Structure of the asn-linked oligosaccharides of apolipophorin III from the insect Locusta migratoria. Carbohydrate-linked 2-aminoethylphosphonate as a constituent of a glycoprotein

Transcriptomic-Guided Phosphonate Utilization Analysis Unveils Evidence of Clathrin-Mediated Endocytosis and Phospholipid Synthesis in the Model Diatom, Phaeodactylum tricornutum

Phosphonates are important components of marine organic phosphorus, but their bioavailability and catabolism by eukaryotic phytoplankton remain enigmatic. Here, diatom Phaeodactylum tricornutum was used to investigate the bioavailability of phosphonates and describe the underlying molecular mechanism. The results showed that 2-aminoethylphosphonic acid (2-AEP) can be utilized as an alternative phosphorus source. Comparative transcriptomics revealed that the utilization of 2-AEP comprised 2 steps, including molecular uptake through clathrin-mediated endocytosis and incorporation into the membrane phospholipids in the form of diacylglyceryl-2-AEP (DAG-2-AEP). In the global ocean, we found the prevalence and dynamic expression pattern of key genes that are responsible for vesicle formation (CLTC, AP-2) and DAG-AEP synthesis (PCYT2, EPT1) in diatom assemblages. This study elucidates a distinctive mechanism of phosphonate utilization by diatoms, and discusses the ecological implications. IMPORTANCE Phosphonates contribute ~25% of total dissolved organic phosphorus in the ocean, and are found to be important for marine phosphorus biogeochemical cycle. As a type of biogenic phosphonate produced by microorganisms, 2-aminoethylphosphonic acid (2-AEP) widely exists in the ocean. It is well known that 2-AEP can be cleaved and utilized by prokaryotes, but its ability to support the growth of eukaryotic phytoplankton remains unclear. Our research identified the bioavailability of 2-AEP for the diatom Phaeodactylum tricornutum, and proposed a distinctive metabolic pathway of 2-AEP utilization. Different from the enzymatic hydrolysis of phosphonates, the results suggested that P. tricornutum utilizes 2-AEP by incorporating it into phospholipid instead of cleaving the C-P bond. Moreover, the ubiquitous distribution of associated representative gene transcripts in the environmental assemblages and the higher gene transcript abundance in the cold regions were observed, which suggests the possible environmental adaption of 2-AEP utilization by diatoms.

Global and seasonal variation of marine phosphonate metabolism

Marine microbial communities rely on dissolved organic phosphorus (DOP) remineralisation to meet phosphorus (P) requirements. We extensively surveyed the genomic and metagenomic distribution of genes directing phosphonate biosynthesis, substrate-specific catabolism of 2-aminoethylphosphonate (2-AEP, the most abundant phosphonate in the marine environment), and broad-specificity catabolism of phosphonates by the C-P lyase (including methylphosphonate, a major source of methane). We developed comprehensive enzyme databases by curating publicly available sequences and then screened metagenomes from TARA Oceans and Munida Microbial Observatory Time Series (MOTS) to assess spatial and seasonal variation in phosphonate metabolism pathways. Phosphonate cycling genes were encoded in diverse gene clusters by 35 marine bacterial and archaeal classes. More than 65% of marine phosphonate cycling genes mapped to Proteobacteria with production demonstrating wider taxonomic diversity than catabolism. Hydrolysis of 2-AEP was the dominant phosphonate catabolism strategy, enabling microbes to assimilate carbon and nitrogen alongside P. Genes for broad-specificity catabolism by the C-P lyase were far less widespread, though enriched in the extremely P-deplete environment of the Mediterranean Sea. Phosphonate cycling genes were abundant in marine metagenomes, particularly from the mesopelagic zone and winter sampling dates. Disparity between prevalence of substrate-specific and broad-specificity catabolism may be due to higher resource expenditure from the cell to build and retain the C-P lyase. This study is the most comprehensive metagenomic survey of marine microbial phosphonate cycling to date and provides curated databases for 14 genes involved in phosphonate cycling.

New insights into the Type A glycan modification of Clostridioides difficile flagellar protein flagellin C by phosphoproteomics analysis

The type A glycan modification found in human pathogen Clostridioides difficile consists of a monosaccharide (GlcNAc) that is linked to an N-methylated threonine through a phosphodiester bond. This structure has previously been described on the flagellar protein flagellin C of several C. difficile strains and is important for bacterial motility. The study of post-translational modifications often relies on some type of enrichment strategy; however, a procedure for enrichment of this modification has not yet been demonstrated. In this study, we show that an approach that is commonly used in phosphoproteomics, Fe³⁺-immobilized metal affinity chromatography, also enriches for peptides with this unique post-translational modification. Using LC–MS/MS analyses of immobilized metal affinity chromatography–captured tryptic peptides, we observed not only type A-modified C. difficile flagellin peptides but also a variety of truncated/modified type A structures on these peptides. Using an elaborate set of mass spectrometry analyses, we demonstrate that one of these modifications consists of a type A structure containing a phosphonate (2-aminoethylphosphonate), a modification that is rarely observed and has hitherto not been described in C. difficile. In conclusion, we show that a common enrichment strategy results in reliable identification of peptides carrying a type A glycan modification, and that the results obtained can be used to advance models about its biosynthesis.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

Fragments of Locusta migratoria apoLp-III provide insight into lipid binding

Apolipophorin III (apoLp-III) from Locusta migratoria is an exchangeable apolipoprotein with a critical role in lipid transport in insects. The protein is composed of a bundle of five amphipathic α-helices which undergo a large conformational change upon lipid binding. To better understand the apoLp-III lipid binding interaction, the protein was cleaved by cyanogen bromide upon introduction of a S92M mutation, generating an N-terminal fragment corresponding to the first three helices (NT_H1–3) and a C-terminal fragment of the last two helices (CT_H4–5). MALDI-TOF analysis of the HPLC purified fragments provided masses of 9863.8 Da for NT_H1–3 and 7497.0 Da for CT_H4–5 demonstrating that the intended fragments were obtained. Circular dichroism spectra revealed a decrease in helical content from 82% for the intact protein to 57% for NT_H1–3 and 41% for CT_H4–5. The fragments adopted considerably higher α-helical structure in the presence of trifluoroethanol or phospholipids. Equimolar mixing of the two fragments did not result in changes in helical content or tryptophan fluorescence, indicating recombination into the native protein fold did not occur. The rate of protein induced dimyristoylphosphatidylcholine vesicle solubilization increased 15-fold for NT_H1–3 and 100-fold for CT_H4–5 compared to the intact protein. Despite the high activity in phospholipid vesicle interaction, CT_H4–5 did not protect phospholipase-treated low-density lipoprotein from aggregation. In contrast, NT_H1–3 provided protection to lipoprotein aggregation similar to the intact protein, indicating that specific amino acid residues in this part of apoLp-III are essential for lipoprotein binding interaction.

