Formation of an intramolecular disulfide bond of glycolipid transfer protein

Identification of Biomarkers of Mercury Contamination in Brachyplatystoma filamentosum of the Madeira River, Brazil, Using Metalloproteomic Strategies

Predator fish can accumulate high levels of mercury, which qualifies them as potential indicators of this toxic metal. The predatory species Brachyplatystoma filamentosum, popularly known as filhote, is among the most consumed species in the Brazilian Amazon. Continuing the metalloproteomic studies of mercury in Amazonian fishes that have been developed in the last 5 years, the present paper provides the data of protein characterization associated with mercury in muscle and liver samples of filhote (Brachyplatystoma filamentosum) collected in the Madeira River, Brazilian Amazon. The mercury concentration in the muscle and liver samples was determined by graphite furnace atomic absorption spectrometry (GFAAS). The protein fraction was extracted in an aqueous medium, and later, a fractional precipitation procedure was performed to obtain the protein pellets. Then, the proteome of the tissue samples of this fish species was separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), and a mercury mapping of the protein spots was carried out by GFAAS after acid digestion. Protein spots that had mercury were characterized by mass spectrometry with electrospray ionization in sequence (ESI-MS/MS) after tryptic digestion. It was possible to characterize 11 mercury-associated protein spots that presented biomarker characteristics and could be used to monitor mercury in fish species of the Amazon region. Thus, the metalloproteomic strategies used in the present study allowed us to characterize 11 mercury-associated protein spots. It should be noted that the protein spots identified as GFRP, TMEM186, TMEM57B, and BHMT, which have coordination sites for elements with characteristics of soft acids, such as mercury, can be used as biomarkers of mercury contamination in monitoring studies of this toxic metal in fish species from the Amazon region.

Sphingolipid transfer proteins defined by the GLTP-fold

Glycolipid transfer proteins (GLTPs) originally were identified as small (~24 kDa), soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. GLTPs and related homologs now are known to adopt a unique, helically dominated, two-layer 'sandwich' architecture defined as the GLTP-fold that provides the structural underpinning for the eukaryotic GLTP superfamily. Recent advances now provide exquisite insights into structural features responsible for lipid headgroup selectivity as well as the adaptability of the hydrophobic compartment for accommodating hydrocarbon chains of differing length and unsaturation. A new understanding of the structural versatility and evolutionary premium placed on the GLTP motif has emerged. Human GLTP-motifs have evolved to function not only as glucosylceramide binding/transferring domains for phosphoinositol 4-phosphate adaptor protein-2 during glycosphingolipid biosynthesis but also as selective binding/transfer proteins for ceramide-1-phosphate. The latter, known as ceramide-1-phosphate transfer protein, recently has been shown to form GLTP-fold while critically regulating Group-IV cytoplasmic phospholipase A2 activity and pro-inflammatory eicosanoid production.

Membranes and mammalian glycolipid transferring proteins

Glycolipids are synthesized in and on various organelles throughout the cell. Their trafficking inside the cell is complex and involves both vesicular and protein-mediated machineries. Most important for the bulk lipid transport is the vesicular system, however, lipids moved by transfer proteins are also becoming more characterized. Here we review the latest advances in the glycolipid transfer protein (GLTP) and the phosphoinositol 4-phosphate adaptor protein-2 (FAPP2) field, from a membrane point of view.

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.

Structural Evidence for Adaptive Ligand Binding of Glycolipid Transfer Protein

Glycolipids participate in many important cellular processes and they are bound and transferred with high specificity by glycolipid transfer protein (GLTP). We have solved three different X-ray structures of bovine GLTP at 1.4 angstroms, 1.6 angstroms and 1.8 angstroms resolution, all with a bound fatty acid or glycolipid. The 1.4 angstroms structure resembles the recently characterized apo-form of the human GLTP but the other two structures represent an intermediate conformation of the apo-GLTPs and the human lactosylceramide-bound GLTP structure. These novel structures give insight into the mechanism of lipid binding and how GLTP may conformationally adapt to different lipids. Furthermore, based on the structural comparison of the GLTP structures and the three-dimensional models of the related Podospora anserina HET-C2 and Arabidopsis thaliana accelerated cell death protein, ACD11, we give structural explanations for their specific lipid binding properties.

Human glycolipid transfer protein: probing conformation using fluorescence spectroscopy

Glycolipid transfer protein (GLTP) is a soluble 24 kDa protein that selectively accelerates the intermembrane transfer of glycolipids in vitro. Little is known about the GLTP structure and dynamics. Here, we report the cloning of human GLTP and characterize the environment of the three tryptophans (Trps) of the protein using fluorescence spectroscopy. Excitation at 295 nm yielded an emission maximum (lambda(max)) near 347 nm, indicating a relatively polar average environment for emitting Trps. Quenching with acrylamide at physiological ionic strength or with potassium iodide resulted in linear Stern-Volmer plots, suggesting accessibility of emitting Trps to soluble quenchers. Insights into reversible conformational changes accompanying changes in GLTP activity were provided by addition and rapid dilution of urea while monitoring changes in Trp or 1-anilinonaphthalene-8-sulfonic acid fluorescence. Incubation of GLTP with glycolipid liposomes caused a blue shift in the Trp emission maximum but diminished the fluorescence intensity. The blue-shifted emission maximum, centered near 335 nm, persisted after separation of glycolipid liposomes from GLTP, consistent with formation of a GLTP-glycolipid complex at a glycolipid-liganding site containing Trp. The results provide the first insights into human GLTP structural dynamics by fluorescence spectroscopy, including global conformational changes that accompany GLTP folding into an active conformational state as well as more subtle conformational changes that play a role in GLTP-mediated transfer of glycolipids between membranes, and establish a foundation for future studies of membrane rafts using GLTP.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Cloning and expression of glycolipid transfer protein from bovine and porcine brain

Glycolipid transfer protein (GLTP) is a small (23-24 kDa), basic protein (pI congruent with 9.0) that accelerates the intermembrane transfer of various glycolipids. Here, we report the first cloning of cDNAs that encode the bovine and porcine GLTPs. The cDNA open reading frame for bovine GLTP was constructed by bridge-overlapping extension polymerase chain reaction (PCR) after obtaining partial coding cDNA clones by hot start, seminested, and rapid amplification of cDNA ends-PCR. The cDNA open reading frame for porcine GLTP was constructed by reverse transcriptase-PCR. The encoded amino acid sequences in the full-length bovine and porcine cDNAs were identical, consisting of 209 amino acid residues, and were nearly the same as the published sequence determined by Edman degradation. The cDNA encoded one additional amino acid at the N terminus (methionine), arginine at positions 10 and 200 instead of lysine, and threonine at position 65 instead of alanine. Expression of GLTP-cDNA in Escherichia coli using pGEX-6P-1 vector resulted in glutathione S-transferase (GST)-GLTP fusion protein. Regulation of growth and induction conditions led to approximately 50% of expressed fusion protein being soluble and active. Proteolytic cleavage of GST-GLTP fusion protein (bound to GST-Sepharose) and affinity purification resulted in fully active GLTP. Northern blot analyses of bovine tissues showed a single transcript of approximately 2.2 kilobases and the following hierarchy of mRNA levels: cerebrum > kidney > spleen congruent with lung congruent with cerebellum > liver > heart muscle. Reverse transcriptase-PCR analyses of mRNA levels supported the Northern blot results.

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.