Purification and characterization of an outer membrane-bound protein involved in long-chain fatty acid transport in Escherichia coli

Characteristics of oleate binding to liver plasma membranes and its uptake by isolated hepatocytes

To clarify mechanisms of hepatic free fatty acid uptake, [3H]oleate uptake by isolated rat hepatocytes was studied, using solutions of 150 microM bovine serum albumin at oleate:albumin molar ratios of 0.033-6.7:1. Oleate partitioning between liver plasma membranes and albumin was also studied, and used to ascertain the membrane binding function for oleate. The experimental uptake curve was complex, but could be resolved by computer fitting into a sum of two components, one a saturable and the second a linear function of the unbound oleate concentration. The saturable component comprises > 90% of total oleate uptake when the oleate:albumin molar ratio is < 2.5, but < 50% when this ratio is > 5. Membrane binding also consisted of a sum of a saturable and a linear component. By comparison of the computer-fitted uptake and binding functions, separate rate constants for the transfer into the cell of the saturably and non-saturably bound oleate were estimated to be 0.7 s-1 and 0.05 s-1, respectively. The former is compatible with a specific, protein-mediated process. It is 15-times greater than the corresponding rate constant for transfer of non-saturably bound oleate into the cell, which in turn is similar to reported rates of non-specific 'flip-flop' of fatty acids across lipid bilayers. The observed kinetics are not consistent with models in which uptake occurs principally from the albumin-bound pool of oleate, or solely from the oleate which has partitioned passively into the lipid bilayer of the plasma membrane.

Stable, conserved outer membrane epitope of strains of Haemophilus influenzae biogroup aegyptius associated with Brazilian purpuric fever

Brazilian purpuric fever is a rapidly fatal childhood disease associated with a clonal strain of Haemophilus influenzae biogroup aegyptius. We describe a conserved, surface-exposed epitope present on 95% of H. influenzae biogroup aegyptius isolates that are associated with Brazilian purpuric fever. This epitope, defined by reaction with the monoclonal antibody 8G3, is on or associated with the 48-kDa heat-modifiable P1 protein. The epitope is absent on strains of H. influenzae biogroup aegyptius that are not associated with Brazilian purpuric fever but is present on one strain of H. influenzae biotype II. None of 81 other Haemophilus strains tested reacted with 8G3. The sensitivity and specificity of the 8G3 monoclonal antibody in detecting Brazilian case-clone strains of H. influenzae biogroup aegyptius associated with Brazilian purpuric fever are 95 and 99%, respectively. Immunoelectron microscopy revealed that the epitope is surface exposed, and N-terminal amino acid sequencing of an 8G3-reactive P1 protein from a strain of H. influenzae biogroup aegyptius showed 100% correlation with the published N-terminal amino acid sequence of a P1 protein of H. influenzae type b. The virulence of the organism in an infant rat model of bacteremia was not dependent on the expression of this epitope.

Primary sequence of the Escherichia coli fadL gene encoding an outer membrane protein required for long-chain fatty acid transport

The fadL gene of Escherichia coli encodes an outer membrane protein (FadL) that plays a central role in the uptake of exogenous long-chain fatty acids. The nucleotide sequence of the fadL gene revealed a single open reading frame of 1,344 bp encoding a protein with 448 amino acid residues and a molecular weight of 48,831. The transcriptional start, analyzed by primer extension, was shown to be 95 bp upstream from the translational start. Apparent -10 and -35 regions were found at -12 and -37 bp upstream from the transcriptional start. Three regions with hyphenated dyad symmetry (two between the transcriptional start and the translational start and one upstream from the -10 and -35 regions) were identified that may play a role in the expression of fadL. The protein product of the fadL gene contained a signal sequence and signal peptidase I cleavage site similar to that defined for other E. coli outer membrane proteins. The N-terminal sequence of mature FadL protein was determined by automated amino acid sequencing of protein purified from the outer membrane of a strain harboring fadL under the control of a T7 RNA polymerase-responsive promoter. This amino acid sequence, Ala-Gly-Phe-Gln-Leu-Asn-Glu-Phe-Ser-Ser, verified the signal peptidase I cleavage site on pre-FadL and confirmed the N-terminal amino acid sequence of FadL predicted from the DNA sequence. Mature FadL contained 421 amino acid residues, giving a molecular weight of 45,969. The amino acid composition of FadL deduced from the DNA sequence suggested that this protein contained an abundance of hydrophobic amino acid residues and lacked cysteinyl residues. The hydrophobic amino acids within FadL were predicted to contribute to at least five regions of the protein with an overall hydrophobic character. The amino acid sequence of FadL was used to search GenBank for other proteins with amino acid sequence homology. These data demonstrated that FadL and the heat-modifiable outer membrane protein P1 of Haemophilus influenzae type b were 60.5% conserved and 42.0% identical over 438 amino acid residues.