Tissue-specific glycosylation in the honeybee: Analysis of the N-glycomes of Apis mellifera larvae and venom

BACKGROUND

Previous glycophylogenetic comparisons of dipteran and lepidopteran species revealed variations in the anionic and zwitterionic modifications of their N-glycans; therefore, we wished to explore whether species- and order-specific glycomic variations would extend to the hymenoptera, which include the honeybee Apis mellifera, an agriculturally- and allergologically-significant social species.

METHODS

In this study, we employed an off-line liquid chromatography/mass spectrometry approach, in combination with enzymatic and chemical treatments, to analyse the N-glycans of male honeybee larvae and honeybee venom in order to facilitate definition of isomeric structures.

RESULTS

The neutral larval N-glycome was dominated by oligomannosidic and paucimannosidic structures, while the neutral venom N-glycome displayed more processed hybrid and complex forms with antennal N-acetylgalactosamine, galactose and fucose residues including Lewis-like epitopes; the anionic pools from both larvae and venom contained a wide variety of glucuronylated, sulphated and phosphoethanolamine-modified N-glycans with up to three antennae. In comparison to honeybee royal jelly, there were more fucosylated and fewer Man_4/5-based hybrid glycans in the larvae and venom samples as well as contrasting antennal lengths.

CONCLUSIONS

Combining the current data on venom and larvae with that we previously published on royal jelly, a total honeybee N-glycomic repertoire of some 150 compositions can be proposed in addition to the 20 previously identified on specific venom glycoproteins.

SIGNIFICANCE

Our data are indicative of tissue-specific modification of the core and antennal regions of N-glycans in Apis mellifera and reinforce the concept that insects are capable of extensive processing to result in rather complex anionic oligosaccharide structures.

Anionic and zwitterionic moieties as widespread glycan modifications in non-vertebrates

Glycan structures in non-vertebrates are highly variable; it can be assumed that this is a product of evolution and speciation, not that it is just a random event. However, in animals and protists, there is a relatively limited repertoire of around ten monosaccharide building blocks, most of which are neutral in terms of charge. While two monosaccharide types in eukaryotes (hexuronic and sialic acids) are anionic, there are a number of organic or inorganic modifications of glycans such as sulphate, pyruvate, phosphate, phosphorylcholine, phosphoethanolamine and aminoethylphosphonate that also confer a 'charged' nature (either anionic or zwitterionic) to glycoconjugate structures. These alter the physicochemical properties of the glycans to which they are attached, change their ionisation when analysing them by mass spectrometry and result in different interactions with protein receptors. Here, we focus on N-glycans carrying anionic and zwitterionic modifications in protists and invertebrates, but make some reference to O-glycans, glycolipids and glycosaminoglycans which also contain such moieties. The conclusion is that 'charged' glycoconjugates are a widespread, but easily overlooked, feature of 'lower' organisms.

Comparisons of N-glycans across invertebrate phyla

Many invertebrates are either parasites themselves or vectors involved in parasite transmission; thereby, the interactions of parasites with final or intermediate hosts are often mediated by glycans. Therefore, it is of interest to compare the glycan structures or motifs present across invertebrate species. While a typical vertebrate modification such as sialic acid is rare in lower animals, antennal and core modifications of N-glycans are highly varied and range from core fucose, galactosylated fucose, fucosylated galactose, methyl groups, glucuronic acid and sulphate through to addition of zwitterionic moieties (phosphorylcholine, phosphoethanolamine and aminoethylphosphonate). Only in some cases are the enzymatic bases and the biological function of these modifications known. We are indeed still in the phase of discovering invertebrate glycomes primarily using mass spectrometry, but molecular biology and microarraying techniques are complementary to the determination of novel glycan structures and their functions.

Lipid-bound apoLp-III is less effective in binding to lipopolysaccharides and phosphatidylglycerol vesicles compared to the lipid-free protein

Apolipophorin III (apoLp-III) is an insect apolipoprotein that is predominantly present in a lipid-free state in the hemolymph. ApoLp-III from Galleria mellonella is able to interact with membrane components of Gram-negative bacteria, as part of an innate immune response to infection. The protein also exists in a lipoprotein-associated state when large amounts of lipids are mobilized. Therefore, lipid-bound apoLp-III was generated to analyze the binding interaction with lipopolysaccharides and phosphatidylglycerol, both abundantly present in membranes of Gram-negative bacteria. G. mellonella apoLp-III was lipidated with palmitoyl-2-oleoyl-glycero-3-phosphocholine to form lipid-protein complexes. The particle shape was discoidal with a 16.4 nm diameter, a molecular mass of 460 kDa, and contained 4 apoLp-III molecules. These discoidal lipoproteins were used to compare the lipopolysaccharide and phosphatidylglycerol binding activity with lipid-free apoLp-III. Lipopolysaccharide binding interaction was analyzed by non-denaturing PAGE, showing reduced ability of the lipid-bound protein to form lipopolysaccharide-protein complexes and to disaggregate lipopolysaccharide micelles. The apoLp-III-induced release of calcein from phosphatidylglycerol vesicles was decreased approximately fivefold when the protein was in the lipid-bound form, indicating reduced binding interaction with the phosphatidylglycerol membrane surface. These results show that when apoLp-III adopts a lipid-bound conformation, it is markedly less effective in interacting with lipopolysaccharides and phosphatidylglycerol vesicles. Thus, in order to be an effective antimicrobial protein, apoLp-III needs to be in a lipid-free state.

Isomeric Separation and Recognition of Anionic and Zwitterionic N-glycans from Royal Jelly Glycoproteins

Royal jelly has received attention because of its necessity for the development of queen honeybees as well as claims of benefits on human health; this product of the hypopharyngeal glands of worker bees contains a large number of proteins, some of which have been claimed to have various biological effects only in their glycosylated state. However, although there have been glycomic and glycoproteomic analyses in the past, none of the glycan structures previously defined would appear to have potential to trigger specific biological functions. In the current study, whole royal jelly as well as single protein bands were subject to off-line LC-MALDI-TOF MS glycomic analyses, complemented by permethylation, Western blotting and arraying data. Similarly to recent in-depth studies on other insect species, previously overlooked glucuronic acid termini, sulfation of mannose residues and core β-mannosylation of the N-glycans were found; additionally, a relatively rare zwitterionic modification with phosphoethanolamine is present, in contrast to the phosphorylcholine occurring in lepidopteran species. Indicative of tissue-specific remodelling of glycans in the Golgi apparatus of hypopharyngeal gland cells, only a low amount of fucosylated or paucimannosidic glycans were detected as compared with other insect samples or even bee venom. The unusual modifications of hybrid and multiantennary structures defined here may not only have a physiological role in honeybee development, but represent epitopes recognized by pentraxins with roles in animal innate immunity.

