C-mannosylation and O-fucosylation of the thrombospondin type 1 module

Cloning, expression and functional characterization of the full-length murine ADAMTS13

Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts13 gene from BALB/c mice liver poly A+ mRNA. Murine ADAMTS13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS13 was inhibited by EDTA and the plasma from a TTP patient.

Chaperone activity of protein O-fucosyltransferase 1 promotes notch receptor folding

Notch proteins are receptors for a conserved signaling pathway that affects numerous cell fate decisions. We found that in Drosophila, Protein O-fucosyltransferase 1 (OFUT1), an enzyme that glycosylates epidermal growth factor-like domains of Notch, also has a distinct Notch chaperone activity. OFUT1 is an endoplasmic reticulum protein, and its localization was essential for function in vivo. OFUT1 could bind to Notch, was required for the trafficking of wild-type Notch out of the endoplasmic reticulum, and could partially rescue defects in secretion and ligand binding associated with Notch point mutations. This ability of OFUT1 to facilitate folding of Notch did not require its fucosyltransferase activity. Thus, a glycosyltransferase can bind its substrate in the endoplasmic reticulum to facilitate normal folding.

Stereocontrolled syntheses of α-C-mannosyltryptophan and its analogues

The total synthesis of alpha-C-mannosyltryptophan (C-Man-Trp), a naturally occurring C-glycosylamino acid, was achieved from a commercially available alpha-methyl-D-mannoside in 10 steps including the following key steps: the C-glycosidation of a mannose derivative with a stannylacetylene, Castro indole synthesis, and Sc(ClO4)3-promoted coupling with L-serine-derived aziridine carboxylate. The glucose- and galactose-analogues of C-Man-Trp were also synthesized in a similar manner. Conformational analyses of the synthesized C-glycosyltryptophan and its synthetic intermediate are briefly discussed.

Methods in enzymology: O-glycosylation of proteins

Cell surface and extracellular proteins are O-glycosylated, where the most abundant type of O-glycosylation in proteins is the GalNAc attachment to serine (Ser) or threonine (Thr) in the protein chain by an a-glycosidic linkage. Most eukaryotic nuclear and cytoplasmic proteins modified by a-linked O-GlcNAc to Ser or Thr exhibit reciprocal O-GlcNAc glycosylation and phosphorylation during the cell cycle, cell stimulation, and/or cell growth. Less-investigated types of O-glycosylation are O-fucosylation, O-mannosylation, and O-glucosylation, but they are functionally of high relevance for early stages of development and for vital physiological functions of proteins. Glycosaminoglycans are a-linked to proteoglycans via a xylose-containing tetrasaccharide, represented by linear chains of repetitive disaccharides modified by carboxylates and O- or/and N-linked sulfates. Analysis of O-glycosylation by mass spectrometry (MS) is a complex task due to the high structural diversity of glycan and protein factors. The parameters in structural analysis of O-glycans include determination of (i) O-glycosylation attachment sites in the protein sequence, (ii) the type of attached monosaccharide moiety, (iii) a core type in the case of GalNAc O-glycosylation, (iv) the type and size of the oligosaccharide portion, (v) carbohydrate branching patterns, (vi) the site of monosaccharide glycosidic linkages, (vii) the anomericity of glycosidic linkages, and (viii) covalent modifications of the sugar backbone chains by carbohydrate- and noncarbohydrate-type of substitutents. Classical and novel analytical strategies for identification and sequencing of O-glycans by MS are described. These include methods to analyze O-glycans after total or partial release from the parent protein by chemical or enzymatic approach or to analyze O-glycosylated peptides by mapping and sequencing from proteolytic mixtures. A recombination process of multiply charged glycopeptides with electrons by electron capture dissociation Fourier transform ion cyclotrone resonance (FTICR)-MS has been introduced and is instrumental for nonergodic polypeptide backbone cleavages without losses of labile glycan substituents. A method for O-glycoscreening under increased sensitivity and efficient sequencing as a combination of an on-line coupling of capillary electrophoresis separation, as well as an automated MS-tandem MS (MS/MS) switching under variable energy conditions collision-induced dissociation (CID) protocol, is beneficial for determination of O-acetylation and oversulfation (Bindila et al., 2004a; Zamfir et al., 2004a). O-glycomics by robotized chip-electrospray/ionization (ESI)-MS and MS/MS on the quadrupole time-of-flight (QTOF) and FTICR analyzers, accurate mass determination, and software for assignment of fragmentation spectra represent essentials for high-throughput (HTP) in serial screenings (Bindila et al., 2004b; Froesch et al., 2004; Vakhrushev et al., 2005). Dimerization of intact O-glycosylated proteins can be investigated by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF)-MS after blotting.

