Chemical Synthesis of a Skeleton Structure of Sperm CD52—A GPI-Anchored Glycopeptide

Sensitive Method To Analyze Cell Surface GPI-Anchored Proteins Using DNA Hybridization Chain Reaction-Mediated Signal Amplification

GPI-anchored proteins (GPI-APs) are ubiquitous and essential but exist in low abundances on the cell surface, making their analysis and investigation especially challenging. To tackle the problem, a new method to detect and study GPI-APs based upon GPI metabolic engineering and DNA-facilitated fluorescence signal amplification was developed. In this context, cell surface GPI-APs were metabolically engineered using azido-inositol derivatives to introduce an azido group. This allowed GPI-AP coupling with alkyne-functionalized multifluorophore DNA assemblies generated by hybridization chain reaction (HCR). It was demonstrated that this approach could significantly improve the detection limit and sensitivity of GPI-APs, thereby enabling various biological studies, including the investigation of live cells. This new, enhanced GPI-AP detection method has been utilized to successfully explore GPI-AP engineering, analyze GPI-APs, and profile GPI-AP expression in different cells.

Recent research progress in glycosylphosphatidylinositol-anchored protein biosynthesis, chemical/chemoenzymatic synthesis, and interaction with the cell membrane

Glycosylphosphatidylinositol (GPI) attachment to the C-terminus of proteins is a prevalent posttranslational modification in eukaryotic species, and GPIs help anchor proteins to the cell surface. GPI-anchored proteins (GPI-APs) play a key role in various biological events. However, GPI-APs are difficult to access and investigate. To tackle the problem, chemical and chemoenzymatic methods have been explored for the preparation of GPI-APs, as well as GPI probes that facilitate the study of GPIs on live cells. Substantial progress has also been made regarding GPI-AP biosynthesis, which is helpful for developing new synthetic methods for GPI-APs. This article reviews the recent advancements in the study of GPI-AP biosynthesis, GPI-AP synthesis, and GPI interaction with the cell membrane utilizing synthetic probes.

A Biotinylated Glycosylphosphatidylinositol (GPI) as the Universal Platform To Access GPI-Anchored Protein Analogues

A glycosylphosphatidylinositol (GPI) derivative with biotin linked to its mannose III 6-O-position was prepared by a convergent strategy. This biotinylated GPI was demonstrated to bind avidinated proteins readily through biotin-avidin interaction and, therefore, can serve as a universal platform to access various biologically significant GPI-anchored protein analogues.

Chemische Synthese und Semisynthese von lipidierten Proteinen

Lipidierung ist eine ubiquitäre Modifikation von Peptiden und Proteinen, die entweder co‐ oder posttranslational auftreten kann. Für die Vielzahl von Lipidklassen wurde gezeigt, dass diese viele entscheidende biologische Aktivitäten, z. B. die Regulierung der Signalweiterleitung, Zell‐Zell‐Adhäsion sowie die Anlagerung von Proteinen an Lipid‐Rafts und Phospholipidmembranen, beeinflussen. Während die Natur Enzyme nutzt, um Lipidmodifikationen in Proteine einzubringen, ist ihre Nutzung für die chemoenzymatische Herstellung von lipidierten Proteinen häufig ineffizient. Eine Alternative ist die Kombination moderner synthetischer und semisynthetischer Techniken, um lipidierte Proteine in reiner und homogen modifizierter Form zu erhalten. Dieser Aufsatz erörtert Fortschritte in der Entwicklung der Lipidierungs‐ und Ligationschemie und deren Anwendung in der Synthese und Semisynthese homogen lipidierter Proteine, die es ermöglichen, den Einfluss dieser Modifikationen auf die Proteinstruktur und ‐funktion zu untersuchen.

