acid and retinoid transport proteins

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.

Presence of intestinal, liver and heart/adipocyte fatty-acid-binding protein types in the liver of a chimaera fish

Five fatty-acid-binding proteins from the liver of the elephant fish (Callorhynchus callorhynchus), a chimaera fish that belongs--together with the elasmobranchs--to the ancient chondrichthyes class were isolated and characterized. The purification procedures for these proteins involved gel filtration, anion-exchange chromatography, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a last step. They were submitted to "in gel" tryptic or cyanogen bromide digestion and the resulting peptides were separated by high performance liquid chromatography and then sequenced by Edman degradation. According to their partial amino acid sequences, one of them presents the highest identity with fatty-acid-binding proteins from human and catfish liver, another three with those from mammalian heart or adipose tissue and the fifth with the mammalian intestinal fatty-acid-binding protein. The presence of various members of this protein family, as now found in elephant fish and previously in catfish (Rhamdia sapo) liver, does not occur in mammalian liver which express only one a characteristic fatty-acid-binding protein.

Probing the folding pathway of a beta-clam protein with single-tryptophan constructs

BACKGROUND

Cellular retinoic acid binding protein I (CRABPI) is a small, predominantly beta-sheet protein with a simple architecture and no disulfides or cofactors. Folding of mutants containing only one of the three native tryptophans has been examined using stopped-flow fluorescence and circular dichroism at multiple wavelengths.

RESULTS

Within 10 ms, the tryptophan fluorescence of all three mutants shows a blue shift, and stopped-flow circular dichroism shows significant secondary structure content. The local environment of Trp7, a completely buried residue located near the intersection of the N and C termini, develops on a 100 ms time scale. Spectral signatures of the other two tryptophan residues (87 and 109) become native-like in a 1 s kinetic phase.

CONCLUSIONS

Formation of the native beta structure of CRABPI is initiated by rapid hydrophobic collapse, during which local segments of chain adopt significant secondary structure. Subsequently, transient yet specific interactions of amino acid residues restrict the arrangement of the chain topology and initiate long-range associations such as the docking of the N and C termini. The development of native tertiary environments, including the specific packing of the beta-sheet sidechains, occurs in a final, highly cooperative step simultaneous with stable interstrand hydrogen bonding.

12-Bromododecanoic acid binds inside the calyx of bovine beta-lactoglobulin

The X-ray structure of bovine beta-lactoglobulin with the ligand 12-bromododecanoic acid as a model for fatty acids has been determined at a resolution of 2.23 A in the trigonal lattice Z form. The ligand binds inside the calyx, resolving a long-standing controversy as to where fatty-acid like ligands bind. The carboxylate head group lies at the surface of the molecule, and the lid to the calyx is open at the pH of crystallization (pH 7.3), consistent with the conformation observed in ligand-free bovine beta-lactoglobulin in lattice Z at pH 7.1 and pH 8.2.

Cloning and chromosomal location of the murine keratinocyte lipid-binding protein gene

The keratinocyte lipid-binding protein (KLBP) is a member of a large multigene family of intracellular fatty-acid-binding proteins. It is expressed in skin and tongue epithelia, adipose, lung and mammary tissue and has been found upregulated in several skin cell carcinomas and papillomas (Krieg et al., 1993). In order to study the regulation of KLBP expression, the murine gene has been cloned. Southern analysis using an exon 2 specific cDNA probe indicated the presence of multiple copies of the gene in the murine genome. Based on the highly conserved structure of the fatty-acid-binding protein genes, the third intron of the KLBP gene was PCR-amplified utilizing murine genomic DNA. Southern analysis with the intron 3 probe identified one unique gene in the murine genome. A full-length genomic clone of KLBP was obtained from a P1 library, and the structural gene was sequenced. Similar to the other FABP genes, the functional KLBP gene contains four exons separated by three introns and maintains the conservation of size and placement of each exon. A functional minimal promoter was demonstrated by transient transfections of 5' upstream KLBP-luciferase reporter constructs into line 308 keratinocyte cells as well as in primary adipocytes. RT-PCR on primary adipocyte RNA demonstrated expression of this KLBP gene by amplification of intron 3 from the primary transcript. Fluorescence in-situ hybridization identified the murine KLBP gene as the fourth FABP gene on chromosome 3, along with myelin P2, ALBP, and intestinal FABP. These studies provide a framework for analysis of KLBP expression in normal and pathophysiological conditions.

Intracellular transport of fatty acids in muscle. Role of cytoplasmic fatty acid-binding protein

Long-chain fatty acids represent a major substrate for energy production in striated muscles, especially in those muscles which have a high oxidative enzymatic capacity. Following their uptake from the extracellular compartment the fatty acids have to translocate through the aqueous cytoplasm of the myocytes to reach the mitochondria where they undergo oxidative degradation. This intracellular transport is assisted by cytoplasmic fatty acid-binding protein (FABPc), a small (15 kD) protein which shows a high affinity for the non-covalent binding of long-chain fatty acids, and of which several types occur. So-called heart-type or muscle-type FABPc is found in muscle cells, and is abundant especially in oxidative fibers. The muscular FABPc content appears to relate to the rate of fatty acid utilization, and also changes in concert to modulations in fatty acid utilization induced by (patho)physiological stimuli (e.g. endurance training, diabetes). The facilitation of intracellular fatty acid transport by FABPc is accomplished by increasing the concentration of the diffusing fatty acids in the aqueous cytoplasm and, most likely, also by interacting directly with membranes to promote transfer of fatty acids to and from the cytosolic binding protein.

The helical domain of intestinal fatty acid binding protein is critical for collisional transfer of fatty acids to phospholipid membranes

Fatty acid binding proteins (FABPs) exhibit a beta-barrel topology, comprising 10 antiparallel beta-sheets capped by two short alpha-helical segments. Previous studies suggested that fatty acid transfer from several FABPs occurs during interaction between the protein and the acceptor membrane, and that the helical domain of the FABPs plays an important role in this process. In this study, we employed a helix-less variant of intestinal FABP (IFABP-HL) and examined the rate and mechanism of transfer of fluorescent anthroyloxy fatty acids (AOFA) from this protein to model membranes in comparison to the wild type (wIFABP). In marked contrast to wIFABP, IFABP-HL does not show significant modification of the AOFA transfer rate as a function of either the concentration or the composition of the acceptor membranes. These results suggest that the transfer of fatty acids from IFABP-HL occurs by an aqueous diffusion-mediated process, i.e., in the absence of the helical domain, effective collisional transfer of fatty acids to membranes does not occur. Binding of wIFABP and IFABP-HL to membranes was directly analyzed by using a cytochrome c competition assay, and it was shown that IFABP-HL was 80% less efficient in preventing cytochrome c from binding to membranes than the native IFABP. Collectively, these results indicate that the alpha-helical region of IFABP is involved in membrane interactions and thus plays a critical role in the collisional mechanism of fatty acid transfer from IFABP to phospholipid membranes.

