ADAMTS13 is a plasma metalloproteinase that cleaves von Willebrand factor to smaller, less thrombogenic forms. Deficiency of ADAMTS13 activity in plasma leads to thrombotic thrombocytopenic purpura. ADAMTS13 contains eight thrombospondin type 1 repeats (TSR), seven of which contain a consensus sequence for the direct addition of fucose to the hydroxyl group of serine or threonine. Mass spectral analysis of tryptic peptides derived from human ADAMTS13 indicate that at least six of the TSRs are modified with an O-fucose disaccharide. Analysis of [3 H]fucose metabolically incorporated into ADAMTS13 demonstrated that the disaccharide has the structure glucose-␤1,3-fucose. Mutation of the modified serine to alanine in TSR2, TSR5, TSR7, and TSR8 reduced the secretion of ADAMTS13. Mutation of more than one site dramatically reduced secretion regardless of the sites mutated. When the expression of protein O-fucosyltransferase 2 (POFUT2), the enzyme that transfers fucose to serines in TSRs, was reduced using siRNA, the secretion of ADAMTS13 decreased. A similar outcome was observed when ADAMTS13 was expressed in a cell line unable to synthesize the donor for fucose addition, GDPfucose. Although overexpression of POFUT2 did not affect the secretion of wild-type ADAMTS13, it did increase the secretion of the ADAMTS13 TSR1,2 double mutant but not that of ADAMTS13 TSR1-8 mutant. Together these findings indicate that O-fucosylation is functionally significant for secretion of ADAMTS13.ADAMTS13 is a plasma metalloprotease that cleaves von Willebrand factor to smaller, less thrombogenic fragments. ADAMTS13 is a member of the ADAMTS family of metalloproteases that are characterized by a conserved domain structure. These include a metalloprotease domain, a disintegrin domain, a thrombospondin type 1 repeat (TSR), 2 a cysteinerich domain, and a spacer domain and conclude with a variable number of additional TSRs (1). ADAMTS13 uniquely contains seven additional TSRs and two CUB1 domains at its carboxyl end (2-4) (Fig. 1A). TSRs contain ϳ60 amino acids with conserved tryptophans and cysteines. They were first described in thrombospondin type 1 and are homologous to the properdin repeat found in many components of the complement system. Thrombospondin type 1 is a protein that is thought to play a role in angiogenesis, cell adhesion, and motility (5, 6). Many interactions of thrombospondin-1 are thought to be mediated through amino acids within the TSRs (7). For example, binding of thrombospondin-1 to the endothelial cell protein, CD36, can be inhibited by the peptide CSVTCG, which is found in the TSRs of thrombospondin-1 (8). The three TSRs of thrombospondin-1 have been shown to contain a fucose directly linked to a serine or threonine within a putative consensus sequence of C 1 XX(S/T)C 2 G (where C 1 and C 2 are the 1st and 2nd conserved cysteines in the TSR (9)), which is the putative CD36 binding region. The fucose on the TSRs was further modified with a glucose in ␤1-3 linkage to form a disaccharide (9, 10). Subsequent analysis ...

Notch activity is regulated by both O-fucosylation and O-glucosylation, and Notch receptors contain multiple predicted sites for both. Here we examine the occupancy of the predicted O-glucose sites on mouse Notch1 (mN1) using the consensus sequence C 1 XSXPC 2 . We show that all of the predicted sites are modified, although the efficiency of modifying O-glucose sites is site-and cell type-dependent. For instance, although most sites are modified at high stoichiometries, the site at EGF 27 is only partially glucosylated, and the occupancy of the site at EGF 4 varies with cell type. O-Glucose is also found at a novel, nontraditional consensus site at EGF 9. Based on this finding, we propose a revision of the consensus sequence for O-glucosylation to allow alanine N-terminal to cysteine 2: C 1 XSX(A/P)C 2 . We also show through biochemical and mass spectral analyses that serine is the only hydroxyamino acid that is modified with O-glucose on EGF repeats. The O-glucose at all sites is efficiently elongated to the trisaccharide Xyl-Xyl-Glc. To establish the functional importance of individual O-glucose sites in mN1, we used a cell-based signaling assay. Elimination of most individual sites shows little or no effect on mN1 activation, suggesting that the major effects of O-glucose are mediated by modification of multiple sites. Interestingly, elimination of the site in EGF 28, found in the Abruptex region of Notch, does significantly reduce activity. These results demonstrate that, like O-fucose, the O-glucose modifications of EGF repeats occur extensively on mN1, and they play important roles in Notch function.The Notch family of single-pass transmembrane receptors is essential for early metazoan development, activating the expression of many genes involved in cell differentiation and tissue morphogenesis (1-3). Defects in Notch signaling have been implicated in a number of human diseases, including several forms of cancer, vascular defects such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (4 -6), multiple sclerosis (6), and a number of developmental syndromes (4, 7-10). The canonical Notch signaling pathway is initiated by the interaction of Notch with its ligand on an apposed cell. Upon ligand binding, Notch undergoes presenilin-1-dependent proteolysis, releasing a soluble Notch intracellular domain, which enters the cell nucleus to interact directly with the transcription factors from the CSL family and regulate Notch target genes (2). There are four members of the Notch family in vertebrates (Notch1 to Notch4) interacting with several classes of Notch ligands: ligands of the DSL family (Delta-like 1, 3, and 4 and Jagged 1 and 2) (1) and newly characterized ligands without the DSL domain, such as DNER and MAGP-1 and -2 (11-13).The Notch proteins consist of a large extracellular domain (ECD), 5 a transmembrane region, and a large intracellular domain (1, 2, 14). The majority of the ECD consists of tandem epidermal growth factor-like (EGF) repeats (36 EGF repeats are ...