Chemical Synthesis and Semisynthesis of Lipidated Proteins

Lipidation is a ubiquitous modification of peptides and proteins that can occur either co‐ or post‐translationally. An array of different lipid classes can adorn proteins and has been shown to influence a number of crucial biological activities, including the regulation of signaling, cell–cell adhesion events, and the anchoring of proteins to lipid rafts and phospholipid membranes. Whereas nature employs a range of enzymes to install lipid modifications onto proteins, the use of these for the chemoenzymatic generation of lipidated proteins is often inefficient or impractical. An alternative is to harness the power of modern synthetic and semisynthetic technologies to access lipid‐modified proteins in a pure and homogeneously modified form. This Review aims to highlight significant advances in the development of lipidation and ligation chemistry and their implementation in the synthesis and semisynthesis of homogeneous lipidated proteins that have enabled the influence of these modifications on protein structure and function to be uncovered.

Chemical Synthesis of Homogeneous Human E-Cadherin N-Linked Glycopeptides: Stereoselective Convergent Glycosylation and Chemoselective Solid-Phase Aspartylation

We report herein an efficient chemical synthesis of homogeneous human E-cadherin N-linked glycopeptides consisting of a heptapeptide sequence adjacent to the Asn-633 N-glycosylation site with representative N-glycan structures, including a conserved trisaccharide, a core-fucosylated tetrasaccharide, and a complex-type biantennary octasaccharide. The key steps are a chemoselective on-resin aspartylation using a pseudoproline-containing peptide and stereoselective glycosylation using glycosyl fluororide as a donor. This synthetic strategy demonstrates potential utility in accessing a wide range of homogeneous N-linked glycopeptides for the examination of their biological function.

Semisynthesis of Functional Glycosylphosphatidylinositol-Anchored Proteins

Glypiation is a common posttranslational modification of eukaryotic proteins involving the attachment of a glycosylphosphatidylinositol (GPI) glycolipid. GPIs contain a conserved phosphoglycan that is modified in a cell‐ and tissue‐specific manner. GPI complexity suggests roles in biological processes and effects on the attached protein, but the difficulties to get homogeneous material have hindered studies. We disclose a one‐pot intein‐mediated ligation (OPL) to obtain GPI‐anchored proteins. The strategy enables the glypiation of folded and denatured proteins with a natural linkage to the glycolipid. Using the strategy, glypiated eGFP, Thy1, and the Plasmodium berghei protein MSP1₁₉ were prepared. Glypiation did not alter the structure of eGFP and MSP1₁₉ proteins in solution, but it induced a strong pro‐inflammatory response in vitro. The strategy provides access to glypiated proteins to elucidate the activity of this modification and for use as vaccine candidates against parasitic infections.

Chemical Synthesis of GPI Glycan-Peptide Conjugates by Traceless Staudinger Ligation

A new strategy has been developed for GPI glycan-peptide conjugate synthesis based upon a traceless Staudinger reaction between a peptide phosphinothioester and a GPI glycan azide. The strategy was first studied and optimized with simple peptides and GPI glycans, which offered excellent yields of the desired conjugates in both organic and aqueous solvents. It was then used to successfully synthesize an analogue of the human CD52 antigen containing the whole CD52 peptide sequence and the conserved trimannose motif of all GPI anchors.

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Sortase A-mediated chemoenzymatic synthesis of complex glycosylphosphatidylinositol-anchored protein

Green fluorescent protein and a glycosylphosphatidylinositol (GPI) anchor containing the common core structure and a lipid chain were synthesized and then coupled together in the promotion of bacterial sortase A (SrtA), which was the first example for the synthesis of a full-size GPI-anchored protein by SrtA, demonstrating that this can be a generally useful method for GPI-anchored protein synthesis.

Chemoenzymatic synthesis of the human CD52 and CD24 antigen analogues

Analogs of the human CD52 and CD24 antigens carrying the common core structure of glycosylphosphatidylinositol (GPI) anchors and the intact polypeptide sequences of CD52 and CD24 were chemoenzymatically synthesized. CD52 and CD24 proteins were obtained by solid-phase peptide synthesis and then coupled to chemically synthesized GPI anchors under the influence of a bacterial enzyme, sortase A, to afford the target molecules in good yields.

