The functional glycosyltransferase signature sequence of the human beta 1,3-glucuronosyltransferase is a XDD motif

The biological role of core 1β1-3galactosyltransferase (T-synthase) in mucin-type O-glycosylation in Silkworm, Bombyx mori

O-glycosylation of secreted and membrane-bound proteins is an important post-translational modification that affects recognition of cell surface receptors, protein folding, and stability. However, despite the importance of O-linked glycans, their biological functions have not yet been fully elucidated and the synthetic pathway of O-glycosylation has not been investigated in detail, especially in the silkworm. In this study, we aimed to investigate O-glycosylation in silkworms by analyzing the overall structural profiles of mucin-type O-glycans using LC-MS. We found GalNAc or GlcNAc monosaccharide and core 1 disaccharide (Galβ1-3-GalNAcα1-Ser/Thr) were major components of the O-glycan attached to secreted proteins produced in silkworms. Furthermore, we characterized the 1 b1,3-galactosyltransferase (T-synthase) required for synthesis of the core 1 structure, common to many animals. Five transcriptional variants and four protein isoforms were identified in silkworms, and the biological functions of these isoforms were investigated. We found that BmT-synthase isoforms 1 and 2 were localized in the Golgi apparatus in cultured BmN4 cells and functioned both in cultured cells and silkworms. Additionally, a specific functional domain of T-synthase, called the stem domain, was found to be essential for activity and is presumed to be needed for dimer formation and galactosyltransferase activity. Altogether, our results elucidated the O-glycan profile and function of T-synthase in the silkworm. Our findings allow the practical comprehension of O-glycosylation required for employing silkworms as a productive expression system.

How inverting β-1,4-galactosyltransferase-1 can quench a high charge of the by-product UDP3- in catalysis: a QM/MM study of enzymatic reaction with native and UDP-5' thio galactose substrates

The catalysis of inverting glycosyltransferases consists of several biophysical and biochemical processes during which the transfer of a sugar residue from the purine phosphate donor substrate to an acceptor substrate occurs with stereo-inversion of the anomeric C1 center at a product. During catalysis a highly charged phosphate by-product (UDP3-) is formed and a mechanism of how the enzyme stabilizes it back to the UDP2- form is not known. Using methods of molecular modeling (hybrid DFT-QM/MM calculations) we proposed and validated a catalytic mechanism of bovine inverting β-1,4-galactosyltransferase-1 (β4Gal-T1) with native (UDP-galactose) and thio donor substrates (UDP-5' thio galactose). We focused on three aspects of the mechanism not yet investigated: (i) the formation of an oxocarbenium ion intermediate, which was only found for the retaining glycosyltransferases for the time being; (ii) the mechanism of stabilization of a highly charged phosphate by-product (UDP3-) back to its standard in vivo form (UDP2-); (iii) explanation for why in experimental measurements the rate of catalysis with the thio donor substrate is only 8% of the rate of that with the natural substrate. To understand the differences in the interaction patterns between the complexes enzyme : UDP-Gal and enzyme : UDP-5S-Gal, fragmented molecular orbital (FMO) decomposition energy analysis was carried out at the DFT level.

From the Uncharacterized Protein Family 0016 to the GDT1 family: Molecular insights into a newly-characterized family of cation secondary transporters

The Uncharacterized Protein Family 0016 (UPF0016) gathers poorly studied membrane proteins well conserved through evolution that possess one or two copies of the consensus motif Glu-x-Gly-Asp-(Arg/Lys)-(Ser/Thr). Members are found in many eukaryotes, bacteria and archaea. The interest for this protein family arose in 2012 when its human member TMEM165 was linked to the occurrence of Congenital Disorders of Glycosylation (CDGs) when harbouring specific mutations. Study of the UPF0016 family is undergone through the characterization of the bacterium Vibrio cholerae (MneA), cyanobacterium Synechocystis (SynPAM71), yeast Saccharomyces cerevisiae (Gdt1p), plant Arabidopsis thaliana (PAM71 and CMT1), and human (TMEM165) members. These proteins have all been identified as transporters of cations, more precisely of Mn²⁺, with an extra reported function in Ca²⁺ and/or H⁺ transport for some of them. Apart from glycosylation in humans, the UPF0016 members are required for lactation in humans, photosynthesis in plants and cyanobacteria, Ca²⁺ signaling in yeast, and Mn²⁺ homeostasis in the five aforementioned species. The requirement of the UPF0016 members for key physiological processes most likely derives from their transport activity at the Golgi membrane in human and yeast, the chloroplasts membranes in plants, the thylakoid and plasma membranes in cyanobacteria, and the cell membrane in bacteria. In the light of these studies on various UPF0016 members, this family is not considered as uncharacterized anymore and has been renamed the Gdt1 family according to the name of its S. cerevisiae member. This review aims at assembling and confronting the current knowledge in order to identify shared and distinct features in terms of transported molecules, mode of action, structure, etc., as well as to better understand their corresponding physiological roles.