Electron capture dissociation of O-glycosylated peptides: radical site-induced fragmentation of glycosidic bonds

Glycosylation of proteins represents one of the most important post-translational modifications. The structural characterisation of glycoproteins--especially with respect to the determination of the glycosylation site--by direct mass spectrometric methods still remains an elusive goal. We have applied the low energy dissociation method electron capture dissociation (ECD) in a 9.4 T Fourier transform ion cyclotron resonance mass spectrometer to the structural elucidation of mucin-derived peptides glycosylated with glycans of different core types. Capture of an electron by multiply protonated precursor ions [M + nH](n+) resulted in the formation of reduced odd electron radical cations [M + nH](n-1)+*. Subsequent cleavage of the N-Calpha bonds of the peptide chain, mostly without loss of the labile sugar moiety, represents a major fragmentation pathway allowing unambiguous assignment of the glycosylation site. In addition to peptide backbone cleavages, loss of acetyl radicals from the N-acetyl group of the HexNAc glycans is observed. Radical site induced elimination processes of the glycan moieties initiated by hydrogen transfer, from the glycan to the peptide backbone and vice versa give rise to signals in the ECD spectra. The different sugar core types exhibit different fragmentation patterns driven by the stability of the resulting fragments allowing the discrimination of isomeric glycans.

Increased expression of protein C-mannosylation in the aortic vessels of diabetic Zucker rats

C-Mannosylation is a novel type of glycosylation in proteins. There are several examples of proteins in which the specific motif Trp-X-X-Trp is mannosylated at the first Trp to produce C-mannosylated Trp (CMW). Although C-mannosylation modifies Trp-X-X-Trp, predicted to be a functional motif of various integral proteins such as cytokine receptors, the physiological or pathological relevance of C-mannosylation in the cell is still not known. In this study, to characterize C-mannosylation in biological samples, we generated specific polyclonal antibodies against CMW by using a chemically synthesized CMW as an antigen. Using the antibody, we investigated the effect of hyperglycemic conditions on protein C-mannosylation in cultured cells and diabetic Zucker fatty rats. We found that protein C-mannosylation was increased in macrophage-like RAW264.7 cells under hyperglycemic conditions compared to low-glucose conditions. Furthermore, C-mannosylation was increased in the aortic vessel wall of Zucker fatty rats. Thrombospondin-1 was identified as a protein modified with C-mannosylation, and its expression was also increased in the aortic tissues of Zucker fatty rats. These results indicate that C-mannosylation is increased in specific tissues or cell types under hyperglycemic conditions, suggesting a pathological role for the increased C-mannosylation in the development of diabetic complications.

Fully-automated chip-based nanoelectrospray tandem mass spectrometry of gangliosides from human cerebellum

The introduction of chip-based electrospray (ESI) ion sources into biological mass spectrometry (MS) addressed the fundamental issue of how to analyze minute amounts of complex biological systems. The automation of sample delivery into the MS combined with the chip-based ESI allows for high quality bioanalysis in a high-throughput fashion. These advantages have already been demonstrated in proteomics, direct screening of drugs and drug discovery. As part of our continuing effort to implement automated chip-based mass spectrometry into the field of complex carbohydrate analysis, we hereby report the development of a chipESI MS and MS/MS methodology for the screening of gangliosides. A strategy to characterize a complex ganglioside mixture from human cerebellar tissue, by automated ESIchip-quadrupole time-of-flight (QTOF) MS and MS/MS is presented here. The feasibility of this method, and the general experimental requirements for automated chipESI MS analysis of these carbohydrate species is described.

Thrombospondin 1: a multifunctional protein implicated in the regulation of tumor growth

Thrombospondins belong to a family of extracellular matrix (ECM) proteins widely found from embryonic to adult tissues. The modular structure of thrombospondins contains a series of peptide sequences implicated in a multiplicity of biological functions. Extracellular matrix undergoes important alterations under proteolysis that occurs in pathological processes like tumorigenesis. An elevated secretion of thrombospondin 1 (TSP1) is often observed in tumors and is sometimes considered as a predictive factor. However, the role of TSP1 in cancer progression remains controversial and must be carefully apprehended. The regulation of cell adhesion, proliferation, apoptosis by TSP1 is examined in the present review and it is clear from the literature and from our investigations that TSP1 presents both stimulatory and inhibitory effects. The exposition of cryptic sites upon conformational changes can partially explain this contradiction. More interestingly, the analysis of TSP1-directed intracellular signaling pathways activated through specific receptors or supramolecular receptors docking systems may be useful to discriminate the precise function of TSP1 in tumor progression. The central role played by TSP1 in the control of matrix-degrading enzyme activation and catabolism reveals attractive tracks of research and highlights the involvement of the lipoprotein receptor-related protein (LRP) receptor in these events. Therefore, TSP1-derived peptides constitute a source of potentially active matrikins which could provide essential tools in cancer therapy.