Recent progress in synthetic and biological studies of GPI anchors and GPI-anchored proteins

Covalent attachment of glycosylphosphatidylinositols (GPIs) to the protein C-terminus is one of the most common posttranslational modifications in eukaryotic cells. In addition to anchoring surface proteins to the cell membrane, GPIs also have many other important biological functions, determined by their unique structure and property. This account has reviewed the recent progress made in disclosing GPI and GPI-anchored protein biosynthesis, in the chemical and chemoenzymatic synthesis of GPIs and GPI-anchored proteins, and in understanding the conformation, organization, and distribution of GPIs in the lipid membrane.

Synthetic Studies of Glycosylphosphatidylinositol (GPI) Anchors and GPI-Anchored Peptides, Glycopeptides, and Proteins

Glycosylphosphatidylinositol (GPI) anchorage of proteins and glycoproteins onto the cell surface is ubiquitous in eukaryotes, and GPI-anchored proteins and glycoproteins play an important role in many biological processes. To study GPI anchorage and explore the functions of GPIs and GPI-anchored proteins and glycoproteins, it is essential to have access to these molecules in homogeneous and structurally defined forms. This review is focused on the progress that our laboratory has made towards the chemical and chemoenzymatic synthesis of structurally defined GPI anchors and GPI-anchored peptides, glycopeptides, and proteins. Briefly, highly convergent strategies were developed for GPI synthesis and were employed to successfully synthesize a number of GPIs, including those carrying unsaturated lipids and other useful functionalities such as the azido and alkynyl groups. The latter enabled further site-specific modification of GPIs by click chemistry. GPI-linked peptides, glycopeptides, and proteins were prepared by regioselective chemical coupling of properly protected GPIs and peptides/glycopeptides or through site-specific ligation of synthetic GPIs and peptides/glycopeptides/proteins under the influence of sortase A. The investigation of interactions between GPI anchors and pore-forming bacterial toxins by means of synthetic GPI anchors and GPI analogs is also discussed.

Chemical biology of glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositols (GPIs) are complex glycolipids that are covalently linked to the C-terminus of proteins as a posttranslational modification. They anchor the attached protein to the cell membrane and are essential for normal functioning of eukaryotic cells. GPI-anchored proteins are structurally and functionally diverse. Many GPIs have been structurally characterized but comprehension of their biological functions, beyond the simple physical anchoring, remains largely speculative. Work on functional elucidation at a molecular level is still limited. This Review focuses on the roles of GPI unraveled by using synthetic molecules and summarizes the structural diversity of GPIs, as well as their biological and chemical syntheses.

Total synthesis of a glycosylphosphatidylinositol anchor of the human lymphocyte CD52 antigen

The first total synthesis of a glycosylphosphatidylinositol (GPI) anchor bearing a polyunsaturated arachidonoyl fatty acid is reported. This lipid is found in mammalian GPIs that do not undergo lipid remodeling, a process that has important implications in the localization and function of GPI-anchored proteins. Incorporation of the oxidation- and reduction-sensitive arachidonoyl lipid in the target GPI was accomplished by using the para-methoxybenzyl (PMB) group for permanent hydroxyl group protection, which featured a selective, rapid, and efficient global deprotection protocol. The flexibility of this synthetic strategy was further highlighted by the inclusion of two additional GPI core structural modifications present in the GPI anchor of the human lymphocyte CD52 antigen.

Sortase-Mediated Transpeptidation for Site-Specific Modification of Peptides, Glycopeptides, and Proteins

Sortases are a family of transpeptidases found in Gram-positive bacteria responsible for covalent anchoring of cell surface proteins to bacterial cell walls. It has been discovered that sortase A (SrtA) of Staphylococcus aureus origin is rather promiscuous and can accept various molecules as substrates. As a result, SrtA has been widely used to ligate peptides and proteins with a variety of nucleophiles, and the ligation products are useful for research in chemical biology, proteomics, biomedicine, etc. This review summarizes the recent applications of SrtA with special emphasis on SrtA-catalyzed ligation of carbohydrates with peptides and proteins.