Folding mechanism of three structurally similar beta-sheet proteins

The folding mechanism of cellular retinoic acid binding protein I (CRABP I), cellular retinol binding protein II (CRBP II), and intestinal fatty acid binding protein (IFABP) were investigated to determine if proteins with similar native structures have similar folding mechanisms. These mostly beta-sheet proteins have very similar structures, despite having as little as 33% sequence similarity. The reversible urea denaturation of these proteins was characterized at equilibrium by circular dichroism and fluorescence. The data were best fit by a two-state model for each of these proteins, suggesting that no significant population of folding intermediates were present at equilibrium. The native states were of similar stability with free energies (linearly extrapolated to 0 M urea, deltaGH2O) of 6.5, 8.3, and 5.5 kcal/mole for CRABP I, CRBP II, and IFABP, respectively. The kinetics of the folding and unfolding processes for these proteins was monitored by stopped-flow CD and fluorescence. Intermediates were observed during both the folding and unfolding of all of these proteins. However, the overall rates of folding and unfolding differed by nearly three orders of magnitude. Further, the spectroscopic properties of the intermediate states were different for each protein, suggesting that different amounts of secondary and/or tertiary structure were associated with each intermediate state for each protein. These data show that the folding path for proteins in the same structural family can be quite different, and provide evidence for different folding landscapes for these sequences.

Cellular retinaldehyde-binding protein ligand interactions. Gln-210 and Lys-221 are in the retinoid binding pocket

Cellular retinaldehyde-binding protein (CRALBP) carries 11-cis-retinal and/or 11-cis-retinol as endogenous ligands in the retinal pigment epithelium (RPE) and Müller cells of the retina and has been linked with autosomal recessive retinitis pigmentosa. Ligand interactions determine the physiological role of CRALBP in the RPE where the protein is thought to function as a substrate carrier for 11-cis-retinol dehydrogenase in the synthesis of 11-cis-retinal for visual pigment regeneration. However, CRALBP is also present in optic nerve and brain where its natural ligand and function are not yet known. We have characterized the interactions of retinoids with native bovine CRALBP, human recombinant CRALBP (rCRALBP) and five mutant rCRALBPs. Efforts to trap and/or identify a Schiff base in the dark, under a variety of reducing, denaturing, and pH conditions were unsuccessful, suggesting the lack of covalent interactions between CRALBP and retinoid. Buried and solvent-exposed lysine residues were identified in bovine CRALBP by reductive methylation of the holoprotein followed by denaturation and reaction with [3H]acetic anhydride. Radioactive lysine residues were identified by Edman degradation and electrospray mass spectrometry following proteolysis and purification of modified peptides. Human rCRALBP mutants K152A, K221A, and K294A were prepared to investigate possible retinoid interactions with buried or partially buried lysines. Two other rCRALBP mutants, I162V and Q210R, were also prepared to identify substitutions altering the retinoid binding properties of a random mutant. The structures of all the mutants were verified by amino acid and mass spectral analyses and retinoid binding properties evaluated by UV-visible and fluorescence spectroscopy. All of the mutants bound 11-cis-retinal essentially like the wild type protein, indicating that the proteins were not grossly misfolded. Three of the mutants bound 9-cis-retinal like the wild type protein; however, Q210R and K221A bound less than stoichiometric amounts of the 9-cis-isomer and exhibited lower affinity for this retinoid relative to wild type rCRALBP. Residues Gln-210 and Lys-221 are located within a region of CRALBP exhibiting sequence homology with the ligand binding cavity of yeast phosphatidylinositol-transfer protein. The data implicate Gln-210 and Lys-221 as components of the CRALBP retinoid binding cavity and are discussed in the context of ligand interactions in structurally or functionally related proteins with known crystallographic structures.

The implications of the structure of the bactericidal/permeability-increasing protein on the lipid-transfer function of the cholesteryl ester transfer protein

The cholesteryl ester transfer protein (CETP) is evolutionarily related to the bactericidal/permeability-increasing protein (BPI). The recently solved structure of BPI shows an elongated, boomerang-shaped molecule, with two hydrophobic pockets opening to its concave side. These pockets each contain a phospholipid molecule. A model of CETP, based on the recently solved crystal structure of BPI, provides the basis for interpreting functional studies on CETP. In this model, C-terminal residues 461-476, which were shown to be required for neutral lipid transfer between plasma lipoproteins, from an amphipathic helix covering the opening of the N-terminal pocket. A possible lipid-transfer mechanism for CETP, with the initial step involving the disordering of lipids in the lipoprotein surface, followed by the flipping and entry of a lipid molecule into the hydrophobic lipid-binding pocket, is hypothesized in light of structural evidence and recent studies.

The design of protein-based catalysts using semisynthetic methods

The combination of site-directed mutagenesis and chemical modification has resulted in the preparation of protein conjugates with new and useful properties. Proteins modified with metal-chelating groups are proving useful for mapping tertiary and quaternary interactions using the technique of affinity cleavage. The attachment of cofactors, including pyridoxal and pyridoxamine, has resulted in the preparation of semisynthetic transaminases that display enzyme-like properties, including enantioselectivity, substrate specificity and reaction-rate acceleration.

Turn scanning by site-directed mutagenesis: application to the protein folding problem using the intestinal fatty acid binding protein

We have systematically mutated residues located in turns between beta-strands of the intestinal fatty acid binding protein (IFABP), and a glycine in a half turn, to valine and have examined the stability, refolding rate constants and ligand dissociation constants for each mutant protein. IFABP is an almost all beta-sheet protein exhibiting a topology comprised of two five-stranded sheets surrounding a large cavity into which the fatty acid ligand binds. A glycine residue is located in seven of the eight turns between the antiparallel beta-strands and another in a half turn of a strand connecting the front and back sheets. Mutations in any of the three turns connecting the last four C-terminal strands slow the folding and decrease stability with the mutation between the last two strands slowing folding dramatically. These data suggest that interactions between the last four C-terminal strands are highly cooperative, perhaps triggered by an initial hydrophobic collapse. We suggest that this trigger is collapse of the highly hydrophobic cluster of amino acids in the D and E strands, a region previously shown to also affect the last stage of the folding process (Kim et al., 1997). Changing the glycine in the strand between the front and back sheets also results in a unstable, slow folding protein perhaps disrupting the D-E strand interactions. For most of the other turn mutations there was no apparent correlation between stability and refolding rate constants. In some turns, the interaction between strands, rather than the turn type, appears to be critical for folding while in others, turn formation itself appears to be a rate limiting step. Although there is no simple correlation between turn formation and folding kinetics, we propose that turn scanning by mutagenesis will be a useful tool for issues related to protein folding.