cpsA regulates mycotoxin production, morphogenesis and cell wall biosynthesis in the fungus Aspergillus nidulans

The model fungus Aspergillus nidulans synthesizes numerous secondary metabolites, including sterigmatocystin (ST). The production of this toxin is positively controlled by the global regulator veA. In the absence of veA (ΔveA), ST biosynthesis is blocked. Previously, we performed random mutagenesis in a ΔveA strain and identified revertant mutants able to synthesize ST, among them RM1. Complementation of RM1 with a genomic library revealed that the mutation occurred in a gene designated as cpsA. While in the ΔveA genetic background cpsA deletion restores ST production, in a veA wild-type background absence of cpsA reduces and delays ST biosynthesis decreasing the expression of ST genes. Furthermore, cpsA is also necessary for the production of other secondary metabolites, including penicillin, affecting the expression of PN genes. In addition, cpsA is necessary for normal asexual and sexual development. Chemical and microscopy analyses revealed that CpsA is found in cytoplasmic vesicles and it is required for normal cell wall composition and integrity, affecting adhesion capacity and oxidative stress sensitivity. The conservation of cpsA in Ascomycetes suggests that cpsA homologs might have similar roles in other fungal species.

A Haploid Genetic Screen Identifies Heparan Sulfate Proteoglycans Supporting Rift Valley Fever Virus Infection

Rift Valley fever virus (RVFV) causes recurrent insect-borne epizootics throughout the African continent, and infection of humans can lead to a lethal hemorrhagic fever syndrome. Deep mutagenesis of haploid human cells was used to identify host factors required for RVFV infection. This screen identified a suite of enzymes involved in glycosaminoglycan (GAG) biogenesis and transport, including several components of the cis-oligomeric Golgi (COG) complex, one of the central components of Golgi complex trafficking. In addition, disruption of PTAR1 led to RVFV resistance as well as reduced heparan sulfate surface levels, consistent with recent observations that PTAR1-deficient cells exhibit altered Golgi complex morphology and glycosylation defects. A variety of biochemical and genetic approaches were utilized to show that both pathogenic and attenuated RVFV strains require GAGs for efficient infection on some, but not all, cell types, with the block to infection being at the level of virion attachment. Examination of other members of the Bunyaviridae family for GAG-dependent infection suggested that the interaction with GAGs is not universal among bunyaviruses, indicating that these viruses, as well as RVFV on certain cell types, employ additional unidentified virion attachment factors and/or receptors.

IMPORTANCE

Rift Valley fever virus (RVFV) is an emerging pathogen that can cause severe disease in humans and animals. Epizootics among livestock populations lead to high mortality rates and can be economically devastating. Human epidemics of Rift Valley fever, often initiated by contact with infected animals, are characterized by a febrile disease that sometimes leads to encephalitis or hemorrhagic fever. The global burden of the pathogen is increasing because it has recently disseminated beyond Africa, which is of particular concern because the virus can be transmitted by widely distributed mosquito species. There are no FDA-licensed vaccines or antiviral agents with activity against RVFV, and details of its life cycle and interaction with host cells are not well characterized. We used the power of genetic screening in human cells and found that RVFV utilizes glycosaminoglycans to attach to host cells. This furthers our understanding of the virus and informs the development of antiviral therapeutics.