Unusual transglycosylation activity of Flavobacterium meningosepticum endoglycosidases enables convergent chemoenzymatic synthesis of core fucosylated complex N-glycopeptides

Structurally well defined, homogeneous glycopeptides and glycoproteins are indispensable tools for functional glycomics studies. By screening of various endo-β-N-acetylglucosaminidases through the use of appropriate synthetic donor and acceptor substrates, we have found that the Flavobacterium meningosepticum endo-β-N-acetyl-glucosaminidases (GH family 18), including Endo-F2 and Endo-F3, were able to glycosylate α-1,6-fucosylated GlcNAc derivative to provide natural, core-fucosylated complex-type N-glycopeptides. The Endo-F2 and Endo-F3 were efficient for transferring both sialylated and asia-lylated glycans and were highly specific for an α-1,6-fucosylated GlcNAc-peptide as acceptor for transglycosylation, showing only marginal activity with non-fucosylated GlcNAc-peptides. In contrast, we found that the commonly used endoglycosidases such as Endo-A and Endo-M, which belong to GH family 85, were unable to take α-1,6-fucosyl-GlcNAc derivative as acceptors for transglycosylation. The novel activity of Endo-F2 and Endo-F3 was successfully applied for a highly convergent chemoenzymatic synthesis of a full-sized CD52 glycopeptide antigen carrying both terminal sialic acid and core fucose. This is the first report on endoglycosidases that are able to glycosylate α-1,6-fucosylated GlcNAc derivatives to form natural core-fucosylated glycopeptides.

Chemical synthesis and functionalization of clickable glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositol (GPI) anchorage is a common posttranslational modification of eukaryotic proteins. Chemical synthesis of structurally defined GPIs and GPI derivatives is a necessary step toward understanding the properties and functions of these molecules in biological systems. In this work, the synthesis of several functionalized GPI anchors was accomplished using the para-methoxybenzyl (PMB) group for permanent hydroxyl protection, which allowed the incorporation of functionalities that are incompatible with permanent protecting groups traditionally used in carbohydrate synthesis. A flexible convergent-divergent assembly strategy enabled efficient access to a diverse set of target structures, including "clickable" Alkynyl-GPIs 1 and 2 and Azido-GPI 3. For global deprotection, a one-pot reaction was employed to afford the target GPIs in excellent yields (85-97%). Fully deprotected clickable GPIs 2 and 3 were readily conjugated to imaging and affinity probes via Cu(I)-catalyzed and Cu-free strain-promoted [3+2] cycloaddition, respectively, resulting in GPI-Fluor 4 and GPI-Biotin 5.

Expeditious chemoenzymatic synthesis of CD52 glycopeptide antigens

CD52 is a glycosylphosphatidylinositol (GPI)-anchored glycopeptide antigen found on sperm cells and human lymphocytes. Recent structural studies indicate that sperm-associated CD52 antigen carries both a complex type N-glycan and an O-glycan on the polypeptide backbone. To facilitate functional and immunological studies of distinct CD52 glycoforms, we report in this paper the first chemoenzymatic synthesis of homogeneous CD52 glycoforms carrying both N- and O-glycans. The synthetic strategy consists of two key steps: monosaccharide primers GlcNAc and GalNAc were first installed at the pre-determined N- and O-glycosylation sites by a facile solid-phase peptide synthesis, and then the N- and O-glycans were extended by respective enzymatic glycosylations. It was found that the endoglycosidase-catalyzed transglycosylation allowed efficient attachment of an intact N-glycan in a single step at the N-glycosylation site, while the recombinant human T-synthase could independently extend the O-linked GalNAc to form the core 1 O-glycan. This chemoenzymatic approach is highly convergent and permits easy construction of various homogeneous CD52 glycoforms from a common polypeptide precursor. In addition, the introduction of a latent thiol group in the form of protected cysteamine at the C-terminus of the CD52 glycoforms will enable site-specific conjugation to a carrier protein to provide immunogens for generating CD52 glycoform-specific antibodies for functional studies.