Role of glycosylation in development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Expression of the type-1 repeats of thrombospondin-1 inhibits tumor growth through activation of transforming growth factor-beta

In the present study, the type-1 repeats of thrombospondin-1 (TSP-1) were transfected into A431 cells. Expression of all three type-1 repeats (3TSR) and expression of just the second type-1 repeat containing the transforming growth factor (TGF)-beta activating sequence KRFK (TSR2 + KRFK) significantly inhibited in vivo tumor angiogenesis and growth in nude mice. These tumors expressed increased levels of both active and total TGF-beta. A431 cells expressing the second type-1 repeat without the KRFK sequence (TSR2 - KRFK) produced tumors that were slightly larger than the 3TSR and TSR2 + KRFK tumors. These tumors expressed elevated levels of active TGF-beta but levels of total TGF-beta were not different from control tumors. Injection of the peptide, LSKL, which blocks TSP-1 activation of TGF-beta, reversed the growth inhibition observed with cells expressing TSR2 + KRFK to a level comparable to controls. Various residues in the WSHWSPW region and the VTCG sequence of both TSR2+/- KRFK were mutated. Although mutation of the VTCG sequence had no significant effect on tumor growth, mutation of the WSHWSPW sequence reduced inhibition of tumor growth. These findings suggest that the inhibition of tumor angiogenesis and growth by endogenous TSP-1 involves regulation of both active and total TGF-beta and the sequences KRFK and WSHWSPW in the second type-1 repeat.

Expression and characterization of recombinant human ADAMTS-13

Thrombotic thrombocytopenic purpura (TTP) is a severe disease associated with unusually large, hemostatically hyperactive von Willebrand factor (VWF) and severe deficiency in ADAMTS-13, the protease responsible for the proteolytic degradation of VWF in plasma. ADAMTS-13 prevents inappropriate microvascular platelet aggregation by cleaving VWF between Tyr1605 and Met1606 thereby producing dimers of 176 kd and 140 kd and smaller multimers. Identification of the ADAMTS13 gene and cloning of the corresponding cDNA allowed for the application of recombinant techniques, such as genetic engineering of ADAMTS13 cDNA, cell culture expression, and in vitro activity studies to analyze the functional relationship between ADAMTS-13 and the pathophysiology of ADAMTS-13 deficiency. In vitro expression and characterization of recombinant ADAMTS-13 (rADAMTS-13) clearly established that ADAMTS-13 is deficient in congenital TTP and inhibited in acquired TTP. Recent studies have contributed greatly to our current understanding of the molecular mechanism leading to congenital and acquired TTP. Apart from being a useful tool, availability of rADAMTS-13 raised the prospect of developing a recombinant substitution therapy to improve TTP treatment and allowing present diagnostic assays to be simplified. Here we report on recent advances in cell culture expression and functional characterization of human rADAMTS-13.

Identification of novel beta1 integrin binding sites in the type 1 and type 2 repeats of thrombospondin-1

In addition to the three known beta(1) integrin recognition sites in the N-module of thrombospondin-1 (TSP1), we found that beta(1) integrins mediate cell adhesion to the type 1 and type 2 repeats. The type 1 repeats of TSP1 differ from typical integrin ligands in that recognition is pan-beta(1)-specific. Adhesion of cells that express one dominant beta(1) integrin on immobilized type 1 repeats is specifically inhibited by antagonists of that integrin, whereas adhesion of cells that express several beta(1) integrins is partially inhibited by each alpha-subunit-specific antagonist and completely inhibited by combining the antagonists. beta(1) integrins recognize both the second and third type 1 repeats, and each type 1 repeat shows pan-beta(1) specificity and divalent cation dependence for promoting cell adhesion. Adhesion to the type 2 repeats is less sensitive to alpha-subunit antagonists, but a beta(1) blocking antibody and two disintegrins inhibit adhesion to immobilized type 2 repeats. beta(1) integrin expression is necessary for cell adhesion to the type 1 or type 2 repeats, and beta(1) integrins bind in a divalent cation-dependent manner to a type 1 repeat affinity column. The widely used TSP1 function blocking antibody A4.1 binds to a site in the third type 2 repeat. A4.1 proximally inhibits beta(1) integrin-dependent adhesion to the type 2 repeats and indirectly inhibits integrin-dependent adhesion mediated by the TSP1 type 1 repeats. Although antibody A4.1 is also an antagonist of CD36 binding to TSP1, these data suggest that some biological activities of A4.1 result from antagonism of these novel beta(1) integrin binding sites.

Quantitative gas chromatography/mass spectrometry determination of C-mannosylation of tryptophan residues in glycoproteins

C-mannosylation of Trp residue is one of the most recently discovered types of glycosylation, but the identification of these mannosylated residues in proteins is rather tedious. In a previous paper, it was reported that the complete analysis of all constituents of glycoproteins (sialic acids, monosaccharides, and amino acids) could be determined on the same sample in three different steps of gas chromatography/mass spectrometry of heptafluorobutyrate derivatives. It was observed that during the acid-catalyzed methanolysis step used for liberation of monosaccharide from classical O- and N-glycans, Trp and His were quantitatively transformed by the addition of a methanol molecule on their indole and imidazole groups, respectively. These derivatives were stable to acid hydrolysis used for the liberation of amino acids. Since monosaccharide derivatives were also stabilized as heptafluorobutyrate derivatives of O-methyl-glycosides, it was suggested that C-mannosides of Trp residues could quantitatively be recovered. Based on the analyses of standard compounds, peptides and RNase 2 from human urine, we report that C((2))-mannosylated Trp could be quantitatively recovered and identified during the step of amino acid analysis. Analyses of different samples indicated that this type of glycosylation is absent in bacteria and yeasts.