Hyaluronan Synthase: The Mechanism of Initiation at the Reducing End and a Pendulum Model for Polysaccharide Translocation to the Cell Exterior

Hyaluronan (HA) biosynthesis has been studied for over six decades, but our understanding of the biochemical details of how HA synthase (HAS) assembles HA is still incomplete. Class I family members include mammalian and streptococcal HASs, the focus of this review, which add new intracellular sugar-UDPs at the reducing end of growing hyaluronyl-UDP chains. HA-producing cells typically create extracellular HA coats (capsules) and also secrete HA into the surrounding space. Since HAS contains multiple transmembrane domains and is lipid-dependent, we proposed in 1999 that it creates an intraprotein HAS-lipid pore through which a growing HA-UDP chain is translocated continuously across the cell membrane to the exterior. We review here the evidence for a synthase pore-mediated polysaccharide translocation process and describe a possible mechanism (the Pendulum Model) and potential energy sources to drive this ATP-independent process. HA synthases also synthesize chitin oligosaccharides, which are created by cleavage of novel oligo-chitosyl-UDP products. The synthesis of chitin-UDP oligomers by HAS confirms the reducing end mechanism for sugar addition during HA assembly by streptococcal and mammalian Class I enzymes. These new findings indicate the possibility that HA biosynthesis is initiated by the ability of HAS to use chitin-UDP oligomers as self-primers.

Sperm mRNAs and microRNAs as candidate markers for the impact of toxicants on human spermatogenesis: an application to tobacco smoking

Spermatozoa contain a complex population of RNAs including messenger RNAs (mRNAs) and small RNAs such as microRNAs (miRNA). It has been reported that these RNAs can be used to understand the mechanisms by which toxicological exposure affects spermatogenesis. The aim of our study was to compare mRNA and miRNA profiles in spermatozoa from eight smokers and eight non-smokers, and search for potential relationships between mRNA and miRNA variation. All men were selected based on their answers to a standard toxic exposure questionnaire, and sperm parameters. Using mRNA and miRNA microarrays, we showed that mRNAs from 15 genes were differentially represented between smokers and non-smokers (p<0.01): five had higher levels and 10 lower levels in the smokers. For the microRNAs, 23 were differentially represented: 16 were higher and seven lower in the smokers (0.004≤p<0.01). Quantitative RT-PCR confirmed the lower levels in smokers compared to non-smokers for hsa-miR-296-5p, hsa-miR-3940, and hsa-miR-520d-3p. Moreover, we observed an inverse relationship between the levels of microRNAs and six potential target mRNAs (B3GAT3, HNRNPL, OASL, ODZ3, CNGB1, and PKD2). Our results indicate that alterations in the level of a small number of microRNAs in response to smoking may contribute to changes in mRNA expression in smokers. We conclude that large-scale analysis of spermatozoa RNAs can be used to help understand the mechanisms by which human spermatogenesis responds to toxic substances including those in tobacco smoke.

Crystal structure of the Golgi casein kinase

The family with sequence similarity 20 (Fam20) kinases phosphorylate extracellular substrates and play important roles in biomineralization. Fam20C is the Golgi casein kinase that phosphorylates secretory pathway proteins within Ser-x-Glu/pSer motifs. Mutations in Fam20C cause Raine syndrome, an osteosclerotic bone dysplasia. Here we report the crystal structure of the Fam20C ortholog from Caenorhabditis elegans. The nucleotide-free and Mn/ADP-bound structures unveil an atypical protein kinase-like fold and highlight residues critical for activity. The position of the regulatory αC helix and the lack of an activation loop indicate an architecture primed for efficient catalysis. Furthermore, several distinct elements, including the presence of disulfide bonds, suggest that the Fam20 family diverged early in the evolution of the protein kinase superfamily. Our results reinforce the structural diversity of protein kinases and have important implications for patients with disorders of biomineralization.