Expression of recombinant psoriasis-associated fatty acid binding protein in Escherichia coli: gel electrophoretic characterization, analysis of binding properties and comparison with human serum albumin

The psoriasis-associated fatty acid binding protein (PA-FABP, also known as FABP5) is a novel keratinocyte protein that is highly up-regulated in psoriatic plaques (P. Madsen, H. H. Rasmussen, H. Leffers, B. Honoré and J. E. Celis, J. Invest. Dermatol. 1992, 99, 299-305). Here we have expressed PA-FABP in Escherichia coli as a fusion protein containing an NH2-terminal hexa-His tag followed by a factor Xa cleavage site. The recombinant protein was expressed at a level of about 30% of the soluble proteins and was purified to homogeneity using a simple two-step protocol consisting of affinity chromatography on Ni2+-nitrilotriacetic acid agarose followed by gel filtration. The recombinant protein was then digested with factor Xa and characterized by two-dimensional gel electrophoresis. The ability of PA-FABP to bind saturated fatty acids ranging from 6 to 16 carbons was determined directly by dialysis and compared to human serum albumin (HSA). The results showed that PA-FABP binds multiple molecules of the fatty acids hexanoate (C6:0), octanoate (C8:0), decanoate (C10:0) and laurate (C12:0), all with a K1 of about 10(4) M(-l), and myristate (C14:0) with a K1 of 4.4 X 10(5) M(-l). Palmitate (C16:0) also bound strongly with multiple molecules. Due to the very low solubility of palmitate its affinity to PA-FABP was measured relatively to HSA and found to be 8.1 times lower. At ligand/protein ratios below 1, all fatty acids bound to PA-FABP with about one to three orders of magnitude lower affinity than to HSA. The difference in the fatty acid binding properties of the two proteins may reflect differences in their three-dimensional structures, which in the case of PA-FABP consists mainly of beta-sheets while HSA contains predominantly alpha-helices.

Molecular cloning and characterization of an invertebrate cellular retinoic acid binding protein

We have cloned a cDNA and gene from the tobacco hornworm, Manduca sexta, which is related to the vertebrate cellular retinoic acid binding proteins (CRABPs). CRABPs are members of the superfamily of lipid binding proteins (LBPs) and are thought to mediate the effects of retinoic acid (RA) on morphogenesis, differentiation, and homeostasis. This discovery of a Manduca sexta CRABP (msCRABP) demonstrates the presence of a CRABP in invertebrates. Compared with bovine/murine CRABP I, the deduced amino acid sequence of msCRABP is 71% homologous overall and 88% homologous for the ligand binding pocket. The genomic organization of msCRABP is conserved with other CRABP family members and the larger LBP superfamily. Importantly, the promoter region contains a motif that resembles an RA response element characteristic of the promoter region of most CRABPs analyzed. Three-dimensional molecular modeling based on postulated structural homology with bovine/murine CRABP I shows msCRABP has a ligand binding pocket that can accommodate RA. The existence of an invertebrate CRABP has significant evolutionary implications, suggesting CRABPs appeared during the evolution of the LBP superfamily well before vertebrate/invertebrate divergence, instead of much later in evolution in selected vertebrates.

The three-dimensional structure of a helix-less variant of intestinal fatty acid-binding protein

Intestinal fatty acid-binding protein (I-FABP) is a cytosolic 15.1-kDa protein that appears to function in the intracellular transport and metabolic trafficking of fatty acids. It binds a single molecule of long-chain fatty acid in an enclosed cavity surrounded by two five-stranded antiparallel beta-sheets and a helix-turn-helix domain. To investigate the role of the helical domain, we engineered a variant of I-FABP by deleting 17 contiguous residues and inserting a Ser-Gly linker (Kim K et al., 1996, Biochemistry 35:7553-7558). This variant, termed delta17-SG, was remarkably stable, exhibited a high beta-sheet content and was able to bind fatty acids with some features characteristic of the wild-type protein. In the present study, we determined the structure of the delta17-SG/palmitate complex at atomic resolution using triple-resonance 3D NMR methods. Sequence-specific 1H, 13C, and 15N resonance assignments were established at pH 7.2 and 25 degrees C and used to define the consensus 1H/13C chemical shift-derived secondary structure. Subsequently, an iterative protocol was used to identify 2,544 NOE-derived interproton distance restraints and to calculate its tertiary structure using a unique distance geometry/simulated annealing algorithm. In spite of the sizable deletion, the delta17-SG structure exhibits a backbone conformation that is nearly superimposable with the beta-sheet domain of the wild-type protein. The selective deletion of the alpha-helical domain creates a very large opening that connects the interior ligand-binding cavity with exterior solvent. Unlike wild-type I-FABP, fatty acid dissociation from delta17-SG is structurally and kinetically unimpeded, and a protein conformational transition is not required. The delta17-SG variant of I-FABP is the only wild-type or engineered member of the intracellular lipid-binding protein family whose structure lacks alpha-helices. Thus, delta17-SG I-FABP constitutes a unique model system for investigating the role of the helical domain in ligand-protein recognition, protein stability and folding, lipid transfer mechanisms, and cellular function.

Fatty acid-binding proteins in the heart

Long-chain fatty acids are important fuel molecules for the heart, their oxidation in mitochondria providing the bulk of energy required for cardiac functioning. The low solubility of fatty acids in aqueous solutions impairs their cellular transport. However, cardiac tissue contains several proteins capable of binding fatty acids non-covalently. These fatty acid-binding proteins (FABPs) are thought to facilitate both cellular uptake and intracellular transport of fatty acids. The majority of fatty acids taken up by the heart seems to pass the sarcolemma through a carrier-mediated translocation mechanism consisting of one or more membrane-associated FABPs. Intracellular transport of fatty acids towards sites of metabolic conversion is most likely accomplished by cytoplasmic FABPs. In this review, the roles of membrane-associated and cytoplasmic FABPs in cardiac fatty acid metabolism under (patho)physiological circumstances are discussed.