α-C-Mannosyltryptophan is not recognized by conventional mannose-binding lectins

Alpha-C-mannosyltryptophan (C-Man-Trp) is a novel, naturally occurring C-linked carbohydrate-protein linkage first found in 1994 from human ribonuclease 2. Since then, a number of C-Man-Trp residue have been found from several important proteins such as interleukin 12 beta, components of complement system, thrombospondin-1, and erythropoietin receptor, however, the biological functions have remained unknown even though its biosynthetic pathway has been revealed. In order to find a clue as to the biological functions, we examined the affinity of C-Man-Trp with conventional mannose lectin such as concanavarin A (Con A) and mannose-binding lectin (MBL). The affinity of C-Man-Trp with Con A, a typical mannose-binding lectin from plant was examined using a Con A-Sepharose column. Unlike p-nitrophenyl-alpha-O-Man, C-Man-Trp was not retained on the column. MBL-C, a major mannose-binding lectin purified from mouse serum, did not bind with N-biotinylated C-Man-Trp, judging from ELISA based assay. These results imply that C-Man-Trp may be recognized with the other specific proteins associated with its unknown biological functions.

Molecular and functional differences induced in thrombospondin-1 by the single nucleotide polymorphism associated with the risk of premature, familial myocardial infarction

A serine (Ser-700) amino acid rather than an asparagine (Asn-700) at residue 700 of thrombospondin-1 has been linked to an increased risk for development of premature, familial heart attacks. We now have identified both functional and structural differences between the Ser-700 and Asn-700 thrombospondin-1 variants. The Ser-700 variant increased the rate and extent of platelet aggregation and showed increased surface expression on platelets compared with the Asn-700 variant. These differences could be ascribed to an enhanced interaction of the Ser-700 variant with fibrinogen on the platelet surface and are consistent with a prothrombotic phenotype in Ser-700 individuals. The Ser-700 variant thrombospondin-1 was conformationally more labile than the Asn-700 variant as demonstrated by increased susceptibility to proteolytic digestion and enhanced susceptibility to unfolding by denaturants. These data suggest a potential molecular and cellular basis for a genetic risk factor associated with early onset myocardial infarction.

Fully Automated Chip-Based Mass Spectrometry for Complex Carbohydrate System Analysis

Carbohydrates represent a major class of biopolymers, which occur in nature either as oligosaccharides or glycoconjugates, in which the sugar moiety is linked to proteins or lipids. The significance of mass spectrometry for highly sensitive analysis of complex carbohydrates increased after the introduction of the electrospray ionization and matrix assisted laser desorption/ionization methods and the possibility of tandem MS for sequencing of single molecular species in complex mixtures. Rapid and sensitive characterization of carbohydrates in biological systems by automated nanoscale liquid delivery and chip-based electrospray interface techniques have not been developed so far. In this contribution, the implementation and optimization of a fully automated chip-based nanoelectrospray assembly (NanoMate system), operating in the negative ion mode, in combination with QTOF-tandem MS for mapping/sequencing and computer-assisted structure assignment for carbohydrate components in complex mixtures is presented.

The WSAWS motif is C-hexosylated in a soluble form of the erythropoietin receptor

The WSXWS motif is a highly conserved structural feature of the type I cytokine receptor family. It has previously been demonstrated that mutations in the (232)WSAWS(236) motif in the erythropoietin receptor (EPOR) can result in strongly inhibited surface expression, due to defective intracellular transport [Hilton, D. J., et al. (1996) J. Biol. Chem. 271, 4699-4708]. Here we report that the first tryptophan in the motif of the recombinant extracellular domain of EPOR (sEPOR) expressed in HEK-EBNA cells carries a C-linked hexosyl residue. The S233A mutation completely abolished secretion of sEPOR, whereas the A234E mutation resulted in enhanced secretion. Comparison of the level of C-hexosylation in the wild-type protein and in the mutant proteins isolated from the conditioned medium and/or the cells suggested that C-hexosylation of the motif did not play a role in the correct intracellular transport of sEPOR.

Differential terminal fucosylation of N-linked glycans versus protein O-fucosylation in leukocyte adhesion deficiency type II (CDG IIc)

LAD II/CDG IIc is a rare autosomal recessive disease characterized by a decreased expression of fucosylated antigens on cell surfaces that results in leukocyte adhesion deficiency and severe neurological and developmental abnormalities. Its molecular basis has been identified as a defect in the transporter of GDP-l-fucose into the Golgi lumen, which reduces the availability of the substrate for fucosyltransferases. During metabolic radiolabeling experiments using [3H]fucose, LAD II fibroblasts incorporated significantly less radiolabel compared with control cells. However, fractionation and analysis of the different classes of glycans indicated that the decrease in [3H]fucose incorporation is not generalized and is mainly confined to terminal fucosylation of N-linked oligosaccharides. In contrast, the total levels of protein O-fucosylation, including that observed in Notch protein, were unaffected. This finding demonstrates that the decrease in GDP-l-fucose levels in the fibroblast Golgi caused by the LAD II defect does not impair bulk protein O-fucosylation, but severely affects the bulk addition of fucose as a terminal modification of N-linked glycans. These data suggest that the severe clinical abnormalities including neurological and developmental ones observed in at least some of the LAD II patients may be related to alteration in recognition systems involving terminal fucose modifications of N-glycans and not be due to a defective O-fucosylation of proteins such as Notch.