Epigenetics: methylation-associated repression of heparan sulfate 3-O-sulfotransferase gene expression contributes to the invasive phenotype of H-EMC-SS chondrosarcoma cells

Heparan sulfate proteoglycans (HSPGs), strategically located at the cell-tissue-organ interface, regulate major biological processes, including cell proliferation, migration, and adhesion. These vital functions are compromised in tumors, due, in part, to alterations in heparan sulfate (HS) expression and structure. How these modifications occur is largely unknown. Here, we investigated whether epigenetic abnormalities involving aberrant DNA methylation affect HS biosynthetic enzymes in cancer cells. Analysis of the methylation status of glycosyltransferase and sulfotransferase genes in H-HEMC-SS chondrosarcoma cells showed a typical hypermethylation profile of 3-OST sulfotransferase genes. Exposure of chondrosarcoma cells to 5-aza-2'-deoxycytidine (5-Aza-dc), a DNA-methyltransferase inhibitor, up-regulated expression of 3-OST1, 3-OST2, and 3-OST3A mRNAs, indicating that aberrant methylation affects transcription of these genes. Furthermore, HS expression was restored on 5-Aza-dc treatment or reintroduction of 3-OST expression, as shown by indirect immunofluorescence microscopy and/or analysis of HS chains by anion-exchange and gel-filtration chromatography. Notably, 5-Aza-dc treatment of HEMC cells or expression of 3-OST3A cDNA reduced their proliferative and invading properties and augmented adhesion of chondrosarcoma cells. These results provide the first evidence for specific epigenetic regulation of 3-OST genes resulting in altered HSPG sulfation and point to a defect of HS-3-O-sulfation as a factor in cancer progression.

Mutational and functional analysis of Large in a novel CHO glycosylation mutant

Inactivating mutations of Large reduce the functional glycosylation of alpha-dystroglycan (alpha-DG) and lead to muscular dystrophy in mouse and humans. The N-terminal domain of Large is most similar to UDP-glucose glucosyltransferases (UGGT), and the C-terminal domain is related to the human i blood group transferase beta1,3GlcNAcT-1. The amino acids at conserved motifs DQD+1 and DQD+3 in the UGGT domain are necessary for mammalian UGGT activity. When the corresponding residues were mutated to Ala in mouse Large, alpha-DG was not functionally glycosylated. A similar result was obtained when a DXD motif in the beta1,3GlcNAcT-1 domain was mutated to AIA. Therefore, the first putative glycosyltransferase domain of Large has properties of a UGGT and the second of a typical glycosyltransferase. Co-transfection of Large mutants affected in the different glycosyltransferase domains did not lead to complementation. While Large mutants were more localized to the endoplasmic reticulum than wild-type Large or revertants, all mutants were in the Golgi, and only very low levels of Golgi-localized Large were necessary to generate functional alpha-DG. When Large was overexpressed in ldlD.Lec1 mutant Chinese hamster ovary (CHO) cells which synthesize few, if any, mucin O-GalNAc glycans and no complex N-glycans, functional alpha-DG was produced, presumably by modifying O-mannose glycans. To investigate mucin O-GalNAc glycans as substrates of Large, a new CHO mutant Lec15.Lec1 that lacked O-mannose and complex N-glycans was isolated and characterized. Following transfection with Large, Lec15.Lec1 cells also generated functionally glycosylated alpha-DG. Thus, Large may act on the O-mannose, complex N-glycans and mucin O-GalNAc glycans of alpha-DG.

[Agrecan and articular cartilage: assessment of glycosyltransferases for the restoration of cartilage matrix in osteoarthritis]

Articular cartilage is a connective tissue containing a single type of cells, chondrocytes, which synthesise a dense extracellular matrix, mainly composed of collagens, hyaluronic acid and proteoglycans. These macromolecules play a major role in the resistance and elastic properties of the tissue. They also favour interactions with small active substances, such as growth factors and cytokines. Chondrocytes have a low metabolic capacity in relatively hypoxic conditions and absence of vascular supply. In physiopathological conditions, such as osteoarthritis (OA), progressive and irreversible degradation of matrix components is occurring. With the aim of developing new and efficient therapies against OA, we investigated the molecular mechanisms that initiate the disease, in order to identify key-proteins. These targets should hopefully lead to the design of new drugs able to stop degradation and restore cartilage. One of the earliest molecular events in OA is the degradation of aggrecan, the most abundant proteoglycan. The glycosaminoglycan (GAG) chains, chondroitin-sulfate, attached on the core protein, are subjected to hydrolysis into smaller fragments. We were interested in the glycosyltransferases that catalyse the formation of the polysaccharidic chains, namely those involved in the common tetrasaccharidic protein linkage region, GlcAbeta1,3Galbeta1,3Galbeta 1,4Xyl-O-Serine. The galactose beta1,3-glucuronosyltransférase-I (GlcAT-I) which catalyses the final step of this primer and which is markedly repressed during OA is an attractive target in that respect. Indeed, the human recombinant enzyme was found to play a pivotal role in GAG synthesis. Moreover, overexpression of GlcAT-I in cartilage explants treated with IL1beta was able to fully counteract proteoglycan depletion induced by the cytokine. These results prompted us to investigate the structure, function and regulation of this enzyme. This study provides the basis for several therapy approaches (gene delivery, design of glycomimetics able to initiate GAG synthesis) to promote cartilage repair.

