A fluorimetric determination of the activity of glycolipid transfer protein and some properties of the protein purified from pig brain

Who moves the sphinx? An overview of intracellular sphingolipid transport

Lipid bilayers function as boundaries that enclose their content from the surrounding media, and the composition of different membrane types is accurately and dynamically tailored so that they can perform their function. To achieve this balance, lipid biosynthetic machinery and lipid trafficking events are intertwined into an elegant network. In this review, we focus on the intracellular movement of sphingolipids mediated by sphingolipid transfer proteins. Additionally, we will focus on the best characterized and understood mammalian sphingolipid transfer proteins and provide an overview of how they are hypothesized to function. Some are already well understood, while others remain enigmatic. A few are actual lipid transfer proteins, moving lipids from membrane to membrane, while others may have more of a sensor role, possibly reacting to changes in the concentrations of their ligands. Considering the substrates available for cytosolic sphingolipid transfer proteins, one open question that is discussed is whether galactosylceramide is a target. Another question is the exact mechanics by which sphingolipid transfer proteins are targeted to different organelles, such as how four phosphate adapter protein-2, FAPP2 is targeted to the endoplasmic reticulum. The aim of this review is to discuss what is known within the field today and to provide a basic understanding of how these proteins may work.

In Vitro Measurement of Sphingolipid Intermembrane Transport Illustrated by GLTP Superfamily Members

Herein, we describe methodological approaches for measuring in vitro transfer of sphingolipids (SLs) between membranes. The approaches rely on direct tracking of the lipid. Typically, direct tracking involves lipid labeling via attachment of fluorophores or introduction of radioactivity. Members of the GlycoLipid Transfer Protein (GLTP) superfamily are used to illustrate two broadly applicable methods for direct lipid tracking. One method relies on Förster resonance energy transfer (FRET) that enables continuous assessment of fluorophore-labeled SL transfer in real time between lipid donor and acceptor vesicles. The second method relies on tracking of radiolabeled SL transfer by separation of lipid donor and acceptor vesicles at discrete time points. The assays are readily adjustable for assessing lipid transfer (1) between various model membrane assemblies (vesicles, micelles, bicelles, nanodiscs), (2) involving other lipid types by other lipid transfer proteins, (3) with protein preparations that are either crudely or highly purified, and (4) that is spontaneous and occurs in the absence of protein.

Glycolipid transfer proteins

Glycolipid transfer proteins (GLTPs) are small (24 kDa), soluble, ubiquitous proteins characterized by their ability to accelerate the intermembrane transfer of glycolipids in vitro. GLTP specificity encompasses both sphingoid- and glycerol-based glycolipids, but with a strict requirement that the initial sugar residue be beta-linked to the hydrophobic lipid backbone. The 3D architecture of GLTP reveals liganded structures with unique lipid-binding modes. The biochemical properties of GLTP action at the membrane surface have been studied rather comprehensively, but the biological role of GLTP remains enigmatic. What is clear is that GLTP differs distinctly from other known glycolipid-binding proteins, such as nonspecific lipid transfer proteins, lysosomal sphingolipid activator proteins, lectins, lung surfactant proteins as well as other lipid-binding/transfer proteins. Based on the unique conformational architecture that targets GLTP to membranes and enables glycolipid binding, GLTP is now considered the prototypical and founding member of a new protein superfamily in eukaryotes.

Glycolipid transfer protein interaction with bilayer vesicles: modulation by changing lipid composition

Glycosphingolipids (GSLs) are important constituents of lipid rafts and caveolae, are essential for the normal development of cells, and are adhesion sites for various infectious agents. One strategy for modulating GSL composition in lipid rafts is to selectively transfer GSL to or from these putative membrane microdomains. Glycolipid transfer protein (GLTP) catalyzes selective intermembrane transfer of GSLs. To enable effective use of GLTP as a tool to modify the glycolipid content of membranes, it is imperative to understand how the membrane regulates GLTP action. In this study, GLTP partitioning to membranes was analyzed by monitoring the fluorescence resonance energy transfer from tryptophans and tyrosines of GLTP to N-(5-dimethyl-aminonaphthalene-1-sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phospho-ethanolamine present in bilayer vesicles. GLTP partitioned to POPC vesicles even when no GSL was present. GLTP interaction with model membranes was nonpenetrating, as assessed by protein-induced changes in lipid monolayer surface pressure, and nonperturbing in that neither membrane fluidity nor order were affected, as monitored by anisotropy of 1,6-diphenyl-1,3,5-hexatriene and 6-dodecanoyl-N,N-dimethyl-2-naphthylamine, even though the tryptophan anisotropy of GLTP increased in the presence of vesicles. Ionic strength, vesicle packing, and vesicle lipid composition affected GLTP partitioning to the membrane and led to the following conclusion: Conditions that increase the ratio of bound/unbound GLTP do not guarantee increased transfer activity, but conditions that decrease the ratio of bound/unbound GLTP always diminish transfer. A model of GLTP interaction with the membrane, based on the partitioning equilibrium data and consistent with the kinetics of GSL transfer, is presented and solved mathematically.