Fucose: biosynthesis and biological function in mammals

Fucose is a deoxyhexose that is present in a wide variety of organisms. In mammals, fucose-containing glycans have important roles in blood transfusion reactions, selectin-mediated leukocyte-endothelial adhesion, host-microbe interactions, and numerous ontogenic events, including signaling events by the Notch receptor family. Alterations in the expression of fucosylated oligosaccharides have also been observed in several pathological processes, including cancer and atherosclerosis. Fucose deficiency is accompanied by a complex set of phenotypes both in humans with leukocyte adhesion deficiency type II (LAD II; also known as congenital disorder of glycosylation type IIc) and in a recently generated strain of mice with a conditional defect in fucosylated glycan expression. Fucosylated glycans are constructed by fucosyltransferases, which require the substrate GDP-fucose. Two pathways for the synthesis of GDP-fucose operate in mammalian cells, the GDP-mannose-dependent de novo pathway and the free fucose-dependent salvage pathway. In this review, we focus on the biological functions of mammalian fucosylated glycans and the biosynthetic processes leading to formation of the fucosylated glycan precursor GDP-fucose.

TSRC1, a widely expressed gene containing seven thrombospondin type I repeats

The thrombospondin type 1 repeat domain is found in nearly 100 mammalian proteins with diverse biological functions that include cellular adhesion, angiogenesis, and patterning of the developing nervous system. We have characterized a novel thrombospondin type 1 repeat containing gene, TSRC1, encoding a predicted protein with seven thrombospondin repeats, six of which are clustered at the C-terminus. The 17 coding exons and two nontranslated exons of TSRC1 span 10 kb of genomic DNA. The human and mouse genes encode proteins of 1074 and 1036 amino acids, respectively, with 76% amino acid sequence identity. Thirty of the extra amino acids in the human protein are encoded by exon 6. Mouse Tsrc1 is expressed in all fetal and adult tissues tested. Three conserved noncoding sequence elements with potential regulatory function are located in intron 1. Mouse Tsrc1 was genetically mapped to chromosome 3 within the nonrecombinant region for the sodium channel modifier locus Scnm1. The sensitive and resistant alleles of Scnm1 did not differ in Tsrc1 protein sequence, transcript length, or transcript abundance. Human TSRC1 is located on chromosome 1q21 within an 11.7 Mb segment of conserved synteny. TSRC1 and the closely linked gene ADAM15 appear to be derived by a chromosomal inversion that interrupted an ancestral ADAMTS gene.

Thrombospondins 1 and 2 function as inhibitors of angiogenesis

Thrombospondins (TSPs) 1 and 2 are matricellular proteins with the well-characterized ability to inhibit angiogenesis in vivo, and the migration and proliferation of cultured microvascular endothelial cells (ECs). Angiogenesis in developing tumors and in various models of wound healing is diminished or delayed by the presence of TSP1 or 2. Sequences within the type I repeats of TSP1 and 2 have been demonstrated to mediate the anti-migratory effects of TSPs on microvascular EC, although, paradoxically, sequences in the N- and C-terminal domains have pro-angiogenic effects. A scavenger receptor, CD36, recognizes the active sequences in the type I repeats, and is required for the anti-angiogenic effects of TSP1 in the corneal neovascularization assay. However, interactions of TSPs with growth factors, proteases, histidine-rich glycoprotein, and other cell-surface receptors on EC have the potential to modulate CD36-mediated effects. Binding of TSP1 to CD36 has been shown to activate apoptosis by inducing p38 and Jun N-terminal kinase, members of the mitogen-activated protein kinase superfamily, and subsequently the cell-surface expression of FasL. Ligation of Fas by FasL then induces a caspase cascade and apoptotic cell death. However, we have recently shown that inhibition of proliferation of microvascular EC by TSPs can occur in the absence of cell death. This finding raises the possibility that TSPs can activate separate cell death and anti-proliferative pathways.

Biophysical characterization, including disulfide bond assignments, of the anti-angiogenic type 1 domains of human thrombospondin-1