Characterization of a novel alpha1,2-fucosyltransferase of Escherichia coli O128:b12 and functional investigation of its common motif

The wbsJ gene from Escherichia coli O128:B12 encodes an alpha1,2-fucosyltransferase responsible for adding a fucose onto the galactose residue of the O-antigen repeating unit via an alpha1,2 linkage. The wbsJ gene was overexpressed in E. coli BL21 (DE3) as a fusion protein with glutathione S-transferase (GST) at its N-terminus. GST-WbsJ fusion protein was purified to homogeneity via GST affinity chromatography followed by size exclusion chromatography. The enzyme showed broad acceptor specificity with Galbeta1,3GalNAc (T antigen), Galbeta1,4Man and Galbeta1,4Glc (lactose) being better acceptors than Galbeta-O-Me and galactose. Galbeta1,4Fru (lactulose), a natural sugar, was furthermore found to be the best acceptor for GST-WbsJ with a reaction rate four times faster than that of lactose. Kinetic studies showed that GST-WbsJ has a higher affinity for lactose than lactulose with apparent Km values of 7.81 mM and 13.26 mM, respectively. However, the kcat/appKm value of lactose (6.36 M(-1) x min(-1)) is two times lower than that of lactulose (13.39 M(-1) x min(-1)). In addition, the alpha1,2-fucosyltransferase activity of GST-WbsJ was found to be independent of divalent metal ions such as Mn2+ or Mg2+. This activity was competitively inhibited by GDP with a Ki value of 1.41 mM. Site-directed mutagenesis and a GDP-bead binding assay were also performed to investigate the functions of the highly conserved motif H152xR154R155xD157. In contrast to alpha1,6-fucosyltransferases, none of the mutants of WbsJ within this motif exhibited a complete loss of enzyme activity. However, residues R154 and D157 were found to play critical roles in donor binding and enzyme activity. The results suggest that the common motif shared by both alpha1,2-fucosyltransferases and alpha1,6-fucosyltransferases have similar functions. Enzymatic synthesis of fucosylated sugars in milligram scale was successfully performed using Galbeta-O-Me and Galbeta1,4Glcbeta-N3 as acceptors.

Molecular basis for acceptor substrate specificity of the human β1,3-glucuronosyltransferases GlcAT-I and GlcAT-P involved in glycosaminoglycan and HNK-1 carbohydrate epitope biosynthesis, respectively