Exploring the Glycans of Euglena gracilis

Euglena gracilis is an alga of great biotechnological interest and extensive metabolic capacity, able to make high levels of bioactive compounds, such as polyunsaturated fatty acids, vitamins and β-glucan. Previous work has shown that Euglena expresses a wide range of carbohydrate-active enzymes, suggesting an unexpectedly high capacity for the synthesis of complex carbohydrates for a single-celled organism. Here, we present an analysis of some of the carbohydrates synthesised by Euglena gracilis. Analysis of the sugar nucleotide pool showed that there are the substrates necessary for synthesis of complex polysaccharides, including the unusual sugar galactofuranose. Lectin- and antibody-based profiling of whole cells and extracted carbohydrates revealed a complex galactan, xylan and aminosugar based surface. Protein N-glycan profiling, however, indicated that just simple high mannose-type glycans are present and that they are partially modified with putative aminoethylphosphonate moieties. Together, these data indicate that Euglena possesses a complex glycan surface, unrelated to plant cell walls, while its protein glycosylation is simple. Taken together, these findings suggest that Euglena gracilis may lend itself to the production of pharmaceutical glycoproteins.

The underestimated N-glycomes of lepidopteran species

BACKGROUND

Insects are significant to the environment, agriculture, health and biotechnology. Many of these aspects display some relationship to glycosylation, e.g., in case of pathogen binding or production of humanised antibodies; for a long time, it has been considered that insect N-glycosylation potentials are rather similar and simple, but as more species are glycomically analysed in depth, it is becoming obvious that there is indeed a large structural diversity and interspecies variability.

METHODS

Using an off-line LC-MALDI-TOF MS approach, we have analysed the N-glycomes of two lepidopteran species (the cabbage looper Trichoplusia ni and the gypsy moth Lymantria dispar) as well as of the commonly-used T. ni High Five cell line.

RESULTS

We detected not only sulphated, glucuronylated, core difucosylated and Lewis-like antennal fucosylated structures, but also the zwitterion phosphorylcholine on antennal GlcNAc residues, a modification otherwise familiar from nematodes; in L. dispar, N-glycans with glycolipid-like antennae containing α-linked N-acetylgalactosamine were also revealed.

CONCLUSION

The lepidopteran glycomes analysed not only display core α1,3-fucosylation, which is foreign to mammals, but also up to 5% anionic and/or zwitterionic glycans previously not found in these species.

SIGNIFICANCE

The occurrence of anionic and zwitterionic glycans in the Lepidoptera data is not only of glycoanalytical and evolutionary interest, but is of biotechnological relevance as lepidopteran cell lines are potential factories for recombinant glycoprotein production.

Phosphonate Biochemistry

Organophosphonic acids are unique as natural products in terms of stability and mimicry. The C-P bond that defines these compounds resists hydrolytic cleavage, while the phosphonyl group is a versatile mimic of transition-states, intermediates, and primary metabolites. This versatility may explain why a variety of organisms have extensively explored the use organophosphonic acids as bioactive secondary metabolites. Several of these compounds, such as fosfomycin and bialaphos, figure prominently in human health and agriculture. The enzyme reactions that create these molecules are an interesting mix of chemistry that has been adopted from primary metabolism as well as those with no chemical precedent. Additionally, the phosphonate moiety represents a source of inorganic phosphate to microorganisms that live in environments that lack this nutrient; thus, unusual enzyme reactions have also evolved to cleave the C-P bond. This review is a comprehensive summary of the occurrence and function of organophosphonic acids natural products along with the mechanisms of the enzymes that synthesize and catabolize these molecules.

Analysis of zwitterionic and anionic N-linked glycans from invertebrates and protists by mass spectrometry

Glycomic analyses over the years have revealed that non-vertebrate eukaryotes express oligosaccharides with inorganic and zwitterionic modifications which are either occurring in different contexts as compared to, or are absent from, mammals. Examples of anionic N-glycans (carrying sulphate or phosphate) are known from amoebae, fungi, molluscs and insects, while zwitterionic modifications by phosphorylcholine, phosphoethanolamine and aminoethylphosphonate occur on N-, O- and lipid-linked glycans from trichomonads, annelids, fungi, molluscs, insects, cestodes and nematodes. For detection of zwitterionic and anionic glycans, mass spectrometry has been a key method, but their ionic character affects the preparation and purification; therefore, as part of a glycomic strategy, the possibility of their presence must be considered in advance. On the other hand, their ionisation and fragmentation in positive and negative ion mode mass spectrometry as well as specific chemical or enzymatic treatments can prove diagnostic to their analysis. In our laboratory, we combine solid-phase extraction, reversed and normal phase HPLC, MALDI-TOF MS, exoglycosidase digests and hydrofluoric acid treatment to reveal N-glycans modified with anionic and zwitterionic moieties in a wide range of organisms. It is to be anticipated that, as more species are glycomically analysed, zwitterionic and anionic modifications of N-glycans will prove rather widespread. This knowledge is - in the longer term - then the basis for understanding the function of this cornucopia of glycan modifications.

Multistep Fractionation and Mass Spectrometry Reveal Zwitterionic and Anionic Modifications of the N- and O-glycans of a Marine Snail

Various studies in the past have revealed that molluscs can produce a wide range of rather complex N-glycan structures, which vary from those occurring in other invertebrate animals; particularly methylated glycans have been found in gastropods, and there are some reports of anionic glycans in bivalves. Due to the high variability in terms of previously described structures and methodologies, it is a major challenge to establish glycomic workflows that yield the maximum amount of detailed structural information from relatively low quantities of sample. In this study, we apply differential release with peptide:N-glycosidases F and A followed by solid-phase extraction on graphitized carbon and reversed-phase materials to examine the glycome of Volvarina rubella (C. B. Adams, 1845), a margin snail of the clade Neogastropoda. The resulting four pools of N-glycans were fractionated on a fused core RP-HPLC column and subject to MALDI-TOF MS and MS/MS in conjunction with chemical and enzymatic treatments. In addition, selected N-glycan fractions, as well as O-glycans released by β-elimination, were analyzed by porous graphitized carbon-LC-MS and MS(n). This comprehensive approach enabled us to determine a number of novel modifications of protein-linked glycans, including N-methyl-2-aminoethylphosphonate on mannose and N-acetylhexosamine residues, core β1,3-linked mannose, zwitterionic moieties on core Galβ1,4Fuc motifs, additional mannose residues on oligomannosidic glycans, and bisubstituted antennal fucose; furthermore, typical invertebrate N-glycans with sulfate and core fucose residues are present in this gastropod.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

2-Aminoethylphosphonate utilization by the cold-adapted Geomyces pannorum P11 strain

Cold-adapted strain of Geomyces pannorum P11 was found to mineralize of phosphorus–carbon bond-containing compound—2-aminoethylphosphonic acid (2-AEP, ciliatine). The biodegradation process proceeded in the phosphate-independent manner. Ciliatine-metabolizing enzymes' activity was detectable in cell-free extracts prepared from psychrophilic G. pannorum pregrown on 4 mM 2-AEP. Phosphonoacetaldehyde hydrolase (phosphonatase) activity in a partially purified extract was demonstrated at 10 °C.