Thrombospondin-1 (TSP1), a modular secreted glycoprotein, possesses anti-angiogenic activity both in vitro and in vivo. This activity has been localized to the thrombospondin type 1 repeats/domains (TSR). A TSP1 monomer contains three TSRs, each with a hydrophobic cluster with three conserved tryptophans (WxxWxxW), a basic cluster with two conserved arginines (RxR), and six conserved cysteines. Using the baculovirus system, we expressed TSRs of human TSP1 as either the three domains in tandem (P123) or the third domain alone (P3) and demonstrated that both P123 and P3 at nanomolar concentrations inhibit either basic fibroblast-growth-factor or sphingosine-1-phosphate induced endothelial cell migration. Far-UV circular dichroism (CD) indicated that P123 and P3 have a common global fold that is very similar to properdin, a protein with six TSRs. Near-UV CD and fluorescence quenching studies indicated the conserved tryptophans are in a structured, partially solvent-accessible, positively charged environment. N-terminal sequence and mass spectrometry analysis of trypsin-digested TSRs indicated that the RFK linker sequence between P1 and P2 is readily proteolyzed and the conserved arginines are solvent accessible. By a combination of proteolysis and mass spectrometry, the recombinant TSRs were determined to be fully disulfide bonded with a connectivity of 1-5, 2-6, and 3-4 (cysteines are numbered sequentially from N- to C-terminus). TSRs are found in numerous extracellular proteins. These TSRs share the hydrophobic and basic clusters of the TSP TSRs but some have quite different placement of cysteine residues. We propose a sorting of TSRs into six groups that reconciles our results with information about other TSRs.

Disseminated candidiasis and hepatic malarial infection in mannose-binding-lectin-A-deficient mice

To examine the physiological functions of mannose-binding lectin A (MBL-A), we generated mice that were deficient in MBL-A and examined their susceptibilities to the microbial pathogens Candida albicans and Plasmodium yoelii, an accepted experimental malaria model in mouse. We found no differences in the survival rates and fungal burdens of wild-type and MBL-A(-/-) mice with disseminated C. albicans infection. The two mouse strains were also similar in their abilities to resist hepatic accumulation of P. yoelii parasites. We conclude that MBL-A deficiency does not alter resistance to disseminated candidiasis or initial hepatic invasion by P. yoelii.

O-glycosylation of EGF repeats: identification and initial characterization of a UDP-glucose: protein O-glucosyltransferase

O-Glucose is an unusual form of posttranslational modification consisting of glucose directly attached to protein through O-linkage. Several serum proteins (factor VII, factor IX, protein Z, and thrombospondin) contain this unique modification on their epidermal growth factor (EGF)-like repeats. Comparison of the glycosylation sites on these proteins revealed a putative consensus sequence for O-glucose modification: C(1)XSXPC(2), where C(1) and C(2) are the first and second conserved cysteines of the EGF repeat. We identify and characterize an enzymatic activity capable of adding glucose to EGF repeats: UDP-glucose: protein O-glucosyltransferase. Using extracts of Chinese hamster ovary cells as the enzyme source, recombinant factor VII EGF repeat as the acceptor, and UDP-[(3)H]glucose as the donor, we show that the activity is linearly dependent on time, enzyme amount, and substrate concentration. As with most glycosyltransferases, metal ions (such as manganese) are required for activity. Analysis demonstrated that the glucose is added in O-linkage to the EGF repeat. Mutation of the serine to alanine in the predicted glycosylation site abrogates glycosylation, as does reduction and alkylation of the EGF repeat, suggesting that the enzyme recognizes not only the consensus sequence but also the 3D structure of the EGF repeat. Detection of O-glucosyltransferase activity in extracts of cell lines from insects to humans and a variety of rat tissues suggests that O-glucose modification is widespread in biology. These studies lay the foundation for future work on the biological role of the O-glucose modification.

Crystal structure of the TSP-1 type 1 repeats: a novel layered fold and its biological implication

Thrombospondin-1 (TSP-1) contains three type 1 repeats (TSRs), which mediate cell attachment, glycosaminoglycan binding, inhibition of angiogenesis, activation of TGFbeta, and inhibition of matrix metalloproteinases. The crystal structure of the TSRs reported in this article reveals a novel, antiparallel, three-stranded fold that consists of alternating stacked layers of tryptophan and arginine residues from respective strands, capped by disulfide bonds on each end. The front face of the TSR contains a right-handed spiral, positively charged groove that might be the "recognition" face, mediating interactions with various ligands. This is the first high-resolution crystal structure of a TSR domain that provides a prototypic architecture for structural and functional exploration of the diverse members of the TSR superfamily.

ADAMTS13 and TTP

Thrombotic thrombocytopenic purpura (TTP) has been a mysterious and deadly disease that often could be treated effectively by plasma exchange, but without real understanding of the underlying pathophysiology. Recent advances now suggest that deficiency of a specific von Willebrand factor (VWF) cleaving protease promotes tissue injury in TTP. VWF multimers participate in the formation of platelet thrombi. Proteolytic cleavage of VWF multimers normally limits platelet thrombus growth, and failure to cleave VWF appears to encourage microvascular thrombosis. The VWF cleaving protease proves to be a new member of the ADAMTS family of metalloproteases, designated ADAMTS13. Autoantibodies that inhibit ADAMTS13 cause sporadic TTP, and mutations in the ADAMTS13 gene cause an autosomal recessive form of chronic relapsing TTP. Further studies of ADAMTS13 seem likely to change our approach to the diagnosis and treatment of TTP and other thrombotic microangiopathies.