The human beta1,3-glucuronosyltransferases galactose-beta1,3-glucuronosyltransferase I (GlcAT-I) and galactose-beta1,3-glucuronosyltransferase P (GlcAT-P) are key enzymes involved in proteoglycan and HNK-1 carbohydrate epitope synthesis, respectively. Analysis of their acceptor specificity revealed that GlcAT-I was selective toward Galbeta1,3Gal (referred to as Gal2-Gal1), whereas GlcAT-P presented a broader profile. To understand the molecular basis of acceptor substrate recognition, we constructed mutants and chimeric enzymes based on multiple sequence alignment and structural information. The drastic effect of mutations of Glu227, Arg247, Asp252, and Glu281 on GlcAT-I activity indicated a key role for the hydrogen bond network formed by these four conserved residues in dictating Gal2 binding. Investigation of GlcAT-I determinants governing Gal1 recognition showed that Trp243 could not be replaced by its counterpart Phe in GlcAT-P. This result combined with molecular modeling provided evidence for the importance of stacking interactions with Trp at position 243 in the selectivity of GlcAT-I toward Galbeta1,3Gal. Mutation of Gln318 predicted to be hydrogen-bonded to 6-hydroxyl of Gal1 had little effect on GlcAT-I activity, reinforcing the role of Trp243 in Gal1 binding. Substitution of Phe245 in GlcAT-P by Ala selectively abolished Galbeta1,3Gal activity, also highlighting the importance of an aromatic residue at this position in defining the specificity of GlcAT-P. Finally, substituting Phe245, Val320, or Asn321 in GlcAT-P predicted to interact with N-acetylglucosamine (GlcNAc), by their counterpart in GlcAT-I, moderately affected the activity toward the reference substrate of GlcAT-P, N-acetyllactosamine, indicating that its active site tolerates amino acid substitutions, an observation that parallels its promiscuous substrate profile. Taken together, the data clearly define key residues governing the specificity of beta1,3-glucuronosyltransferases.

Structural Effects of Naturally Occurring Human Blood Group B Galactosyltransferase Mutations Adjacent to the DXD Motif

Human blood group A and B antigens are produced by two closely related glycosyltransferase enzymes. An N-acetylgalactosaminyltransferase (GTA) utilizes UDP-GalNAc to extend H antigen acceptors (Fuc alpha(1-2)Gal beta-OR) producing A antigens, whereas a galactosyltransferase (GTB) utilizes UDP-Gal as a donor to extend H structures producing B antigens. GTA and GTB have a characteristic (211)DVD(213) motif that coordinates to a Mn(2+) ion shown to be critical in donor binding and catalysis. Three GTB mutants, M214V, M214T, and M214R, with alterations adjacent to the (211)DVD(213) motif have been identified in blood banking laboratories. From serological phenotyping, individuals with the M214R mutation show the B(el) variant expressing very low levels of B antigens, whereas those with M214T and M214V mutations give rise to A(weak)B phenotypes. Kinetic analysis of recombinant mutant GTB enzymes revealed that M214R has a 1200-fold decrease in k(cat) compared with wild type GTB. The crystal structure of M214R showed that DVD motif coordination to Mn(2+) was disrupted by Arg-214 causing displacement of the metal by a water molecule. Kinetic characterizations of the M214T and M214V mutants revealed they both had GTA and GTB activity consistent with the serology. The crystal structure of the M214T mutant showed no change in DVD coordination to Mn(2+). Instead a critical residue, Met-266, which is responsible for determining donor specificity, had adopted alternate conformations. The conformation with the highest occupancy opens up the active site to accommodate the larger A-specific donor, UDP-GalNAc, accounting for the dual specificity.

Phylogenetic and mutational analyses reveal key residues for UDP-glucuronic acid binding and activity of beta1,3-glucuronosyltransferase I (GlcAT-I)

The beta1,3-glucuronosyltransferases are responsible for the completion of the protein-glycosaminoglycan linkage region of proteoglycans and of the HNK1 epitope of glycoproteins and glycolipids by transferring glucuronic acid from UDP-alpha-D-glucuronic acid (UDP-GlcA) onto a terminal galactose residue. Here, we develop phylogenetic and mutational approaches to identify critical residues involved in UDP-GlcA binding and enzyme activity of the human beta1,3-glucuronosyltransferase I (GlcAT-I), which plays a key role in glycosaminoglycan biosynthesis. Phylogeny analysis identified 119 related beta1,3-glucuronosyltransferase sequences in vertebrates, invertebrates, and plants that contain eight conserved peptide motifs with 15 highly conserved amino acids. Sequence homology and structural information suggest that Y84, D113, R156, R161, and R310 residues belong to the UDP-GlcA binding site. The importance of these residues is assessed by site-directed mutagenesis, UDP affinity and kinetic analyses. Our data show that uridine binding is primarily governed by stacking interactions with the phenyl group of Y84 and also involves interactions with aspartate 113. Furthermore, we found that R156 is critical for enzyme activity but not for UDP binding, whereas R310 appears less important with regard to both activity and UDP interactions. These results clearly discriminate the function of these two active site residues that were predicted to interact with the pyrophosphate group of UDP-GlcA. Finally, mutation of R161 severely compromises GlcAT-I activity, emphasizing the major contribution of this invariant residue. Altogether, this phylogenetic approach sustained by biochemical analyses affords new insight into the organization of the beta1,3-glucuronosyltransferase family and distinguishes the respective importance of conserved residues in UDP-GlcA binding and activity of GlcAT-I.