Mucin-type proteins produced in the Trichoplusia ni and Spodoptera frugiperda insect cell lines carry novel O-glycans with phosphocholine and sulfate substitutions

The O-glycans of a recombinant mucin-type protein expressed in insect cell lines derived from Trichoplusia ni (Hi-5) and Spodoptera frugiperda (Sf9) were characterized. The P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b) fusion protein carrying 106 potential O-glycosylation sites and 6 potential N-glycosylation sites was expressed and purified from the Hi-5 and Sf9 cell culture medium using affinity chromatography and gel filtration. Liquid chromatography mass spectrometry (LC-MS) of O-glycans released from PSGL-1/mIgG2b revealed a large repertoire of structurally diverse glycans, which is in contrast to previous reports of only simple glycans. O-Glycans containing hexuronic acid (HexA, here glucuronic acid and galacturonic acid) were found to be prevalent. Also sulfate (Hi-5 and Sf9) and phosphocholine (PC; Sf9) O-glycan substitutions were detected. Western blotting confirmed the presence of O-linked PC on PSGL-1/mIG2b produced in Sf9 cells. To our knowledge, this is the first structural characterization of PC-substituted O-glycans in any species. The MS analyses revealed that Sf9 oligosaccharides consisted of short oligosaccharides (<6 residues) low in hexose (Hex) and with terminating N-acetylhexosamine (HexNAc) units, whereas Hi-5 produced a family of large O-glycans with (HexNAc-HexA-Hex) repeats and sulfate substitution on terminal residues. In both cell lines, the core N-acetylgalactosamine was preferentially non-branched, but small amounts of O-glycan cores with single fucose or hexose branches were found.

Complicated N-linked glycans in simple organisms

Although countless genomes have now been sequenced, the glycomes of the vast majority of eukaryotes still present a series of unmapped frontiers. However, strides are being made in a few groups of invertebrate and unicellular organisms as regards their N-glycans and N-glycosylation pathways. Thereby, the traditional classification of glycan structures inevitably approaches its boundaries. Indeed, the glycomes of these organisms are rich in surprises, including a multitude of modifications of the core regions of N-glycans and unusual antennae. From the actually rather limited glycomic information we have, it is nevertheless obvious that the biotechnological, developmental and immunological relevance of these modifications, especially in insect cell lines, model organisms and parasites means that deciphering unusual glycomes is of more than just academic interest.

The N-glycans of Trichomonas vaginalis contain variable core and antennal modifications

Trichomonad species are widespread unicellular flagellated parasites of vertebrates which interact with their hosts through carbohydrate-lectin interactions. In the past, some data have been accumulated regarding their lipo(phospho)glycans, a major glycoconjugate on their cell surfaces; on the other hand, other than biosynthetic aspects, few details about their N-linked oligosaccharides are known. In this study, we present both mass spectrometric and high-performance liquid chromatography data about the N-glycans of different strains of Trichomonas vaginalis, a parasite of the human reproductive tract. The major structure in all strains examined is a truncated oligomannose form (Man(5)GlcNAc(2)) with α1,2-mannose residues, compatible with a previous bioinformatic examination of the glycogenomic potential of T. vaginalis. In addition, dependent on the strain, N-glycans modified by pentose residues, phosphate or phosphoethanolamine and terminal N-acetyllactosamine (Galβ1,4GlcNAc) units were found. The modification of N-glycans by N-acetyllactosamine in at least some strains is shared with the lipo(phospho)glycan and may represent a further interaction partner for host galectins, thereby playing a role in binding of the parasite to host epithelia. On the other hand, the variation in glycosylation between strains may be the result of genetic diversity within this species.

Apolipoprotein-induced conversion of phosphatidylcholine bilayer vesicles into nanodisks

Apolipoprotein mediated formation of nanodisks was studied in detail using apolipophorin III (apoLp-III), thereby providing insight in apolipoprotein-lipid binding interactions. The spontaneous solubilization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles occured only in a very narrow temperature range at the gel-liquid-crystalline phase transition temperature, exhibiting a net exothermic interaction based on isothermal titration calorimetry analysis. The resulting nanodisks were protected from proteolysis by trypsin, endoproteinase Glu-C, chymotrypsin and elastase. DMPC solubilization and the simultaneous formation of nanodisks were promoted by increasing the vesicle diameter, protein to lipid ratio and concentration. Inclusion of cholesterol in DMPC dramatically enhanced the rate of nanodisk formation, presumably by stabilization of lattice defects which form the main insertion sites for apolipoprotein α-helices. The presence of fully saturated acyl chains with a length of 13 or 14 carbons in phosphatidylcholine allowed the spontaneous vesicle solubilization upon apolipoprotein addition. Nanodisks with C13:0-phosphatidylcholine were significantly smaller with a diameter of 11.7 ± 3.1nm compared to 18.5 ± 5.6 nm for DMPC nanodisks determined by transmission electron microscopy. Nanodisk formation was not observed when the phosphatidylcholine vesicles contained acyl chains of 15 or 16 carbons. However, using very high concentrations of lipid and protein (>10mg/ml), 1,2,-dipalmitoyl-sn-glycero-3-phosphocholine nanodisks could be produced spontaneously although the efficiency remained low.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Apolipophorin III lysine modification: Effect on structure and lipid binding

Apolipophorin III (apoLp-III) from Locusta migratoria was used as a model to investigate apolipoprotein lipid binding interactions. ApoLp-III contains eight lysine residues, of which seven are located on one side of the protein. To investigate the role of positive charges on lipid binding, lysine residues were acetylated by acetic anhydride. The degree of acetylation was analyzed by SDS-PAGE and MALDI-TOF, indicating a maximum of eight acetyl additions. Modified apoLp-III remained alpha-helical, but displayed a decreased alpha-helical content (from 78 to 54%). Acetylation resulted in a slight increase in protein stability, as indicated by a change in the midpoint of guanidine-HCl induced denaturation from 0.55 (unmodified) to 0.65 M (acetylated apoLp-III). Lipid bound apoLp-III, either acetylated or unmodified, displayed similar increases in helical content and midpoint of guanidine-HCl-induced denaturation of approximately 4 M. The ability to solubilize vesicles of dimyristoylphosphatidylcholine remained unchanged. However, the rate to solubilize dimyristoylphosphatidylglycerol vesicles was reduced two-fold. In addition, a decreased ability to stabilize diacylglycerol-enriched low density lipoproteins was observed. This indicated that lysine residues are not critical for the protein's ability to bind to zwitterionic phospholipids. Since binding interactions with ionic phospholipids and lipoproteins were affected by acetylation, lysine side-chains may play a modulating role in the interaction with more complex lipid surfaces encountered in vivo.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