Chemoenzymatic synthesis of glycosylphosphatidylinositol-anchored glycopeptides

MUC1 glycopeptide was efficiently coupled to glycosylphosphatidylinositol (GPI) derivatives by sortase A (SrtA), verifying that SrtA can accept sterically hindered glycopeptide as substrate for ligation with GPIs. This work has established a practical method for the chemoenzymatic synthesis of GPI-linked glycopeptides.

Sortase-catalyzed peptide-glycosylphosphatidylinositol analogue ligation

It is demonstrated that sortase A (SrtA) can catalyze efficient coupling of peptides to GPI analogues with a glycine residue attached to the phosphoethanolamine moiety at the nonreducing end to form GPI-linked peptides. This represents the first chemoenzymatic synthesis of GPI-peptide conjugates and is a proof-of-concept for the potential application of SrtA to the synthesis of more complex GPI-anchored peptides/glycopeptides and GPI-anchored proteins/glycoproteins.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

The glycosylphosphatidylinositol anchor: a complex membrane-anchoring structure for proteins

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified protein in the outer leaflet of the cell membrane. The GPI anchor is a complex structure comprising a phosphoethanolamine linker, glycan core, and phospholipid tail. GPI-anchored proteins are structurally and functionally diverse and play vital roles in numerous biological processes. While several GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. This review discusses the structural diversity of the GPI anchor and its putative cellular functions, including involvement in lipid raft partitioning, signal transduction, targeting to the apical membrane, and prion disease pathogenesis. We specifically highlight studies in which chemically synthesized GPI anchors and analogues have been employed to study the roles of this unique posttranslational modification.

Synthetic analogues of glycosylphosphatidylinositol-anchored proteins and their behavior in supported lipid bilayers

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified proteins in the outer leaflet of the plasma membrane. GPI-anchored proteins play vital roles in signal transduction, the vertebrate immune response, and the pathobiology of trypanosomal parasites. While many GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. We synthesized a series of GPI-protein analogues bearing modified anchor structures that were designed to dissect the contribution of various glycan components to the GPI-protein's membrane behavior. These anchor analogues were similar in length to native GPI anchors and included mimics of the native structure's three domains. A combination of expressed protein ligation and native chemical ligation was used to attach these analogues to the green fluorescent protein (GFP). These modified GFPs were incorporated in supported lipid bilayers, and their mobilities were analyzed using fluorescence correlation spectroscopy. The data from these experiments suggest that the GPI anchor is more than a simple membrane-anchoring device; it also may prevent transient interactions between the attached protein and the underlying lipid bilayer, thereby permitting rapid diffusion in the bilayer. The ability to generate chemically defined analogues of GPI-anchored proteins is an important step toward elucidating the molecular functions of this interesting post-translational modification.

Recent progress in the field of glycopeptide synthesis

Glycosylation is a common post-translational modification of proteins. Although its significance in biological system is well recognized, approaches to analyze carbohydrate function are limited. This is because of difficulty in obtaining homogeneous glycoproteins from natural sources. Due to the progress of the carbohydrate and peptide chemistry, syntheses of various homogeneous glycopeptides and glycoproteins, which are suitable for biological studies, have been achieved by chemical means. In this review, we briefly summarize recent advances in the field of glycopeptide synthesis after 1999.

Carbohydrate diversity: synthesis of glycoconjugates and complex carbohydrates

The fundamental role of glycoconjugates in many biological processes is now well appreciated and has intensified the development of innovative and improved synthetic strategies. All areas of synthetic methodology have seen major advances and many complex, highly branched carbohydrates and glycoproteins have been prepared using solution- and/or solid-phase approaches. The development of an automated oligosaccharide synthesizer provides rapid access to biologically relevant compounds. These chemical approaches help to produce sufficient quantities of defined oligosaccharides for biological studies. Synthetic chemistry also supports an improved understanding of glycobiology and will eventually result in the discovery of new therapeutics.