Notch signaling is a component of a wide variety of developmental processes in many organisms. Notch activity can be modulated by O-fucosylation (mediated by protein O-fucosyltransferase-1) and Fringe, a ␤1,3-N-acetylglucosaminyltransferase that modifies O-fucose in the context of epidermal growth factor-like (EGF) repeats. Fringe was initially described in Drosophila, and three mammalian homologues have been identified, Manic fringe, Lunatic fringe, and Radical fringe. Here for the first time we have demonstrated that, similar to Manic and Lunatic, Radical fringe is also a fucose-specific ␤1,3-N-acetylglucosaminyltransferase. The fact that three Fringe homologues exist in mammals raises the question of whether and how these enzymes differ. Although Notch contains numerous EGF repeats that are predicted to be modified by O-fucose, previous studies in our laboratory have demonstrated that not all O-fucosylated EGF repeats of Notch are further modified by Fringe, suggesting that the Fringe enzymes can differentiate between them. In this work, we have sought to identify specificity determinants for the recognition of an individual O-fucosylated EGF repeat by the Fringe enzymes. We have also sought to determine differences in the biochemical behavior of the Fringes with regard to their in vitro enzymatic activities. Using both in vivo and in vitro experiments, we have found two amino acids that appear to be important for the recognition of an O-fucosylated EGF repeat by all three mammalian Fringes. These amino acids provide an initial step toward defining sequences that will allow us to predict which O-fucosylated EGF repeats are modified by the Fringes.The Notch protein is a transmembrane receptor involved in various cell fate decisions. It was initially characterized in Drosophila and has subsequently been found in all known metazoans (1). Notch becomes activated upon binding to its ligands located on apposing cells. These ligands, known collectively as the DSL (for Delta-Serrate-Lag2) family, fall into two classes, Serrate/Jagged and Delta. A large portion of the extracellular domain of Notch is composed of epidermal growth factor (EGF) 3 -like repeats, and many of these EGF repeats contain consensus sites for modification by O-fucose (a process mediated by the protein O-fucosyltransferase-1 (O-FucT-1) (2, 3). Some of these O-fucosylated EGF repeats can be further modified by the actions of Fringe, a ␤1,3-GlcNAc transferase (4 -6). The modification of Notch by Fringe plays an important role in the modulation of Notch signaling in various contexts (7,8).Fringe was initially described in Drosophila as a gene that modulates dorsal-ventral cell interactions in the developing wing (9). Further studies reveal that Fringe functions specifically by modulating the response of Notch to its ligands, potentiating Delta signaling while inhibiting Serrate signaling (10 -12). Three mammalian homologues to Drosophila Fringe (Dfng) have been identified, Manic fringe (Mfng), Lunatic fringe (Lfng), and Radical fringe (Rfng). Lfng a...