Crystal structure of apo-cellular retinoic acid-binding protein type II (R111M) suggests a mechanism of ligand entry

The crystal structure of unliganded mutant R111M of human cellular retinoic acid-binding protein type II (apo-CRABPII (R111M)) has been determined at 2.3 A and refined to a crystallographic R-factor of 0. 18. Although the mutant protein has lower affinity for all-trans-retinoic acid (RA) than the wild-type, it is properly folded, and its conformation is very similar to the wild-type. apo-CRABPII (R111M) crystallizes in space group P1 with two molecules in the unit cell. The two molecules have high structural similarity except that their alpha2 helices differ strikingly. Analyses of the molecular conformation and crystal packing environment suggest that one of the two molecules assumes a conformation compatible with RA entry. Three structural elements encompassing the opening of the binding pocket exhibit large conformational changes, when compared with holo-CRABPII, which include the alpha2 helix and the betaC-betaD and betaE-betaF hairpin loops. The alpha2 helix is unwound at its N terminus, which appears to be essential for the opening of the RA-binding pocket. Three arginine side-chains (29, 59, and 132) are found with their guanidino groups pointing into the RA-binding pocket. A three-step mechanism of RA entry has been proposed, addressing the opening of the RA entrance, the electrostatic potential that directs entry of RA into the binding pocket, and the intramolecular interactions that stabilize the RA.CRABPII complex via locking the three flexible structural elements when RA is bound.

Effects of metal ions on the rates and enantioselectivities of reactions catalyzed by a series of semisynthetic transaminases created by site directed mutagenesis

Fatty acid binding proteins are a class of small 15 kDa proteins with a simple architecture that forms a large solvent sequestered cavity. In previous work, we demonstrated that reductive amination reactions could be performed in this cavity by covalent attachment of a pyridoxamine cofactor and that the rate, enantioselectivity and substrate specificity of these reactions could be altered by site directed mutagenesis. Herein, we show that the chemistry performed by these conjugates can be extended to include catalytic transamination and describe the effects of added metal ions on reaction rate and enantioselectivity. We conclude that metal ions can be used to increase the rate of reactions catalyzed by semisynthetic transaminases; however, the addition of metal ions can also retard the reaction rate. Furthermore, it appears that the presence of metal ions almost always results in an erosion of reaction enantioselectivity. This limits their utility as a practical means of increasing reaction rate. The results reported here, for four independent systems, should be considered in future designs of artificial transaminases.

Presence of a fatty acid-binding protein in the armadillo Harderian gland

A fatty acid-binding protein from the cytosolic fraction of the armadillo Chaetophractus villosus Harderian gland was purified to homogeneity by a procedure based on gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein has an apparent molecular mass of 14 kDa. N-terminal sequence analysis showed that the protein has a blocked N-terminus. For internal amino acid sequencing, the protein was digested in-gel and the resulting peptides were fractionated by reverse-phase high performance liquid chromatography and subjected to automated Edman degradation. Partial amino acid sequencing suggests that it belongs to the heart type. Moreover, it cross-reacted with anti-serum to rat heart fatty acid-binding protein but not with rat intestinal and liver anti-sera. A very slow cross-reaction was also found with anti-serum to rat ALBP. This is the first time that a fatty acid-binding protein has been reported in a Harderian gland.

Thermodynamics of fatty acid binding to engineered mutants of the adipocyte and intestinal fatty acid-binding proteins

We constructed 18 single amino acid mutants of the adipocyte fatty acid-binding protein (A-FABP) and 17 of the intestinal fatty acid-binding protein (I-FABP), at locations in the fatty acid (FA) binding sites. For each mutant protein, we measured thermodynamic parameters that characterize FA binding. Binding affinities ranged from about 200-fold smaller to 30-fold larger than the wild type (WT) proteins. Thermodynamic parameters revealed that binding affinities often inaccurately reported changes in protein-FA interactions because changes in the binding entropy and enthalpy were usually compensatory and larger than the binding free energy. FA-FABP interactions were quite different for I-FABP and A-FABP proteins. Binding affinities were larger and decreased to a greater degree with increasing FA solubility for most of the I-FABP as compared with the A-FABP proteins, consistent with a more hydrophobic binding site for the I-FABP proteins. In A-FABP, Ala substitutions for Arg106 and Arg126, which interact with the FA carboxylate, reduce affinities by about 100-fold, but in I-FABP, R106A increases affinities up to 30-fold. Moreover, in A-FABP, the thermodynamic parameters predict that the FA carboxylate location switches from the 126-position in R106A to the 106 position in R126A. Finally, the A-FABP proteins, in contrast to the I-FABP proteins, reveal significant heat capacity changes (DeltaCp) upon FA binding, and substitutions at residues Arg106 and Arg126 reduce the magnitude of DeltaCp.

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.

Fatty acid binding protein isoforms: structure and function

Although structural aspects of cytosolic fatty acid binding proteins (FABPs) in mammalian tissues are now well understood, significant advances regarding the physiological function(s) of these proteins have been slow in forthcoming. Part of the difficulty lies in the complexity of the multigene FABP family with nearly twenty identified members. Furthermore, isoelectric focusing and ion exchange chromatography operationally resolve many of the mammalian native FABPs into putative isoforms. However, a more classical biochemical definition of an isoform, i.e. proteins differing by a single amino acid, suggests that the operational definition is too broad. Because at least one putative heart H-FABP isoform, the mammary derived growth inhibitor, was an artifact (Specht et al. (1996) J. Biol. Chem. 271: 1943-49), the ensuing skepticism and confusion cast doubt on the existence of FABP isoforms in general. Yet, increasing data suggest that several FABPs, e.g. human intestinal I-FABP, bovine and mouse heart H-FABP, rabbit myelin P2 protein and bovine liver L-FABP may exist as true isoforms. In contrast, the rat liver L-FABP putative isoforms may actually be due either to bound ligand, post-translational S-thiolation and/or structural conformers. In any case, almost nothing is known regarding possible functions of either the true or putative isoforms in vitro or in vivo. The objective of this article is to critically evaluate which FABPs form biochemically defined or true isoforms versus FABPs that form additional forms, operationally defined as isoforms. In addition, recent developments in the molecular basis for FABP true isoform formation, the processes leading to additional operationally defined putative isoforms and insights into potential function(s) of this unusual aspect of FABP heterogeneity will be examined.

Micellar lipoproteins as the possible storage and translocation form of intracellular diacylglycerol

Previous work indicated that diacylglycerol (DG) molecules translocate across the cytoplasm of mammalian cells, a process relevant to the signalling role of this lipid as protein kinase C activator. Here we investigated the possible mechanism underlying DG translocation. We examined the interaction between 1,2-di-[1-14C]oleoyl-sn-glycerol and rat liver cytosol (rlc) using assays based on Lipidex-1000 and on coelution on Sepharose CL 6B. We measured high DG binding activity and found that it resides in cytosolic proteins and not in cytosolic lipids. Chromatography of rlc proteins on Sepharose CL 6B showed profiles in which the activity measured by either method coincided. Further, we showed that the DG-rlc protein interaction results in the stabilization of DG in a micellar form, eluting in the void volume of Sepharose CL 6B. Such stabilized micelles are reminiscent of insect lipophorins and may represent a new, thus far unrecognized, mode of lipid transport within living cells.