Identification of active site residues of the inverting glycosyltransferase Cgs required for the synthesis of cyclic beta-1,2-glucan, a Brucella abortus virulence factor

Brucella abortus cyclic glucan synthase (Cgs) is a 320-kDa (2868-amino acid) polytopic integral inner membrane protein responsible for the synthesis of the virulence factor cyclic beta-1,2-glucan by a novel mechanism in which the enzyme itself acts as a protein intermediate. Cgs functions as an inverting processive beta-1,2-autoglucosyltransferase and has the three enzymatic activities required for the synthesis of the cyclic glucan: initiation, elongation, and cyclization. To gain further insight into the protein domains that are essential for the enzymatic activity, we have compared the Cgs sequence with other glycosyltransferases (GTs). This procedure allowed us to identify in the Cgs region (475-818) the widely spaced D, DxD, E/D, (Q/R)xxRW motif that is highly conserved in the active site of numerous GTs. By site-directed mutagenesis and in vitro and in vivo activity assays, we have demonstrated that most of the amino acid residues of this motif are essential for Cgs activity. These sequence and site-directed mutagenesis analyses also indicate that Cgs should be considered a bi-functional modular GT, with an N-terminal GT domain belonging to a new GT family related to GT-2 (GT-84) followed by a GH-94 glycoside hydrolase C-terminal domain. Furthermore, over-expression of inactive mutants results in wild-type (WT) production of cyclic glucan when bacteria co-express the mutant and the WT form, indicating that Cgs may function in the membrane as a monomeric enzyme. Together, these results are compatible with a single addition model by which Cgs acts in the membrane as a monomer and uses the identified motif to form a single center for substrate binding and glycosyl-transfer reaction.

Genomic organization and expression of the expanded SCG/L/R gene family of Leishmania major: internal clusters and telomeric localization of SCGs mediating species-specific LPG modifications

Stage-specific modifications to the abundant surface lipophosphoglycan (LPG) adhesin of Leishmania play critical roles in binding and release of the parasite during its infectious cycle in the sand fly, and control the ability of different fly species to transmit different parasite strains and species. In Leishmania major Friedlin V1, binding to a sand fly midgut lectin is mediated by side chain galactosyl (scGal) modifications of the LPG phosphoglycan (PG) repeats, while release occurs following arabinose-capping of scGals. Previously we identified a family of six SCG genes encoding PG scbeta-galactosyltransferases, and here we show that the extended SCG gene family (now termed SCG/L/R) encompasses 14 members in three subfamilies (SCG, SCGL and SCGR). Northern blot and RT-PCR analyses suggest that most of the SCG/L/R genes are expressed, with distinct patterns during the infectious cycle. The six SCGR subfamily genes are clustered and interspersed with the two SCA genes responsible for developmentally regulated arabinosylation of PG scGals; relationships amongst the SCGR revealed clear evidence of extensive gene conversion. In contrast, the seven SCG 'core' family members are localized adjacent to telomeres. These telomeres share varying amounts of sequence upstream and/or downstream of the SCG ORFs, again providing evidence of past gene conversions. Multiple SCG1-7 RNAs were expressed simultaneously within parasite populations. Potentially, telomeric localization of SCG genes may function primarily to facilitate gene conversion and the elaboration of functional evolutionary diversity in the degree of PG sc-galactosylation observed in other strains of L. major.