Notch ligands are substrates for protein O-fucosyltransferase-1 and Fringe

O-Fucose has been identified on epidermal growth factor-like (EGF) repeats of Notch, and elongation of O-fucose has been implicated in the modulation of Notch signaling by Fringe. O-Fucose modifications are also predicted to occur on Notch ligands based on the presence of the C(2)XXGG(S/T)C(3) consensus site (where S/T is the modified amino acid) in a number of the EGF repeats of these proteins. Here we establish that both mammalian and Drosophila Notch ligands are modified with O-fucose glycans, demonstrating that the consensus site was useful for making predictions. The presence of O-fucose on Notch ligands raised the question of whether Fringe, an O-fucose specific beta 1,3-N-acetylglucosaminyltransferase, was capable of modifying O-fucose on the ligands. Indeed, O-fucose on mammalian Delta 1 and Jagged1 can be elongated with Manic Fringe in vivo, and Drosophila Delta and Serrate are substrates for Drosophila Fringe in vitro. These results raise the interesting possibility that alteration of O-fucose glycans on Notch ligands could play a role in the mechanism of Fringe action on Notch signaling. As an initial step to begin addressing the role of the O-fucose glycans on Notch ligands in Notch signaling, a number of mutations in predicted O-fucose glycosylation sites on Drosophila Serrate have been generated. Interestingly, analysis of these mutants has revealed that O-fucose modifications occur on some EGF repeats not predicted by the C(2)XXGGS/TC(3) consensus site. A revised, broad consensus site, C(2)X(3-5)S/TC(3) (where X(3-5) are any 3-5 amino acid residues), is proposed.

Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds

Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.

Preparation of glycosylated amino acid derivatives for glycoprotein synthesis by in vitro translation system

General preparation of glycosylated amino acylated nucleotide for in vitro peptide synthesis was described. Both O-glycosylated amino acyl nucleotides and C-glycosylated amino acyl nucleotide were synthesized by choosing the appropriate protecting group.

Thrombospondins: multifunctional regulators of cell interactions

Thrombospondins are secreted, multidomain macromolecules that act as regulators of cell interactions in vertebrates. Gene knockout mice constructed for two members of this family demonstrate roles in the organization and homeostasis of multiple tissues, with particularly significant activities in the regulation of angiogenesis. This review discusses the functions of thrombospondins with regard to their cellular mechanisms of action and highlights recent advances in understanding how multifactorial molecular interactions, at the cell surface and within extracellular matrix, produce cell-type-specific effects on cell behavior and the organization of matrix and tissues.

C-mannosylation and o-fucosylation of thrombospondin type 1 repeats

The final chemical structure of a newly synthesized protein is often only attained after further covalent modification. Ideally, a comprehensive proteome analysis includes this aspect, a task that is complicated by our incomplete knowledge of the range of possible modifications and often by the lack of suitable analysis methods. Here we present two recently discovered, unusual forms of protein glycosylation, i.e. C-mannosylation and O-fucosylation. Their analysis by a combined mass spectrometric approach is illustrated with peptides from the thrombospondin type 1 repeats (TSRs) of the recombinant axonal guidance protein F-spondin. Nano-electrospray ionization tandem-mass spectrometry of isolated peptides showed that eight of ten Trp residues in the TSRs of F-spondin are C-mannosylated. O-Fucosylation sites were determined by a recently established nano-electrospray ionization quadrupole time-of-flight tandem-mass spectrometry approach. Four of five TSRs carry the disaccharide Hex-dHex-O-Ser/Thr in close proximity to the C-mannosylation sites. In analogy to thrombospondin-1, we assume this to be Glc-Fuc-O-Ser/Thr. Our current knowledge of these glycosylations will be discussed.

Thrombospondin-1 as an endogenous inhibitor of angiogenesis and tumor growth

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein that influences cellular phenotype and the structure of the extracellular matrix. These effects are important components of the tissue remodeling that is associated with angiogenesis and neoplasia. The genetic mutations in oncogenes and tumor suppressor genes that occur within tumor cells are frequently associated with decreased expression of TSP-1. However, the TSP-1 that is produced by stromal fibroblasts, endothelial cells and immune cells suppresses tumor progression. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival and through indirect effects on growth factor mobilization. TSP-1 that is present in the tumor microenvironment also acts to suppress tumor cell growth through activation of transforming growth factor beta in those tumor cells that are responsive to TGF beta. In this review, the molecular basis for the role of TSP-1 in the inhibition of tumor growth and angiogenesis is summarized.

Structure of von Willebrand factor-cleaving protease (ADAMTS13), a metalloprotease involved in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura is associated with acquired or congenital deficiency of a plasma von Willebrand factor-cleaving protease (VWFCP). Based on partial amino acid sequence, VWFCP was identified recently as a new member of the ADAMTS family of metalloproteases and designated ADAMTS13. The 4.6-kilobase pair cDNA sequence for VWFCP has now been determined. By Northern blotting, full-length VWFCP mRNA was detected only in liver. VWFCP consists of 1427 amino acid residues and has a signal peptide, a short propeptide terminating in the sequence RQRR, a reprolysin-like metalloprotease domain, a disintegrin-like domain, a thrombospondin-1 repeat, a Cys-rich domain, an ADAMTS spacer, seven additional thrombospondin-1 repeats, and two CUB domains. VWFCP apparently is made as a zymogen that requires proteolytic activation, possibly by furin intracellularly. Sites for Zn(2+) and Ca(2+) ions are conserved in the protease domain. The Cys-rich domain contains an RGDS sequence that could mediate integrin-dependent binding to platelets or other cells. Alternative splicing gives rise to at least seven potential variants that truncate the protein at different positions after the protease domain. Alternative splicing may have functional significance, producing proteins with distinct abilities to interact with cofactors, connective tissue, platelets, and von Willebrand factor.