Two distinct types of fatty acid-binding protein are expressed in heart ventricle of Antarctic teleost fishes

This report provides the first evidence for the existence of two distinct types of fatty acid-binding protein (FABP) in cardiac tissue of vertebrates. Four species of Antarctic teleost fish (Chaenocephalus aceratus, Cryodraco antarcticus, Gobionotothen gibberifrons and Notothenia coriiceps) exhibited two FABP mRNAs of 1. 0 kb and 0.8 kb, which we have termed Hh-FABP and Had-FABP (isolated from Heart tissue, with similarity to mammalian heart-type FABP or mammalian adipose-type FABP respectively). These FABP types appear to be products of distinct genes. Both FABP transcripts were abundant in cardiac and aerobic pectoral muscle. However, relative abundance of the two types varied distinctly among other tissues such as kidney, brain, spleen and white muscle. Neither FABP type was expressed in liver or intestine. The coding regions of Hh-FABP and Had-FABP cDNAs from the same species are only approximately 60% identical with one another. However, homologues of each FABP species, which exhibit >98% identity to their respective types, were isolated from three other Antarctic teleosts. Phylogenetic analysis of aligned amino-acid sequences places Hh-FABP with other vertebrate heart-type FABPs, and Had with adipose/cutaneous FABPs. Expression of two distinct FABPs in cardiac tissue of Antarctic teleosts may be related to their ability to both utilize fatty acid as the primary metabolic fuel and to store lipid intracellularly.

Structure of Escherichia coli ribokinase in complex with ribose and dinucleotide determined to 1.8 A resolution: insights into a new family of kinase structures

BACKGROUND

D-ribose must be phosphorylated at O5' before it can be used in either anabolism or catabolism. This reaction is catalysed by ribokinase and requires the presence of ATP and magnesium. Ribokinase is a member of a family of carbohydrate kinases of previously unknown structure.

RESULTS

The crystal structure of ribokinase from Escherichia coli in complex with ribose and dinucleotide was determined at 1.84 A resolution by multiple isomorphous replacement. There is one 33 kDa monomer of ribokinase in the asymmetric unit but the protein forms a dimer around a crystallographic twofold axis. Each subunit consists of a central alpha/beta unit, with a new type of nucleotide-binding fold, and a distinct beta sheet that forms a lid over the ribose-binding site. Contact between subunits involves orthogonal packing of beta sheets, in a novel dimer interaction that we call a beta clasp.

CONCLUSIONS

Inspection of the complex indicates that ribokinase utilises both a catalytic base for activation of the ribose in nucleophilic attack and an anion hole that stabilises the transition state during phosphoryl transfer. The structure suggests an ordered reaction mechanism, similar to those proposed for other carbohydrate kinases that probably involves conformational changes. We propose that the beta-clasp structure acts as a lid, closing and opening upon binding and release of ribose. From these observations, an understanding of the structure and catalytic mechanism of related sugar kinases can be obtained.

Crystal structure of p14TCL1, an oncogene product involved in T-cell prolymphocytic leukemia, reveals a novel beta-barrel topology

BACKGROUND

Chromosome rearrangements are frequently involved in the generation of hematopoietic tumors. One type of T-cell leukemia, T-cell prolymphocytic leukemia, is consistently associated with chromosome rearrangements characterized by the juxtaposition of the TCRA locus on chromosome 14q11 and either the TCL1 gene on 14q32.1 or the MTCP1 gene on Xq28. The TCL1 gene is preferentially expressed in cells of early lymphoid lineage; its product is a 14 kDa protein (p14TCL1), expressed in the cytoplasm. p14TCL1 has strong sequence similarity with one product of the MTCP1 gene, p13MTCP1 (41% identical and 61% similar). The functions of the TCL1 and MTCP1 genes are not known yet. They have no sequence similarity to any other published sequence, including those of well-documented oncogene families responsible for leukemia. In order to gain a more fundamental insight into the role of this particular class of oncogenes, we have determined the three-dimensional structure of p14TCL1.

RESULTS

The crystal structure of p14TCL1 has been determined at 2.5 A resolution. The structure was solved by molecular replacement using the solution structure of p13MTCP1, revealing p14TCL1 to be an all-beta protein consisting of an eight-stranded antiparallel beta barrel with a novel topology. The barrel consists of two four-stranded beta-meander motifs, related by a twofold axis and connected by a long loop. This internal pseudo-twofold symmetry was not expected on basis of the sequence alone, but structure-based sequence analysis of the two motifs shows that they are related. The structures of p13MTCP1 and p14TCL1 are very similar, diverging only in regions that are either flexible and/or involved in crystal packing. p14TCL1 forms a tight crystallographic dimer, probably corresponding to the 28 kDa species identified in solution by gel filtration experiments.

CONCLUSIONS

Structural similarities between p14TCL1 and p13MTCP1 suggest that their (unknown) function may be analogous. This is confirmed by the fact that these proteins are implicated in analogous diseases. Their structure does not show similarity to other oncoproteins of known structure, confirming their classification as a novel class of oncoproteins.

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.

Simulations of fatty acid-binding proteins. II. Sites for discrimination of monounsaturated ligands

Fatty acid binding proteins (FABPs) can discriminate between saturated and unsaturated fatty acids via molecular mechanisms that are not understood. Molecular dynamics computer calculations are used to suggest the relationship between tertiary structure and binding specificity. Three separate 1-ns simulations, with explicit solvent, are presented: 1) oleic acid (C18:1 cis) bound to adipocyte FABP, 2) oleic acid bound to human muscle FABP, and 3) elaidic acid (C18:1 trans) bound to human muscle FABP. The average structural, dynamic, and energetic properties of the trajectory were analyzed, as were the motional correlations. The molecular dynamics trajectories reveal intriguing differences among all three systems. For example, the two proteins have different strengths of interaction energy with the ligand and different motional coupling, as seen with covariance analysis. This suggests distinctive molecular behavior of monounsaturated fatty acids in the two similar proteins. An importance scale, based on motional correlation and interaction energy between protein and ligand, is proposed, to help identify amino acids involved with the discrimination of ligand saturation state or geometric isomerization.