Point mutational analysis of the liganding site in human glycolipid transfer protein. Functionality of the complex

Mammalian glycolipid transfer proteins (GLTPs) facilitate the selective transfer of glycolipids between lipid vesicles in vitro. Recent structural determinations of the apo- and glycolipid-liganded forms of human GLTP have provided the first insights into the molecular architecture of the protein and its glycolipid binding site (Malinina, L., Malakhova, M. L., Brown, R. E., and Patel, D. J. (2004) Nature 430, 1048-1053). In the present study, we have evaluated the functional consequences of point mutation of the glycolipid liganding site of human GLTP within the context of a carrier-based mechanism of glycolipid intermembrane transfer. Different approaches were developed to rapidly and efficiently assess the uptake and release of glycolipid by GLTP. They included the use of glass-immobilized, glycolipid films to load GLTP with glycolipid and separation of GLTP/glycolipid complexes from vesicles containing glycolipid (galactosylceramide or lactosylceramide) or from monosialoganglioside dispersions by employing nickel-nitrilotriacetic acid-based affinity or gel filtration strategies. Point mutants of the sugar headgroup recognition center (Trp-96, Asp-48, Asn-52) and of the ceramide-accommodating hydrophobic tunnel (Phe-148, Phe-183, Leu-136) were analyzed for their ability to acquire and release glycolipid ligand. Two manifestations of point mutation within the liganding site were apparent: (i) impaired formation of the GLTP/glycolipid complex; (ii) impaired acquisition and release of bound glycolipid by GLTP. The results are consistent with a carrier-based mode of GLTP action to accomplish the intermembrane transfer of glycolipid. Also noteworthy was the inefficient release of glycolipid by wtGLTP into phosphatidylcholine acceptor vesicles, raising the possibility of a function other than intermembrane glycolipid transfer in vivo.

Glycolipid transfer protein mediated transfer of glycosphingolipids between membranes: a model for action based on kinetic and thermodynamic analyses

Glycolipid transfer protein (GLTP) catalyzes the intermembrane transfer of lipids that have sugars beta-linked to either diacylglycerol or ceramide backbones, including simple glycosphingolipids (GSLs) and gangliosides. The present study provides a quantitative understanding of GLTP action involving bilayer vesicles that have high and low curvature stress, i.e., small and large unilamellar vesicles (SUVs and LUVs). When the GSL intervesicular transfer was monitored in real time using an established fluorescence resonance energy approach, the initial GSL transfer rates (v(0)) and net transfer equilibrium (K(eq)) were determined for GLTP-mediated transfer from SUVs and LUVs over the temperature range of 30-44 degrees C. v(0) exhibited a linear dependence with respect to varying GLTP concentrations (0-143 nM range) in SUVs and LUVs, suggesting a first order dependence on the GLTP bulk concentration. Thermodynamic parameters associated with the GLTP-GSL transition-state complex and GSL net transfer were determined from linear Arrhenius and van't Hoff plots, respectively. Although initial transfer rates were lower for LUVs than for SUVs, the activation energy barriers were higher for LUVs, while the Gibbs's free energy of the transition states were similar. The formation of a transition-state complex was predominantly enthalpy driven, whereas the net transfer of GSLs was mainly entropy driven. The rate-limiting step for GLTP action appeared to be the surface processes leading to the GLTP-GSL complex formation and release associated with a shuttle/carrier mode of action. Because surface processes leading to the GLTP-GSL complex formation were limiting for GLTP action with SUVs and LUVs, it was concluded that GLTP is likely to be a valuable tool to probe and manipulate GSL environments in membranes.

Glycolipid transfer protein and intracellular traffic of glucosylceramide

Glycolipid transfer protein (GL-TP), a nonglycosylated protein with a molecular weight of 22,000 K, has been purified from pig brain. The protein transfers, by a carrier mechanism, glycolipids with a beta-glucosyl or beta-galactosyl residue directly linked to either ceramide or diacylglycerol. GL-TP appears to be present in most animal cells, and evidence has been obtained which indicates that it is a cytoplasmic protein. Little is known about the function of GL-TP. Current evidence indicates that glycosphingolipid glycosylation occurs at the luminal side of the Golgi apparatus, except for the glucosylation of ceramide, which has been shown to occur at the cytoplasmic side of the Golgi or endoplasmic membrane. It appears most likely that GL-TP participates in the intracellular traffic of glucosylceramide.