Major O-glycans from the nest of Vespula germanica contain phospho-ethanolamine

We describe here the structural deciphering of four wasp O-glycans. Following purification of a mixture of glycoproteins from nests of the common wasp Vespula germanica L. (Hymenoptera), their substituting O-glycans were liberated by reducing beta-elimination and characterised using a combination of high resolution NMR and mass spectrometry analyses. Besides ubiquitously found in the insect cells GalNAc-ol and Gal(beta1-3)GalNAc-ol compounds, two novel O-glycans carrying a 2-aminoethyl phosphate group were described for the first time here. We suggest that they present the following structures: Etn-P-(O-->6)-GalNAc-ol and Etn-P-(O-->6)-[Gal(beta1-3)]GalNAc-ol. In conjunction with previous studies, these results suggest that a 2-aminoethyl phosphate group may act as an alternative to sialic acid for conferring charges to glycoproteins.

Apolipophorin III: a lipid-triggered molecular switch

Apolipophorin III (apoLp-III) is a low molecular weight exchangeable apolipoprotein that plays an important role in the enhanced neutral lipid transport during insect flight. The protein exists in lipid-free and lipid-bound states. The lipid-bound state is the active form of the protein and occurs when apoLp-III associates with lipid-enriched lipophorins. ApoLp-III is well characterized in two evolutionally divergent species: Locusta migratoria and Manduca sexta. The two apolipoproteins interact in a similar manner with model phospholipid vesicles, and transform them into discoidal particles. Their low intrinsic stability in the lipid-free state likely facilitates interaction with lipid surfaces. Low solution pH also favors lipid binding interaction through increased exposure of hydrophobic surfaces on apoLp-III. While secondary structure is maintained under acidic conditions, apoLp-III tertiary structure is altered, adopting molten globule-like characteristics. In studies of apoLp-III interaction with natural lipoproteins, we found that apoLp-III is readily displaced from the surface of L. migratoria low-density lipophorin by recombinant apoLp-III proteins from either L. migratoria or M. sexta. Thus, despite important differences between these two apoLp-IIIs (amino acid sequence, presence of carbohydrate), their functional similarity is striking. This similarity is also illustrated by the recently published NMR solution structure of M. sexta apoLp-III wherein its molecular architecture closely parallels that of L. migratoria apoLp-III.

Lipid-triggered conformational switch of apolipophorin III helix bundle to an extended helix organization

Apolipophorin III (ApoLp-III) from the Sphinx moth, Manduca sexta, is an 18kDa protein that binds reversibly to hydrophobic surfaces generated on metabolizing lipoprotein particles. It is comprised of amphipathic alpha-helices (H1-H5) organized in an up-and-down topology forming a helix bundle in the lipid-free state. Upon interaction with lipids, apoLp-III has been proposed to undergo a dramatic conformational change, involving helix bundle opening about putative hinge loops such that H1, H2 and H5 move away from H3 and H4. In the present study, we examine the relative spatial disposition of H1 and H5 on discoidal phospholipid complexes and spherical lipoproteins. Cysteine residues were engineered at position 8 in H1 and/or at position 138 in H5 in apoLp-III (which otherwise lacks Cys) yielding A8C-, A138C- and A8C/A138C-apoLp-III. Tethering of H1 and H5 by a disulfide bond between A8C and A138C abolished the ability of apoLp-III to transform phospholipid vesicles to discoidal particles, or to interact with lipoproteins, demonstrating that these helices are required to reposition during lipid interaction. Site-specific labeling of A8C/A138C-apoLp-III with N-(1-pyrene)maleimide in the lipid-free state resulted in intramolecular pyrene "excimer" fluorescence emission indicative of spatial proximity between these sites. Upon association with dimyristoylphosphatidylcholine (DMPC) discoidal complexes, the intramolecular excimer was replaced by intermolecular excimer fluorescence due to proximity between pyrene moieties on A8C and A138C in neighboring apoLp-III molecules on the discoidal particle. No excimer emission was observed in the case of pyrene-A8C-apoLp-III/DMPC or pyrene-A138C-apoLp-III/DMPC complexes. However, equimolar mixing of the two labeled single-cysteine mutants prior to disc formation resulted in excimer emission. In addition, intramolecular pyrene excimer formation was diminished upon binding of pyrene-A8C/A138C-apoLp-III to spherical lipoproteins. The data are consistent with repositioning of H1 away from H5 upon encountering a lipid surface, resulting in an extended conformation of apoLp-III that circumscribes the discoidal bilayer particle.

The 2-aminoethylphosphonate-specific transaminase of the 2-aminoethylphosphonate degradation pathway

The 2-aminoethylphosphonate transaminase (AEPT; the phnW gene product) of the Salmonella enterica serovar Typhimurium 2-aminoethylphosphonate (AEP) degradation pathway catalyzes the reversible reaction of AEP and pyruvate to form phosphonoacetaldehyde (P-Ald) and L-alanine (L-Ala). Here, we describe the purification and characterization of recombinant AEPT. pH rate profiles (log V(m) and log V(m)/K(m) versus pH) revealed a pH optimum of 8.5. At pH 8.5, K(eq) is equal to 0.5 and the k(cat) values of the forward and reverse reactions are 7 and 9 s(-1), respectively. The K(m) for AEP is 1.11 +/- 0.03 mM; for pyruvate it is 0.15 +/- 0.02 mM, for P-Ald it is 0.09 +/- 0.01 mM, and for L-Ala it is 1.4 +/- 0.03 mM. Substrate specificity tests revealed a high degree of discrimination, indicating a singular physiological role for the transaminase in AEP degradation. The 40-kDa subunit of the homodimeric enzyme is homologous to other members of the pyridoxalphosphate-dependent amino acid transaminase superfamily. Catalytic residues conserved within well-characterized members are also conserved within the seven known AEPT sequences. Site-directed mutagenesis demonstrated the importance of three selected residues (Asp168, Lys194, and Arg340) in AEPT catalysis.