Transfection of L6 myoblasts with adipocyte fatty acid-binding protein cDNA does not affect fatty acid uptake but disturbs lipid metabolism and fusion

We studied the involvement of fatty acid-binding protein (FABP) in growth, differentiation and fatty acid metabolism of muscle cells by lipofection of rat L6 myoblasts with rat heart (H) FABP cDNA or with rat adipocyte (A) FABP cDNA in a eukaryotic expression vector which contained a puromycin acetyltransferase cassette. Stable transfectants showed integration into the genome for all constructs and type-specific overexpression at the mRNA and protein level for the clones with H-FABP and A-FABP cDNA constructs. The rate of proliferation of myoblasts transfected with rat A-FABP cDNA was 2-fold higher compared with all other transfected cells. In addition, these myoblasts showed disturbed fusion and differentiation, as assessed by morphological examination and creatine kinase activity. Uptake rates of palmitate were equal for all clone types, in spite of different FABP content and composition. Palmitate oxidation over a 3 h period was similar in all clones from growth medium. After being cultured in differentiation medium, mock- and H-FABP-cDNA-transfected cells showed a lower fatty acid-oxidation rate, in contrast with A-FABP-cDNA-transfected clones. The ratio of [14C]palmitic acid incorporation into phosphatidylcholine and phosphatidylethanolamine of A-FABP-cDNA-transfected clones changed in the opposite direction in differentiation medium from that of mock- and H-FABP-cDNA-transfected clones. In conclusion, transfection of L6 myoblasts with A-FABP cDNA does not affect H-FABP content and fatty acid uptake, but changes fatty acid metabolism. The latter changes may be related to the observed fusion defect.

Cloning and sequencing of complementary DNA for fatty acid binding protein from rainbow trout heart

A cDNA encoding a rainbow trout homologue of mammalian heart fatty acid binding protein (H-FABP) was isolated. The deduced protein sequence is 75% identical to that of rat H-FABP. The structural conservation of H-FABPs and their evolutionary relationship are discussed.

Regulation of gene expression in adipose cells by polyunsaturated fatty acids

In fat cells polyunsaturated fatty acids are both substrates for, and products of, triacylglycerol metabolism. Dietary fatty acids are efficiently incorporated into the triacylglycerol droplet under lipogenic conditions while rapidly mobilizing them during lipolytic stimulation. Hence, the flux and magnitude of the fatty acid pool in adipocytes is constantly changing in response to hormonal, metabolic and genetic determinants. Due to the rapidly changing flux of fatty acids, the majority of genes encoding enzymes and proteins of lipid metabolism are largely refractory to long-term regulatory control by fatty acids. Only at extremes of high or low lipid levels, or under pathophysiological conditions, do adipose genes respond by up- or down-regulating gene expression. Despite the lack of responsiveness to lipids in adipose tissue, a surprisingly large number of genes have been characterized recently as lipid responsive when assayed in heterologous systems. These observations suggest an endogenous negative element exists in the lipid signaling pathway in adipocytes. The major intracellular lipid binding protein in adipose cells is the adipocyte lipid binding protein (ALBP), the product of the aP2 gene. This protein is 15 kDa, abundant and found exclusively in the cytoplasm of adipocytes. The protein binds fatty acids and related lipids in a 1:1 stoichiometry within a large water filled interior cavity. The lipid binding protein forms high affinity associations with polyunsaturated fatty acids such as arachidonic acid (Kd approximately 250 nM) but not with prostaglandins of the E, D or J series (Kd > 4 microM). The upstream region of the aP2 gene contains a peroxisome-proliferator activated receptor response element which associates with PPARs to regulate its expression. A positive autoregulatory circuit exists to upregulate lipid binding protein expression when polyunsaturated fatty acid levels are increased. Analysis of adipose tissue from aP2 null animals generated by a targeted disruption revealed that the partial loss of ALBP expression in heterozygotes and complete lack of ALBP in the nulls was accompanied by a compensatory up-regulation of the keratinocyte lipid binding protein. However, the total amount of lipid binding protein in the nulls was less than 15% that in the wild type littermates. No evidence was found for upregulation of other lipid binding proteins such as the heart FABP or liver FABP. In aP2 nulls, the fatty acid composition was unaltered but the mass of fatty acid per gram tissue more than doubled relative to wild type. In heterozygotes, the level of fatty acid was intermediate to that of wild-type and nulls, consistent with an intermediate level of lipid binding protein. These results indicate that the fatty acid pool level in adipocytes is inversely correlated with the amount of lipid binding protein. Since prostaglandin biosynthesis is dependent upon polyunsaturated fatty acid substrates, the intracellular lipid binding proteins control accessibility of substrates of the prostanoid pathway. Intracellular lipid binding proteins therefore are negative elements in polyunsaturated fatty acid control of gene expression.

Cavity formation before stable hydrogen bonding in the folding of a beta-clam protein

The time course of folding of a small beta-sheet protein reveals formation of a central ligand binding cavity before the consolidation of the native hydrogen bonding network. These results suggest that side chain interactions and not stable hydrogen bonding determine the beta-sheet architecture and play crucial roles in the overall chain topology.

Lipid specificity and location of the sterol carrier protein-2 fatty acid-binding site: a fluorescence displacement and energy transfer study

Although it was recently recognized that sterol carrier protein-2 (SCP-2) interacts with fatty acids, little is known regarding the specificity of SCP-2 for long-chain fatty acids or branched-chain fatty-acid-like molecules. Likewise the location of the fatty-acid binding site within SCP-2 is unresolved. A fluorescent cis-parinaric acid displacement assay was used to show that SCP-2 optimally interacted with 14-22 carbon chain lipidic molecules: polyunsaturated fatty acids > monounsaturated, saturated > branched-chain isoprenoids > branched-chain phytol-derived fatty acids. In contrast, the other major fatty-acid binding protein in liver, fatty-acid binding protein (L-FABP), displayed a much narrower carbon chain preference in general: polyunsaturated fatty acids > branched-chain phytol-derived fatty acids > 14- and 16-carbon saturated > branched-chain isoprenoids. However, both SCP-2 and L-FABP displayed a very similar unsaturated fatty-acid specificity profile. The presence and location of the SCP-2 lipid binding site were investigated by fluorescence energy transfer. The distance between the SCP-2 Trp50 and bound cis-parinaric acid was determined to be 40 A. Thus, the SCP-2 fatty-acid binding site appeared to be located on the opposite side of the SCP-2 Trp50. These findings not only contribute to our understanding of the SCP-2 ligand binding site but also provide evidence suggesting a potential role for SCP-2 and/or L-FABP in metabolism of branched-chain fatty acids and isoprenoids.

Amino acid sequence, binding properties and evolutionary relationships of the basic liver fatty-acid-binding protein from the catfish Rhamdia sapo

The complete amino acid sequence of a basic liver fatty acid-binding protein (L-FABP) from catfish (Rhamdia sapo) was determined. Alignment of sequences shows that it has more similarity to chicken basic L-FABP than to mammalian L-FABP. The phylogenetic analysis suggests that basic L-FABP from catfish, chicken and iguana diverged from the mammalian protein before the fish-tetrapod divergence, thus implying that the two types are encoded by different genes. Supporting this conclusion, a 14-kDa protein, structurally closely related to mammalian L-FABP, was isolated from catfish intestine, indicating the presence of the two genes in the same species. The catfish basic L-FABP binds only one fatty acid/molecule, while mammalian L-FABP bind two. The former has more affinity for trans-parinaric acid than for cis-parinaric acid, in constrast to the latter proteins.