Modification of epidermal growth factor-like repeats with O-fucose. Molecular cloning and expression of a novel GDP-fucose protein O-fucosyltransferase

The O-fucose modification is found on epidermal growth factor-like repeats of a number of cell surface and secreted proteins. O-Fucose glycans play important roles in ligand-induced receptor signaling. For example, elongation of O-fucose on Notch by the beta1,3-N-acetylglucosaminyltransferase Fringe modulates the ability of Notch to respond to its ligands. The enzyme that adds O-fucose to epidermal growth factor-like repeats, GDP-fucose protein O-fucosyltransferase (O-FucT-1), was purified previously from Chinese hamster ovary (CHO) cells. Here we report the isolation of a cDNA that encodes human O-FucT-1. A probe deduced from N-terminal sequence analysis of purified CHO O-FucT-1 was used to screen a human heart cDNA library and expressed sequence tag and genomic data bases. The cDNA contains an open reading frame encoding a protein of 388 amino acids with a predicted N-terminal transmembrane sequence typical of a type II membrane orientation. Likewise, the mouse homolog obtained from an expressed sequence tag and 5'-rapid amplification of cDNA ends of a mouse liver cDNA library encodes a type II transmembrane protein of 393 amino acids with 90.4% identity to human O-FucT-1. Homologs were also found in Drosophila and Caenorhabditis elegans with 41.2 and 29.4% identity to human O-FucT-1, respectively. The human gene (POFUT1) is on chromosome 20 between PLAGL2 and KIF3B, near the centromere at 20p11. The mouse gene (Pofut1) maps near Plagl2 on a homologous region of mouse chromosome 2. POFUT1 gene transcripts were expressed in all tissues examined, consistent with the widespread localization of the modification. Expression of a soluble form of human O-FucT-1 in insect cells yielded a protein of the predicted molecular weight with O-FucT-1 kinetic and enzymatic properties similar to those of O-FucT-1 purified from CHO cells. The identification of the gene encoding protein O-fucosyltransferase I now makes possible mutational strategies to examine the functions of the unusual O-fucose post-translational modification.

Fucosylation of Cripto is required for its ability to facilitate nodal signaling

O-linked fucose modification is rare and has been shown to occur almost exclusively within epidermal growth factor (EGF)-like modules. We have found that the EGF-CFC family member human Cripto-1 (CR) is modified with fucose and through a combination of peptide mapping, mass spectrometry, and sequence analysis localized the site of attachment to Thr-88. The identification of a fucose modification on human CR within its EGF-like domain and the presence of a consensus fucosylation site within all EGF-CFC family members suggest that this is a biologically important modification in CR, which functionally distinguishes it from the EGF ligands that bind the type 1 erbB growth factor receptors. A single CR point mutation, Thr-88 --> Ala, results in a form of the protein that is not fucosylated and has substantially weaker activity in cell-based CR/Nodal signaling assays, indicating that fucosylation is functionally important for CR to facilitate Nodal signaling.

Identification of GlcNAcylation sites of peptides and alpha-crystallin using Q-TOF mass spectrometry

The addition of a single N-acetylglucosamine residue O-linked to serine and threonine residues of nuclear and cytoplasmic proteins is a widespread modification throughout all eukaryotes. The conventional method for detecting and locating sites of modification is a multi-step radioactivity-based protocol. In this paper we show that using quadrupole time-of-flight (Q-TOF) mass spectrometry, modification sites can be identified at a significantly higher sensitivity than previous approaches. This is the first demonstration that sites of O-GlcNAcylation can be identified directly using mass spectrometry.

Direct determination of glycosylation sites in O-fucosylated glycopeptides using nano-electrospray quadrupole time-of-flight mass spectrometry

O-Fucosylation is an unusual posttranslational modification present in several proteins that play important roles in physiological processes such as coagulation, cell signaling and metastasis. Although the exact function of the modification is still unclear, the number of proteins found to be modified is increasing, and there is a need for further structural and functional analyses. Here we report on a rapid and straightforward approach in the analysis of glycosylation status and determination of glycosylation sites in O-fucosylated glycopeptides using nano-electrospray quadrupole time-of-flight (nano-ESI Q-TOF) mass spectrometry. In a single measurement of previously chemically untreated O-fucosylated peptides originating from the thrombospondin-1 repeats, we were able to determine the glycosylation status of the analyzed peptide, the glycosylation site, and the glycan structure. The abundance of glycosylated peptide fragment ions in MS(2) spectra suggests that nano-ESI Q-TOF mass spectrometry can be used as a general approach in structural studies of O-fucosylation in proteins.