Adipokinetic hormones of insect: release, signal transduction, and responses

Flight activity of insects provides an attractive yet relatively simple model system for regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a well-accepted model insect. Peptide adipokinetic hormones (AKHs) are synthesized and stored by neurosecretory cells of the corpus cardiacum, a neuroendocrine gland connected with the insect brain. The actions of these hormones on their fat body target cells trigger a number of coordinated signal transduction processes which culminate in the mobilization of both carbohydrate (trehalose) and lipid (diacylglycerol). These substrates fulfill differential roles in energy metabolism of the contracting flight muscles. The molecular mechanism of diacylglycerol transport in insect blood involving a reversible conversion of lipoproteins (lipophorins) has revealed a novel concept for lipid transport in the circulatory system. In an integrative approach, recent advances are reviewed on the consecutive topics of biosynthesis, storage, and release of insect AKHs, AKH signal transduction mechanisms and metabolic responses in fat body cells, and the dynamics of reversible lipophorin conversions in the insect blood.

Partial characterization of oligosaccharides expressed on midgut microvillar glycoproteins of the mosquito, Anopheles stephensi Liston

Midguts of the malaria-transmitting mosquito, Anopheles stephensi, were homogenized and microvillar membranes prepared by calcium precipitation and differential centrifugation. Oligosaccharides present on the microvillar glycoproteins were identified by lectin blotting before and after in vitro and in situ treatments with endo- and exo-glycosidases. Twenty-eight glycoproteins expressed a structurally restricted range of terminal sugars and oligosaccharide linkages. Twenty-three glycoproteins expressed oligomannose and/or hybrid N-linked oligosaccharides, some with alpha1-6 linked fucose as a core residue. Complex-type N-linked oligosaccharides on eight glycoproteins all possessed terminal N-acetylglucosamine, and alpha- and beta-linked N-acetylgalactosamine. Eight glycoproteins expressed O-linked oligosaccharides all containing N-acetylgalactosamine with or without further substitutions of fucose and/or galactose. Galactosebeta1-3/4/6N-acetylglucosamine-, sialic acidalpha2-3/6galactose-, fucosealpha1-2galactose- and galactosealpha1-3galactose- were not detected. Terminal alpha-linked N-acetylgalactosamine residues on N-linked oligosaccharides are described for the first time in insects. The nature and function of these midgut glycoproteins have yet to be identified, but the oligosaccharide side chains are candidate receptors for ookinete binding and candidate targets for transmission blocking strategies.

N-glycan patterns of human transferrin produced in Trichoplusia ni insect cells: effects of mammalian galactosyltransferase

The N-glycans of human serum transferrin produced in Trichopulsia ni cells were analyzed to examine N-linked oligosaccharide processing in insect cells. Metabolic radiolabeling of the intra- and extracellular protein fractions revealed the presence of multiple transferrin glycoforms with molecular weights lower than that observed for native human transferrin. Consequently, the N-glycan structures of transferrin in the culture medium were determined using three-dimensional high performance liquid chromatography. The attached oligosaccharides included high mannose, paucimannosidic, and hybrid structures with over 50% of these structures containing one fucose, alpha(1,6)-, or two fucoses, alpha(1,6)- and alpha(1,3)-, linked to the Asn-linked N-acetylglucosamine. Neither sialic acid nor galactose was detected on any of the N-glycans. However, when transferrin was coexpressed with beta(1,4)-galactosyltransferase three additional galactose-containing hybrid oligosaccharides were obtained. The galactose attachments were exclusive to the alpha(1, 3)-mannose branch and the structures varied by the presence of zero, one, or two attached fucose residues. Furthermore, the presence of the galactosyltransferase appeared to reduce the number of paucimannosidic structures, which suggests that galactose attachment inhibits the ability of hexosaminidase activity to remove the terminal N-acetylglucosamine. The ability to promote galactosylation and reduce paucimannosidic N-glycans suggests that the oligosaccharide processing pathway in insect cells may be manipulated to mimic more closely that of mammalian cells.

Lipid transport biochemistry and its role in energy production

Recent advances on the biochemistry of flight-related lipid mobilization, transport, and metabolism are reviewed. The synthesis and release of adipokinetic hormones and their function in activation of fat body triacylglycerol lipase to produce diacylglycerol is discussed. The dynamics of reversible lipoprotein conversions and the structural properties and role of the exchangeable apolipoprotein, apolipophorin III, in this process is presented. The nature and structure of hemolymph lipid transfer particle and the potential role of a recently discovered lipoprotein receptor of the low-density lipoprotein receptor family, in lipophorin metabolism and lipid transport is reviewed.

Monosaccharide compositions of Danaus plexippus (monarch butterfly) and Trichoplusia ni (cabbage looper) egg glycoproteins

Monosaccharide compositions of eggs from Danaus plexippus (monarch butterfly) and Trichoplusia ni (cabbage looper) were analyzed. Analyses were performed mainly with high performance anion exchange chromatography (HPAEC) using crude extracts of eggs or SDS-PAGE separated and PVDF-blotted protein bands. Man and GlcN were the major components in all cases, but low levels of Gal and Fuc were possibly present in some samples. Some T. ni egg glycoproteins even contained GalN. Although a peak comigrating with Neu5Ac could be detected with HPAEC-PAD or RP-HPLC (fluorometry) after derivatization with 1,2-diamino-4,5-methylenedioxy-benzene, the quantities were too small to be significant as an integral part of the analyzed glycoproteins. These data suggests that most of glycans on the glycoproteins are pauci-Man type N-glycans, but a small portion of N-glycan may be either hybrid type or complex type.

O-glycosylation potential of lepidopteran insect cell lines

The enzyme activities involved in O-glycosylation have been studied in three insect cell lines, Spodoptera frugiperda (Sf-9), Mamestra brassicae (Mb) and Trichoplusia ni (Tn) cultured in two different serum-free media. The structural features of O-glycoproteins in these insect cells were investigated using a panel of lectins and the glycosyltransferase activities involved in O-glycan biosynthesis of insect cells were measured (i.e., UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, UDP-Gal:core-1 beta1, 3-galactosyltransferase, CMP-NeuAc:Galbeta1-3GalNAc alpha2, 3-sialyltransferase, and UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase activities). First, we show that O-glycosylation potential depends on cell type. All three lepidopteran cell lines express GalNAcalpha-O-Ser/Thr antigen, which is recognized by soy bean agglutinin and reflects high UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase activity. Capillary electrophoresis and mass spectrometry studies revealed the presence of at least two different UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases in these insect cells. Only some O-linked GalNAc residues are further processed by the addition of beta1,3-linked Gal residues to form T-antigen, as shown by the binding of peanut agglutinin. This reflects relative low levels of UDP-Gal:core-1 beta1,3-galactosyltransferase in insect cells, as compared to those observed in mammalian control cells. In addition, we detected strong binding of Bandeiraea simplicifolia lectin-I isolectin B4 to Mamestra brassicae endogenous glycoproteins, which suggests a high activity of a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase. This explains the absence of PNA binding to Mamestra brassicae glycoproteins. Furthermore, our results substantiated that there is no sialyltransferase activity and, therefore, no terminal sialic acid production by these cell lines. Finally, we found that the culture medium influences the O-glycosylation potential of each cell line.