The Notch signaling pathway is involved in a wide variety of highly conserved developmental processes in mammals. Importantly, mutations of the Notch protein and components of its signaling pathway have been implicated in an array of human diseases (T-cell leukemia and other cancers, Multiple Sclerosis, CADASIL, Alagille Syndrome, Spondylocostal Dysostosis). In mammals, Notch becomes activated upon binding of its extracellular domain to ligands (Delta and Jagged/Serrate) that are present on the surface of apposed cells. The extracellular domain of Notch contains up to 36 tandem Epidermal Growth Factor-like (EGF) repeats. Many of these EGF repeats are modified at evolutionarily-conserved consensus sites by an unusual form of O-glycosylation called O-fucose. Work from several groups indicates that O-fucosylation plays an important role in ligand mediated Notch signaling. Recent evidence also suggests that the enzyme responsible for addition of O-fucose to Notch, protein O-fucosyltransferase-1 (POFUT1), may serve a quality control function in the endoplasmic reticulum. Additionally, some of the O-fucose moieties are further elongated by the action of members of the Fringe family of beta-1,3-N-acetylglucosaminyltransferases. The alteration in O-fucose saccharide structure caused by Fringe modulates the response of Notch to its ligands. Thus, glycosylation serves an important role in regulating Notch activity. This review focuses on the role of glycosylation in the normal functioning of the Notch pathway. As well, potential roles for glycosylation in Notch-related human diseases, and possible roles for therapeutic targeting of POFUT1 and Fringe in Notch-related human diseases, are discussed.

Collagen is the most abundant protein in the human body and thereby a structural protein of considerable biotechnological interest. The complex maturation process of collagen, including essential post-translational modifications such as prolyl and lysyl hydroxylation, has precluded large-scale production of recombinant collagen featuring the biophysical properties of endogenous collagen. The characterization of new prolyl and lysyl hydroxylase genes encoded by the giant virus mimivirus reveals a method for production of hydroxylated collagen. The coexpression of a human collagen type III construct together with mimivirus prolyl and lysyl hydroxylases in Escherichia coli yielded up to 90 mg of hydroxylated collagen per liter culture. The respective levels of prolyl and lysyl hydroxylation reaching 25 % and 26 % were similar to the hydroxylation levels of native human collagen type III. The distribution of hydroxyproline and hydroxylysine along recombinant collagen was also similar to that of native collagen as determined by mass spectrometric analysis of tryptic peptides. The triple helix signature of recombinant hydroxylated collagen was confirmed by circular dichroism, which also showed that hydroxylation increased the thermal stability of the recombinant collagen construct. Recombinant hydroxylated collagen produced in E. coli supported the growth of human umbilical endothelial cells, underlining the biocompatibility of the recombinant protein as extracellular matrix. The high yield of recombinant protein expression and the extensive level of prolyl and lysyl hydroxylation achieved indicate that recombinant hydroxylated collagen can be produced at large scale for biomaterials engineering in the context of biomedical applications.

Thrombospondin type 1 repeats (TSRs) are biologically important domains of extracellular proteins. They are modified with a unique Glc␤1,3Fuc␣1-O-linked disaccharide on either serine or threonine residues. Here we identify the putative glycosyltransferase, B3GTL, as the ␤1,3-glucosyltransferase involved in the biosynthesis of this disaccharide. This enzyme is conserved from Caenorhabditis elegans to man and shares 28% sequence identity with Fringe, the ␤1,3-N-acetylglucosaminyltransferase that modifies O-linked fucosyl residues in proteins containing epidermal growth factor-like domains, such as Notch. ␤1,3-Glucosyltransferase glucosylates properly folded TSR-fucose but not fucosylated epidermal growth factor-like domain or the non-fucosylated modules. Specifically, the glucose is added in a ␤1,3-linkage to the fucose in TSR. The activity profiles of ␤1,3-glucosyltransferase and protein O-fucosyltransferase 2, the enzyme that carries out the first step in TSR O-fucosylation, superimpose in endoplasmic reticulum subfractions obtained by density gradient centrifugation. Both enzymes are soluble proteins that efficiently modify properly folded TSR modules. The identification of the ␤1,3-glucosyltransferase gene allows us to manipulate the formation of the rare Glc␤1,3Fuc␣1 structure to investigate its biological function.

Phosphotransacetylase (Pta) [EC 2.3.1.8] is ubiquitous in the carbon assimilation and energy-yielding pathways in anaerobic prokaryotes where it catalyzes the reversible transfer of the acetyl group from acetyl phosphate to CoA forming acetyl CoA and inorganic phosphate. The crystal structure of Pta from the methane-producing archaeon Methanosarcina thermophila, representing the first crystal structure of any Pta, was determined by multiwavelength anomalous diffraction at 2.7 A resolution. In solution and in the crystal, the enzyme forms a homodimer. Each monomer consists of two alpha/beta domains with a cleft along the domain boundary, which presumably contains the substrate binding sites. Comparison of the four monomers present in the asymmetric unit indicates substantial variations in the relative orientation of the two domains and the structure of the putative active site cleft. A search for structural homologs revealed the NADP(+)-dependent isocitrate and isopropylmalate dehydrogenases as the only homologs with a similar two-domain architecture.

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucintype O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the Epidermal Growth Factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that Thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.