Sulfhydryl groups in glycolipid transfer protein: formation of an intramolecular disulfide bond and oligomers by Cu2+-catalyzed oxidation

Glycolipid transfer protein (GLTP) purified from pig brain facilitates the transfer of various glycolipids between lipid bilayers. Purified GLTP migrates as two bands of different mobility in SDS-polyacrylamide gel electrophoresis (SDS-PAGE) under non-reducing conditions. The slower component and the faster component constituted about 80% and about 15%, respectively, of purified GLTP. Treatment of GLTP with 45 microM CuSO4 resulted in a decrease in the slower component, an increase in the faster component, and the formation of oligomeric components. The faster and oligomeric components were quantitatively converted to the slower component by reduction with 2% 2-mercaptoethanol in the presence of 1% SDS. The formation of oligomeric components was enhanced by increasing the concentration of CuSO4 to 450 microM and 4.5 mM. Oxidation of GLTP catalyzed by CuSO4 resulted in a decrease in the transfer activity and an increase in the apparent binding affinity of GLTP to 1-O-(beta-D-galactopyranosyl)-N-[10-(1-pyrenyl)decanoyl]-D-erythro- sphingosine (PyrGalCer). The oligomeric components and the monomeric components were isolated by chromatography on a Sephadex G-75 column. It was found that GLTP in fractions enriched with the monomeric components had very high transfer activity and is responsible for most of the transfer activity in the oxidized GLTP. Treatment of GLTP with 1.27 mM HgCl2 resulted in a formation of components unresolvable on SDS-PAGE and also resulted in a reduction of the transfer activity to one-third. However, no obvious change in the binding affinity of GLTP to PyrGalCer was observed by HgCl2 treatment. Treatment with 2-mercaptoethanol restored the activity of GLTP inactivated by HgCl2, whereas the activity inactivated by CuSO4 was not restored by treatment with 2-mercaptoethanol. These results suggest that the transfer activity depends on the turnover rate of the GLTP-PyrGalCer complex which is affected by modification of sulfhydryl groups of GLTP. The sulfhydryl group content of GLTP was estimated by the use of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). A value of 2.2 mol sulfhydryl groups per mol of GLTP was found in the presence of 0.5% SDS and one sulfhydryl group in a GLTP molecule was very rapidly oxidized in the native state, from which it is assumed that the slower component contains three sulfhydryl groups per GLTP molecule and the faster component contains one sulfhydryl group and one disulfide bond per GLTP molecule.

Formation of an intramolecular disulfide bond of glycolipid transfer protein

Functional and structural differences between the two forms of glycolipid transfer protein (GLTP) with (the faster component) and without (the slower component) an intramolecular disulfide bond were studied. GLTP treated with N-ethylmaleimide (NEM) and Na2S4O6 had a transfer activity of about 70% and 55%, respectively, of the control GLTP. No significant decrease was found in the binding affinity of NEM-treated GLTP to pyrene-labeled galactosylceramide (PyrGalCer). A small decrease in the binding affinity was found in the Na2S4O6-treated GLTP. Oxidation of NEM-treated and Na2S4O6-treated GLTP catalyzed by CuSO4 resulted in a stoichiometric conversion of the slower component to the faster component. The faster component thus formed was quantitatively reduced back to the slower component by treatment with 2-mercaptoethanol in the presence of 1% SDS. These results provided strong evidence for the conversion of the slower component to the faster component as a result of the formation of an intramolecular disulfide bond. The transfer activity of the NEM-treated and oxidized GLTP (the faster component) was 1.7-fold higher than that of the original GLTP and 2.4-fold higher than that of the NEM-treated GLTP. The transfer activity of the Na2S4O6-treated and oxidized GLTP was 2-fold higher than that of the original GLTP and 3.6-fold higher than that of the Na2S4O6-treated GLTP. The binding affinity of the faster components, produced from both the NEM-treated GLTP and the Na2S4O6-treated GLTP, to PyrGalCer was found to be twice that of the respective modified GLTPs before oxidation. By circular dichroism measurements, it was found that a small decrease in the magnitude of mean residue ellipticity but no significant change in ellipticity spectrum was brought about either upon the modification of GLTP by Na2S4O6 or upon the formation of an intramolecular disulfide bond in GLTP. The results suggest that the formation of an intramolecular disulfide bond results in only a small change in the secondary and tertiary structure of GLTP. The results presented in this paper suggest that the transfer activity of GLTP may be regulated by the redox state of sulfhydryl groups present in GLTP.