Insect cells as hosts for the expression of recombinant glycoproteins

Baculovirus-mediated expression in insect cells has become well-established for the production of recombinant glycoproteins. Its frequent use arises from the relative ease and speed with which a heterologous protein can be expressed on the laboratory scale and the high chance of obtaining a biologically active protein. In addition to Spodoptera frugiperda Sf9 cells, which are probably the most widely used insect cell line, other mainly lepidopteran cell lines are exploited for protein expression. Recombinant baculovirus is the usual vector for the expression of foreign genes but stable transfection of - especially dipteran - insect cells presents an interesting alternative. Insect cells can be grown on serum free media which is an advantage in terms of costs as well as of biosafety. For large scale culture, conditions have been developed which meet the special requirements of insect cells. With regard to protein folding and post-translational processing, insect cells are second only to mammalian cell lines. Evidence is presented that many processing events known in mammalian systems do also occur in insects. In this review, emphasis is laid, however, on protein glycosylation, particularly N-glycosylation, which in insects differs in many respects from that in mammals. For instance, truncated oligosaccharides containing just three or even only two mannose residues and sometimes fucose have been found on expressed proteins. These small structures can be explained by post-synthetic trimming reactions. Indeed, cell lines having a low level of N-acetyl-beta-glucosaminidase, e.g. Estigmene acrea cells, produce N- glycans with non-reducing terminal N-acetylglucosamine residues. The Trichoplusia ni cell line TN-5B1-4 was even found to produce small amounts of galactose terminated N-glycans. However, there appears to be no significant sialylation of N-glycans in insect cells. Insect cells expressed glycoproteins may, though, be alpha1,3-fucosylated on the reducing-terminal GlcNAc residue. This type of fucosylation renders the N-glycans on one hand resistant to hydrolysis with PNGase F and on the other immunogenic. Even in the absence of alpha1,3-fucosylation, the truncated N-glycans of glycoproteins produced in insect cells constitute a barrier to their use as therapeutics. Attempts and strategies to "mammalianise" the N-glycosylation capacity of insect cells are discussed.

Characterization of a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase activity in a Mamestra brassicae cell line

The binding of Bandeiraea simplicifolia lectin-I isolectin B4 on the endogenous glycoproteins of different insect cell lines led us to characterize for the first time a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase in a Mamestra brassicae cell line (Mb). The study of the acceptor specificity indicated that the Mb alpha-galactosyltransferase prefers Galbeta1-3-R as acceptor, and among such glycans, the relative substrate activity Vmax/Km was equal to 20 microliters.mg-1.h-1 for Galbetal-3GlcNAcbeta1-O-octyl and to 330 microliters.mg-1.h-1 for Galbeta1-3GalNAcalpha-1-O-benzyl, showing clearly that Galbeta1-3GalNAc disaccharide was the more suitable acceptor substrate for Mb alpha-galactosyltransferase activity. Nuclear magnetic resonance and mass spectrometry data allowed us to establish that the Mb alpha-galactosyltransferase synthesizes one unique product, Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl. The Galbeta1-3GalNAc disaccharide is usually present on O-glycosylation sites of numerous asialoglycoproteins and at the nonreducing end of some glycolipids. We observed that Mb alpha1,4-galactosyltransferase catalyzed the transfer of galactose onto both natural acceptors. Finally, we demonstrated that the trisaccharide Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl was able to inhibit anti-PK monoclonal antibody-mediated hemagglutination of human blood group PK1 and PK2 erythrocytes.

Recombinant locust apolipophorin III: characterization and NMR spectroscopy

Apolipophorin III (apoLp-III) from the locust Locusta migratoria is an exchangeable apolipoprotein that reversibly binds to lipoproteins. During lipid binding the protein has been proposed to undergo a major conformational change. To study the mechanism of lipid binding we have cloned and expressed recombinant protein in bacteria, permitting stable isotope enrichment for heteronuclear NMR spectroscopy and site-directed mutagenesis. The cDNA coding for apoLp-III was subcloned into the pET expression vector and transformed into Escherichia coli cells. Induction of expression resulted in the specific appearance of apoLp-III in the cell culture medium, indicating it escaped the bacteria without lysis. The protein was purified from the cell-free supernatant by reversed-phase HPLC, characterized and compared to the natural protein isolated from locust hemolymph. SDS-PAGE revealed the recombinant protein has a molecular mass of approximately 17 kDa, similar to that of deglycosylated natural apoLp-III. Monoclonal antibodies were used to detect recombinant apoLp-III in the cells as well as in cell-free medium of induced bacterial cultures. Amino acid sequencing and analysis confirmed the identity of the recombinant protein as L. migratoria apoLp-III. Circular dichroism spectroscopy of recombinant and natural apoLp-III showed similar spectra, both displaying high contents of alpha-helical secondary structure. Denaturation studies of lipid-free apoLp-III with guanidine hydrochloride showed that both proteins have similar denaturation midpoints and DeltaG values indicating similar protein stability. The natural and recombinant protein were functional in lipoprotein binding assays. Using recombinant protein, uniformly and specifically labeled with 15N-amino acids, two dimensional 1H-15N heteronuclear single quantum correlation spectra were obtained. The spectra revealed excellent chemical shift dispersion in both the 1H and 15N dimensions with a well defined resonance pattern. Studies with 15N-leucine specifically labeled apoLp-III in the presence and absence of the micelle forming lipid, dodecylphosphocholine, provided evidence for a significant conformational change upon lipid association.

Isolation and characterization of the carbon-phosphorus bond-forming enzyme phosphoenolpyruvate mutase from the mollusk Mytilus edulis

The enzyme phosphoenolpyruvate mutase was purified to homogeneity from the mollusk Mytilus edulis. The subunit size of the native homotetramer was determined to be 34,000 Da. The steady-state kinetic constants for catalysis of the conversion of phosphonopyruvate to phosphoenolpyruvate at pH 7.5 and 25 degrees C were measured at kcat = 34 s-1, phosphonopyruvate Km = 3 microM, and Mg2+ Km = 4 microM. The enzyme displayed a broad specificity for divalent metal ion activation; Co2+, Mn2+, Zn2+, and Ni2+ are activators, whereas Ca2+ is not. Analysis of the pH dependence of the Mg2+-activated mutase-catalyzed reaction of phosphonopyruvate revealed one residue that must be protonated (apparent pKa = 8.3) and a second residue that must be unprotonated (apparent pKa = 7.7) for maximal catalytic activity.