Role of membrane-associated and cytoplasmic fatty acid-binding proteins in cellular fatty acid metabolism

A number of membrane-associated and cytoplasmic fatty acid-binding proteins (FABPs) are now being implicated in the cellular uptake and intracellular transport of long-chain fatty acids (FA). These proteins each have the capacity of non-covalent binding of FA, are present in tissues actively involved in FA metabolism, and are upregulated in conditions of increased cellular FA metabolism. To date, five distinct membrane FABPs have been described, ranging in mass from 22 to 88 kDa and each showing a characteristic tissue distribution. Evidence for involvement in cellular fatty acid uptake has been provided for several of them, because it was recently found that isolated cell lines transfected with 88-kDa putative fatty acid translocase (FAT; homologous to CD36) or with 63-kDa fatty acid-transport protein show an increased rate of FA uptake. The (at least nine) FABPs of cytoplasmic origin belong to a family of small (14-15 kDa) lipid binding proteins, all having a similar tertiairy structure but differing in binding properties and in tissue occurrence. The biological functions of the various FABPs, possibly exerted in a concerted action among them, comprise solubilization and compartmentalization of FA, facilitation of the cellular uptake and intracellular trafficking of FA, and modulation of mitosis, cell growth, and cell differentiation. In addition, the FABPs have been suggested to participate in and/or modulate FA-mediated signal transduction pathways and FA regulation of gene expression, and to prevent local high FA concentrations thereby contributing to the protection of cells against the toxic effects of FA. In conclusion, long-chain fatty acids are subject to continuous interaction with multiple proteins, which interplay influences their cellular metabolism.

Comparison of straight chain and cyclic unnatural amino acids embedded in the core of staphylococcal nuclease

We have determined by X-ray crystallography the structures of several variants of staphylococcal nuclease with long flexible straight chain and equivalent length cyclic unnatural amino acid side chains embedded in the protein core. The terminal atoms in the straight side chains are not well defined by the observed electron density even though they remain buried within the protein interior. We have previously observed this behavior and have suggested that it may arise from the addition of side-chain vibrational and oscillational motions with each bond as a side chain grows away from the relatively rigid protein main chain and/or the population of multiple rotamers (Wynn R, Harkins P, Richards FM. Fox RO. 1996. Mobile unnatural amino acid side chains in the core of staphylococcal nuclease. Protein Sci 5:1026-1031). Reduction of the number of degrees of freedom by cyclization of a side chain would be expected to constrain these motions. These side chains are in fact well defined in the structures described here. Over-packing of the protein core results in a 1.0 A shift of helix 1 away from the site of mutation. Additionally, we have determined the structure of a side chain containing a single hydrogen to fluorine atom replacement on a methyl group. A fluorine atom is intermediate in size between methyl group and a hydrogen atom. The fluorine atom is observed in a single position indicating it does not rotate like methyl hydrogen atoms. This change also causes subtle differences in the packing interactions.

Mutants of rat intestinal fatty acid-binding protein illustrate the critical role played by enthalpy-entropy compensation in ligand binding

Site-specific variants of rat intestinal fatty acid-binding protein were constructed to identify the molecular interactions that are important for binding to fatty acids (FAs). Several variants displayed affinities that appeared incompatible with the crystal structure of the protein-FA complex. Thermodynamic measurements provided an explanation for these apparent inconsistencies and revealed that binding affinities often inaccurately reported changes in protein-FA interactions because changes in the binding entropy and enthalpy were usually compensatory. These results demonstrate that understanding the effects of amino acid replacements on ligand binding requires measurements of enthalpy and entropy, in addition to affinity.

Photoacoustic analysis of proteins: volumetric signals and fluorescence quantum yields

A series of proteins has been examined using time-resolved, pulsed-laser volumetric photoacoustic spectroscopy. Photoacoustic waveforms were collected to measure heat release for calculation of fluorescence quantum yields, and to explore the possibility of photoinduced nonthermal volume changes occurring in these protein samples. The proteins studied were the green fluorescent protein (GFP); intestinal fatty acid binding protein (IFABP), and adipocyte lipid-binding protein (ALBP), each labeled noncovalently with 1-anilinonaphthalene-8-sulfonate (1,8-ANS) and covalently with 6-acryloyl-2-(dimethylamino)naphthalene (acrylodan); and acrylodan-labeled IFABP and ALBP with added oleic acid. Of this group of proteins, only the ALBP labeled with 1,8-ANS showed significant nonthermal volume changes at the beta = 0 temperature (approximately 3.8 degrees C) for the buffer used (10 mM Tris-HCI, pH 7.5) (beta is the thermal cubic volumetric expansion coefficient). For all of the proteins except for acrylodan-labeled IFABP, the fluorescence quantum yields calculated assuming simple energy conservation were anomalously high, i.e., the apparent heat signals were lower than those predicted from independent fluorescence measurements. The consistent anomalies suggest that the low photoacoustic signals may be characteristic of fluorophores buried in proteins, and that photoacoustic signals derive in part from the microenvironment of the absorbing chromophore.

Heart-type fatty acid binding protein - involvement in growth inhibition and differentiation

Fatty acid binding proteins (FABPs) comprise a well-established family of cytoplasmic hydrophobic ligand binding proteins and are thought to be involved in lipid metabolism by binding and intracellular transport of long-chain fatty acids. However, from other studies role for FABPs in cell signalling, growth inhibition and differentiation has also been implied. In particular, the heart-type (H-FABP) is abundantly expressed in differentiated mammary gland and its relationship with a very homologous (95%) mammary derived growth inhibitor (MDGI) was disputed. Here we give a survey on the experimental evidence for the existence of such protein with growth inhibitory function. After cloning of the bovine adipocyte-type (A-)FABP cDNA from mammary gland we conclude that the reported MDGI sequence actually represents a mixture of bovine H- and A-FABP and that the MDGI function is exerted by H-FABP. We also monitored the H-FABP level during differentiation of C2C12 muscle cells from myoblasts to multiply nucleated myotubes. H-FABP expression is clearly detected after that of the transcription factor myogenin which is upregulated immediately upon onset of differentiation and after that of the typical muscle enzyme creatine kinase. This argues against an active role of H-FABP in muscle development unlike the situation in the mammary gland.