Characterization of FadL-specific fatty acid binding in Escherichia coli

The product of the fadL gene (FadL) is a central component of the long-chain fatty acid transport system of Escherichia coli. When fatty acid activation is blocked by a mutation in the structural gene for acyl CoA synthetase (fadD) transport is inhibited allowing a FadL-specific fatty acid binding activity to be measured. This binding activity was 4- to 6-fold greater in the fadL+ fadD strain LS6928 when compared to the delta fadLfadD strain LS6929. With long-chain fatty acids, this binding activity was saturable and it was estimated that there were approx. 35,000 FadL-specific oleic acid binding sites per cell in the fadL+ strain LS6928. The FadL-specific fatty acid binding affinity was highest for oleic acid (18:1) and palmitic acid (16:0) giving apparent KD values of 2.3.10(-7) M and 8.8.10(-7) M, respectively. FadL-specific binding affinity of myristic acid (14:0) was nearly an order of magnitude less and no FadL-specific binding of decanoic acid (10:0) could be measured. Two lines of evidence suggest that FadL-fatty acid binding occurs by a hydrophobic interaction: (1) There was a preference for the long-chain substrates oleic acid and palmitic acid; and (2) oleic acid binding activity was not significantly changed over the pH range 5.0 to 8.0. The FadL-specific binding of oleic acid in the fadL+ strain LS6928 could be blocked by preincubation with antisera raised against purified FadL providing a clear correlation between the activity and identity of FadL. The binding activity associated with FadL was measured in vesicles of the outer membrane following passage over the hydrophobic resin Lipidex 1000. The KD of oleic acid binding attributable to FadL in outer membranes vesicles (6.0.10(-7) M) was in close agreement with that determined in whole cells. Overall, these studies demonstrated that FadL binds long-chain fatty acids with a relatively high affinity prior to their transport across the outer membrane.

The fadL gene product of Escherichia coli is an outer membrane protein required for uptake of long-chain fatty acids and involved in sensitivity to bacteriophage T2

The fadL+ gene of Escherichia coli encodes an outer membrane protein (FadL) essential for the uptake of long-chain fatty acids (C12 to C18). The present study shows that in addition to being required for uptake of and growth on the long-chain fatty acid oleate (C18:1), FadL acts as a receptor of bacteriophage T2. Bacteriophage T2-resistant (T2r) strains lacked FadL and were unable to take up and grow on long-chain fatty acids. Upon transformation with the fadL+ clone pN103, T2r strains became sensitive to bacteriophage T2 (T2s), became able to take up long-chain fatty acids at wild-type levels, and contained FadL in the outer membrane.

Uptake of fatty acids by Mycoplasma capricolum

The energy requirements for fatty acid uptake by Mycoplasma capricolum were studied. Fatty acid transport and esterification to phospholipid appeared to be tightly coupled, since there was little intracellular accumulation of free fatty acid. Uptake was blocked by iodoacetate, n-ethylmaleimide, and p-chloromercuribenzoate. Glucose, glycerol, and potassium ions were necessary for fatty acid uptake by whole cells. A reduction in uptake was observed in cells treated with valinomycin or dicyclohexylcarbodiimide. The effect of temperature on the rate of oleate uptake showed a discontinuity at 24 degrees C. Above 24 degrees C an energy of activation of 4.6 kcal (ca. 19.2 kJ)/mol was obtained. The data suggest that uptake of fatty acid by M. capricolum is an energy-linked, protein-mediated process. A membrane-bound enzyme activity that catalyzed the synthesis of fatty acyl-hydroxamate was demonstrated. This activity was virtually independent or only marginally dependent on coenzyme A, depending on the assay system, but was stimulated approximately twofold by ATP.

Nucleotide sequencing and expression of the fadL gene involved in long-chain fatty acid transport in Escherichia coli

The fadL gene of Escherichia coli codes for an outer membrane protein involved in long-chain fatty acid transport. Its product was purified from outer membrane proteins. We determined the nucleotide sequence of a 2.8-kb chromosomal DNA segment that contains the fadL gene. The fadL gene consists of a 1149-nucleotide coding region and contains a highly hydrophobic polypeptide of 383 amino acids with a calculated molecular weight of 42,380. We have used S1-mapping analysis to identify the transcription initiation site. A region exhibiting extensive dyad symmetry and perfect homology to the catabolite activator protein binding site was detected.

Fatty acid transport through the blood-brain barrier

Across the cerebral capillaries, the anatomical locus of the blood-brain barrier, the unidirectional influxes of the saturated fatty acids, octanoic and myristic acids, and the unsaturated essential fatty acid, linoleic acid, were measured. Employing an in situ rat brain perfusion technique that allows control of perfusate composition and accurate measurement of perfusate-to-brain fatty acid transport, we found that both [14C]octanoic and [14C]myristic acids were transported through the blood-brain barrier in vivo, in large part, by a specific, probenecid-sensitive transport system. However, the transport of [14C]linoleic acid was not probenecid sensitive. With 0.5 microM fatty acid but no plasma proteins in the perfusate, the permeability-surface area constant was higher for myristic acid (4.8 X 10(-2) X s-1) than for octanoic and linoleic acids (1.5 and 1.2 X 10(-2) X s-1, respectively). Approximately 70, 30, and 25% of the [14C]myristic, [14C]octanoic, or [14C]linoleic acids, respectively, were extracted from the perfusate.