Role of glycosylation in the lipid-binding activity of the exchangeable apolipoprotein, apolipophorin-III

Non-glycosylated recombinant Locusta migratoria apolipophorin-III, apoLp-III, was expressed in E. coli and its physical-chemical properties were compared to those of the glycosylated native apoLp-III. Fluorescence quantum yield and acrylamide quenching studies indicated a slightly higher accessibility of the Trp residues in the recombinant apoLp-III. Far-UV CD spectroscopy indicated that the recombinant apoLp-III has a lower alpha-helical content than the glycosylated apoLp-III. Both proteins spontaneously formed discoidal recombinant lipoprotein particles when incubated with dimyristoylphosphatidylcholine (DMPC). Interaction with lipid promotes an increase in alpha-helical content. CD and fluorescence studies indicate that both proteins adopt the same conformation in the lipid-bound state. However, the kinetics of association of the recombinant protein with DMPC is 5-fold faster than that of the native protein. The results suggest that glycosylation inhibits the lipid binding activity by preventing the exposure of hydrophobic domains and/or decreasing the conformational flexibility of the protein.

Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins

Apolipophorin-III (apoLp-III) from the insect, Manduca sexta, is a 166-residue exchangeable apolipoprotein that plays a critical role in the dynamics of plasma lipoprotein interconversions. Our previous work indicated that a 36-residue C-terminal peptide fragment, generated by cyanogen bromide digestion of apoLp-III, was unable to bind to lipid surfaces (Narayanaswami V, Kay CM, Oikawa K, Ryan RO, 1994, Biochemistry 33:13312-13320), and showed no secondary structure in aqueous solution. In this paper, we have performed structural studies of this peptide (E131-Q166) complexed with SDS detergent micelles, or in the presence of the helix-inducing solvent trifluoroethanol (TFE), by two-dimensional 1H NMR spectroscopy. The peptide adopts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-bound structure of the peptide, generated from the NMR NOE data, showed an elongated, slightly curved alpha-helix. Despite its high alpha-helix forming propensity, the peptide requires alpha helix-promoting environment to adopt an alpha-helical structure. This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, tertiary contacts in the folded protein play a role in maintaining its structural integrity. Furthermore, the data suggest that the amphipathic helix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-III.

N-glycosylation of a baculovirus-expressed recombinant glycoprotein in three insect cell lines

The capacity of two Trichoplusia ni (TN-368 and BTI-Tn-5b 1-4) and a Spodoptera frugiperda (IPLB-SF-21A) cell lines to glycosylate recombinant, baculovirus-encoded, secreted, placental alkaline phosphatase was compared. The alkaline phosphatase from serum-containing, cell culture medium was purified by phosphate affinity column chromatography. The N-linked oligosaccharides were released from the purified protein with PNGase F and analyzed by fluorophore-assisted carbohydrate electrophoresis. The majority of oligosaccharide structures produced by the three cell lines contained two or three mannose residues, with and without core fucosylation, but there were structures containing up to seven mannose residues. The oligosaccharides that were qualitatively or quantitatively different between the cell lines were sequenced with glycosidase digestions. The S. frugiperda cells produced more fucosylated oligosaccharides than either of the T. ni cell lines. The smallest oligosaccharide produced by S. frugiperda cells was branched trimannose. In contrast, both T. ni cell lines produced predominantly dimannose and linear trimannose structures devoid of alpha 1-3-linked mannose.

Primary structure of 21 novel monoantennary and diantennary N-linked carbohydrate chains from alphaD-hemocyanin of Helix pomatia

The primary structures of 21 novel monoantennary and diantennary N-glycans of the glycoprotein alphaD-hemocyanin (alphaD-Hc) of Helix pomatia have been determined. Outer oligosaccharide fragments (antennae) were released from the glycoprotein by Smith degradation of an alphaD-Hc pronase digest. The major antenna, obtained following HPLC fractionation on Lichrosorb-NH2, was characterized using 1H-NMR spectroscopy, fast-atom-bombardment mass spectrometry, and linkage analysis, and corresponds to a pentasaccharide fragment. The intact carbohydrate chains of alphaD-Hc were released with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase-F digestion, separated from the protein on Bio-Gel P-100, and subfractionated on Bio-Gel P-4. A portion of subfractions was reduced with sodium borodeuteride, and the non-reduced and reduced samples were further fractionated on CarboPac PA-1, Lichrosorb-NH2/Lichrosphere-NH2, and/or Lichrosphere-C18. Purified oligosaccharides and oligosaccharide-alditols were analyzed using 500/600-MHz 1H-NMR spectroscopy. In total, four novel types of antenna were identified, namely, [structures: see text] which are all attached to O-2 of alphaMan residues of the trimannosyl-N,N'-diacetylchitobiose core element, which is generally beta-1,2-xylosylated and alpha-1,6-fucosylated, Man(alpha1-6)[Man(alpha1-3)][+/-Xyl(beta1-2)]Man(beta1-4)GlcNAc(beta1-4) [+/-Fuc(alpha1-6)]GlcNAc.

Insight into lipid surface recognition and reversible conformational adaptations of an exchangeable apolipoprotein by multidimensional heteronuclear NMR techniques

Apolipophorin III (apoLp-III) from the insect Manduca sexta is a 166-residue (Mr 18,340) member of the exchangeable apolipoprotein class that functions to stabilize lipid-enriched plasma lipoproteins. In the present study, we present the secondary structure and global fold of recombinant apoLp-III derived from three-dimensional heteronuclear NMR spectroscopy experiments. Five discrete alpha-helical segments (21-30 residues in length) with well defined boundaries were characterized by four NMR parameters: medium range nuclear Overhauser enhancement contacts between proton pairs, chemical shift index, coupling constants, and amide proton exchange rates. An antiparallel arrangement of helical segments has been obtained based on the long range interhelical nuclear Overhauser enhancement contacts. The NMR solution structure reveals a globular, up and down helix bundle organization similar to that of Locusta migratoria apoLp-III (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608). However, a short helix (comprised of 5 amino acids) has been identified in the region between helix 3 and helix 4. This helix is postulated to play a role in lipid surface recognition and/or initiation of binding. Our results also indicate the existence of buried polar and charged residues in the helix bundle, providing a structural basis for the relatively low stability of apoLp-III in its lipid-free state. It is suggested that the intrinsic low stability of lipid-free apoLp-III may be important in terms of its ability to undergo a reversible, lipid binding-induced, conformational change. This study underscores the striking resemblance in molecular architecture between insect apoLp-III and the N-terminal domain of human apolipoprotein E. The potential for application of NMR techniques to studies of the exchangeable apolipoproteins, possibly in their biologically active, lipid-associated state, has broad implications in terms of our understanding of the molecular basis of their physiological functions.