A survey on cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds: from organic anion binders to new protein families

This review deals with recent advances in the research of cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds. Emphasis is given to the important contribution of structural data in the understanding of the functional properties of these proteins and in the emergence of new protein families. The possibility that many of the 'cytosolic' proteins might be structure-bound and structure-forming in the living cell is discussed, with references to so far available structural data and to recent investigations on the architecture and biochemical composition of the cytoplasm. The aim of this review is to present in a condensed form (227 references) the evolution in the study of cytosolic proteins binding and transferring lipophilic compounds and to enable interested investigators to become aware of current concepts and perspectives in this active and steadily growing area of research.

Subcellular distribution and biosynthesis of rat liver gangliosides

Gangliosides have generally been assumed to be localized primarily in the plasma membrane. Analysis of gangliosides from isolated subcellular membrane fractions of rat liver indicated that 76% of the total ganglioside sialic acid was present in the plasma membrane. Mitochondria and endoplasmic reticulum fractions, while containing only low levels of gangliosides on a protein basis, each contained approx. 10% of total ganglioside sialic acid. Gangliosides also were present in the Golgi apparatus and nuclear membrane fractions, and soluble gangliosides were in the supernatant. Individual gangliosides were non-homogeneously distributed and each membrane fraction was characterized by a unique ganglioside composition. Plasma membrane contained only 14 and 28% of the total GD1a and GD3, respectively, but 80-90% of the GM1, GD1b, GT1b and GQ1b. Endoplasmic reticulum, when corrected for plasma membrane contamination, contained only trace amounts of GM1, GD1b, GT1b and GQ1b, but 11 and 5% of the total GD1a and GD3, respectively. The ganglioside composition of highly purified endoplasmic reticulum was similar. Ganglioside biosynthetic enzymes were concentrated in the Golgi apparatus. However, low levels of these enzymes were present in the highly purified endoplasmic reticulum fractions. Pulse-chase experiments with [3H]galactose revealed that total gangliosides were labeled first in the Golgi apparatus, mitochondria and supernatant within 10 min. Labeled gangliosides were next observed at 30 min in the endoplasmic reticulum, plasma membrane and nuclear membrane fractions. Analysis of the individual gangliosides also revealed that GM3, GM1, GD1a and GD1b were labeled first in the Golgi apparatus at 10 min. These studies indicate that gangliosides synthesized in the Golgi apparatus may be transported not only to the plasma membrane, but to the endoplasmic reticulum and to other internal endomembranes as well.

Glycolipid-binding proteins

Proteins which bind glycolipids with high specificity are tentatively divided into two groups. One group consists of activator proteins involved in the catabolism of glycolipids by acid lysosomal hydrolases. Two activator proteins, GM2-activator and sphingolipid activator protein-1, are critically appraised on their glycolipid-binding properties and on their activity to facilitate the transfer of glycolipids. These proteins are glycoproteins localized in the lysosomes. Their molecular weights are in a range of 21 000-27 000, and isoelectric points are 4-5. Glycolipid transfer protein (GLTP) is included in the other group. GLTP purified from pig brain has a molecular weight of about 20 000 and an isoelectric point of 8.3. GLTP facilitates the transfer of various glycosphingolipids and glyceroglycolipids between membranes. The protein does not facilitate the transfer of phospholipids or cholesterol. GLTP binds galactosylceramide. The galactosylceramide-GLTP complex participates in the transfer reaction as the intermediate. Each protein in both groups binds glycolipids with a characteristic specificity to the sugar moiety. A stoichiometry of 1 mol of lipid per mol of protein has been found in all three proteins. Proteins in both groups seem to have a hydrophobic region on their surface, since all three proteins have been efficiently purified by hydrophobic chromatography.

Properties of a specific glycolipid transfer protein from bovine brain

A transfer protein specific for glycolipids has been isolated from bovine brain. As judged by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, the protein is 68% pure and has a molecular weight of 20 000. Three different assays were employed to study the protein's specificity and glycolipid binding properties. The protein transferred several different neutral glycosphingolipids and ganglioside GM1 equally well, but failed to accelerate phosphatidylcholine or sphingomyelin intervesicular movement. The protein's ability to interact with glycolipids was strongly influenced by the physical properties of the matrix phospholipid in which the glycolipids reside. Both the phase state of the phospholipid matrix and bilayer curvature affected glycolipid intervesicular transfer rates. Protein binding to phospholipid vesicles containing either tritium-labeled or pyrene-labeled glucosylceramide could not be demonstrated by density gradient centrifugation or fluorescence energy transfer measurements, respectively. A specific association of the transfer protein for pyrene-labeled glucosylceramide was found when the fluorescence emission of the pyrene excimer-to-monomer ratio was measured suggesting that a portion of the fluorescent glycolipid was being sequestered from the phospholipid vesicles and was binding to the freely soluble protein.