Dramatic enhancement of the specific expression of the heart-type fatty acid binding protein in rat brown adipose tissue by cold exposure

To understand the difference in energy metabolisms in brown (BAT) and white (WAT) adipose tissues, we examined the steady-state transcript levels of the heart-type and adipose-type fatty acid binding proteins (H-FABP and A-FABP, respectively) by Northern blot analysis. The transcript of H-FABP in rat BAT was increased about 100-fold by cold exposure, whereas that in WAT was negligible, and was increased only slightly by cold exposure. The transcript of A-FABP was observed in both BAT and WAT, the level being slightly greater in WAT. However, its transcript level was not affected by cold exposure in either adipose tissue. In addition, on treatment with norepinephrine (NE), transcript level of H-FABP was elevated markedly but that of A-FABP was not changed in rat brown adipocytes. Therefore, the stimulatory effect of cold exposure on the transcript of H-FABP in BAT was concluded to be mediated by NE, like that of the uncoupling protein (UCP). Thus, the expressions of H-FABP and UCP may be controlled by the same mechanism.

Crystal structure of human BPI and two bound phospholipids at 2.4 angstrom resolution

Bactericidal/permeability-increasing protein (BPI), a potent antimicrobial protein of 456 residues, binds to and neutralizes lipopolysaccharides from the outer membrane of Gram-negative bacteria. At a resolution of 2.4 angstroms, the crystal structure of human BPI shows a boomerang-shaped molecule formed by two similar domains. Two apolar pockets on the concave surface of the boomerang each bind a molecule of phosphatidylcholine, primarily by interacting with their acyl chains; this suggests that the pockets may also bind the acyl chains of lipopolysaccharide. As a model for the related plasma lipid transfer proteins, BPI illuminates a mechanism of lipid transfer for this protein family.

Expression of fatty acyl-CoA binding proteins in colon cells: response to butyrate and transformation

Fatty acyl-CoA affect many cellular functions as well as serving as cellular building blocks. Several families of cytosolic fatty acyl-CoA binding proteins may modulate the activities of fatty acyl-CoA. Intestinal enterocytes contain at least three unique families of cytosolic proteins that bind fatty acyl-CoA: acyl-CoA binding protein (ACBP), fatty acid binding proteins (including the liver, L-FABP and intestinal, I-FABP), and sterol carrier protein-2 (SCP-2). Immortalized rat colon epithelial cell lines expressed only ACBP and SCP-2 at levels of 0.75 +/- 0.13 and 0.42 +/- 0.02 ng/microgram protein. Ras and src transformation increased colon cell density and differentially altered ACBP and SCP-2 expression without affecting I-FABP or L-FABP levels. ACBP levels were 1.8-fold and 1.5-fold increased in ras- and src-transformed cells, respectively. In contrast, SCP-2 expression was significantly decreased 55 and 67% in ras- and src-transformed cells, respectively. Butyrate treatment of ras- and src-transformed cells decreased cell proliferation up to 60-85% as compared to 25-30% in control cells. Butyrate treatment decreased ACBP expression in all cell lines but had no effect on the levels of SCP-2, I-FABP, or L-FABP. These studies suggest that the differential expression of ACBP and SCP-2 in rat colonic cell lines, as well as their modulation by butyrate, may be altered by cell transformation.

Isoforms of rat liver fatty acid binding protein differ in structure and affinity for fatty acids and fatty acyl CoAs

Although native rat liver fatty acid binding protein (L-FABP) is composed of isoforms differing in isoelectric point, their comparative structure and function are unknown. These properties of apo- and holo-L-FABP isoforms were resolved by circular dichroism, time-resolved fluorescence spectroscopy, and binding/displacement of fluorescent ligands. Both apo-isoforms had similar hydrodynamic radii of 18.5 A, but apo-isoform I had a greater alpha-helical content and exhibited a longer Tyr lifetime, indicative of secondary and tertiary structural differences from isoform II. Isoforms I and II both had two fatty acid or fatty acyl CoA binding sites. Ligand binding decreased the isoform hydrodynamic radii by 3-4 A and increased Tyr rotational motions in a more restricted range. Fatty acyl CoAs were more effective than fatty acids in altering the isoform structures. Scatchard analysis showed that both isoforms bound cis- parinaric acid with high affinity (Kd values 41 and 60 nM, respectively) and bound trans-parinaric acid with 2- and 7-fold, respectively, higher affinity than for cis-parinaric acid. In contrast, isoform I had higher affinity for cis- and trans-parinaroyl CoAs (Kd values of 33 and 14 nM) than did isoform II (Kd values of 110 and 97 nM), thereby resulting in biphasic plots of parinaroyl-CoA binding to native L-FABP. Finally, displacement studies indicated that each isoform displayed distinct specificities for fatty acid/fatty acyl CoA chain length and unsaturation. Thus, rat L-FABP isoforms differ markedly in both structure and ligand binding function.

Secretion of a novel, developmentally regulated fatty acid-binding protein into the perivitelline fluid of the parasitic nematode, Ascaris suum

Early development of the parasitic nematode, Ascaris suum, occurs inside a highly resistant eggshell, and the developing larva is bathed in perivitelline fluid. Two-dimensional gel analysis of perivitelline fluid from infective larvae reveals seven major proteins; a cDNA encoding one of these, As-p18, has been cloned, sequenced, and protein expressed in Escherichia coli. The predicted amino acid sequence of As-p18 exhibits similarities to the intracellular lipid-binding protein (iLBP) family including retinoid- and fatty acid-binding proteins (FABP). As-p18 is unusual in that it possesses a hydrophobic leader that is not present in the mature protein, the developmental regulation of its expression, and in terms of its predicted structure. Recombinant As-p18 is a functional FABP with a high affinity for both a fluorescent fatty acid analog (11(((5-(dimethylamino)-1-naphthalenyl)sulfonyl)amino) undecanoic acid) and oleic acid, but not retinol. Circular dichroism of rAs-p18 reveals a high beta-sheet content (62%), which is consistent with secondary structure for the protein predicted from sequence algorithms, and the structure of iLBPs. Unusual features are apparent in a structural model of As-p18 generated from existing crystal structures of iLBPs. As-p18 is not found in unembryonated eggs, begins to be synthesized at about day 3 of development, reaches a maximal concentration with the formation of the first-stage larva and remains abundant in the perivitelline fluid of the second-stage larva. Since As-p18 is not present in the post-infective third-stage larva or adult worm tissues, it appears to be exclusive to the egg. Surprisingly, however, Northern blot analysis yields mRNA for As-p18 not only in the early larval stages, but also the unembryonated egg, third-stage larvae, and ovaries of adult worms, even though the protein is not detectable from any of those sources. As-p18 may play a role in sequestering potentially toxic fatty acids and their peroxidation products, or it may be involved in the maintenance of the impermeable lipid layer of the eggshell.