Tissue distribution, subcellular localization and endocrine disruption patterns induced by Cr and Mn in the crab Ucides cordatus

The essential trace elements Cr and Mn are toxic at high concentrations and information about low concentration is insufficient in the literature. In polluted mangroves, the crab Ucides cordatus can represent a useful tool to assess information on the potential impact of trace elements like Cr and Mn on the environment, since this species is comestible and thus, commercially negotiated. Therefore, U. cordatus crabs were exposed in vivo to different concentrations of Cr and Mn solved in seawater and had their tissue distribution and subcellular deposits evaluated. The gill, hepatopancreas and muscle concentrations were determined by atomic absorption spectroscopy and the results showed that Cr and Mn presented the highest values in the gills rather than in the hepatopancreas and muscular tissue. Electron microscopy and analytical X-ray microanalysis revealed Cr precipitates on the gill surface, co-localized with epiphyte bacteria. In addition, since Cr and Mn did not equally accumulate in most of the tissues studied, glycemic rate of animals, which received injections of extracts of eyestalks of the contaminated crabs, were measured in order to evaluate whether the studied concentrations of Cr and Mn could produce any metabolic alteration. The results indicated that extracts of the eyestalks of crabs submitted to Cr and Mn salts and injected into normal crabs markedly influenced crustacean hyperglycemic hormone synthesis and/or release. The results are discussed with respect to sensitivity of the employed methods and the possible significance of the concentrations of Cr and Mn in the organisms.

Phosphorylation and Sulfation of Oligosaccharide Substrates Critically Influence the Activity of Human β1,4-Galactosyltransferase 7 (GalT-I) and β1,3-Glucuronosyltransferase I (GlcAT-I) Involved in the Biosynthesis of the Glycosaminoglycan-Protein Linkage Region of Proteoglycans

We determined whether the two major structural modifications, i.e. phosphorylation and sulfation of the glycosaminoglycan-protein linkage region (GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1), govern the specificity of the glycosyltransferases responsible for the biosynthesis of the tetrasaccharide primer. We analyzed the influence of C-2 phosphorylation of Xyl residue on human beta1,4-galactosyltransferase 7 (GalT-I), which catalyzes the transfer of Gal onto Xyl, and we evaluated the consequences of C-4/C-6 sulfation of Galbeta1-3Gal (Gal2-Gal1) on the activity and specificity of beta1,3-glucuronosyltransferase I (GlcAT-I) responsible for the completion of the glycosaminoglycan primer sequence. For this purpose, a series of phosphorylated xylosides and sulfated C-4 and C-6 analogs of Galbeta1-3Gal was synthesized and tested as potential substrates for the recombinant enzymes. Our results revealed that the phosphorylation of Xyl on the C-2 position prevents GalT-I activity, suggesting that this modification may occur once Gal is attached to the Xyl residue of the nascent oligosaccharide linkage. On the other hand, we showed that sulfation on C-6 position of Gal1 of the Galbeta1-3Gal analog markedly enhanced GlcAT-I catalytic efficiency and we demonstrated the importance of Trp243 and Lys317 residues of Gal1 binding site for enzyme activity. In contrast, we found that GlcAT-I was unable to use digalactosides as acceptor substrates when Gal1 was sulfated on C-4 position or when Gal2 was sulfated on both C-4 and C-6 positions. Altogether, we demonstrated that oligosaccharide modifications of the linkage region control the specificity of the glycosyltransferases, a process that may regulate maturation and processing of glycosaminoglycan chains.

Stimulation of proteoglycan synthesis by glucuronosyltransferase-I gene delivery: a strategy to promote cartilage repair

Osteoarthritis is a degenerative joint disease characterized by a progressive loss of articular cartilage components, mainly proteoglycans (PGs), leading to destruction of the tissue. We investigate a therapeutic strategy based on stimulation of PG synthesis by gene transfer of the glycosaminoglycan (GAG)-synthesizing enzyme, beta1,3-glucuronosyltransferase-I (GlcAT-I) to promote cartilage repair. We previously reported that IL-1beta down-regulated the expression and activity of GlcAT-I in primary rat chondrocytes. Here, by using antisense oligonucleotides, we demonstrate that GlcAT-I inhibition impaired PG synthesis and deposition in articular cartilage explants, emphasizing the crucial role of this enzyme in PG anabolism. Thus, primary chondrocytes and cartilage explants were engineered by lipid-mediated gene delivery to efficiently overexpress a human GlcAT-I cDNA. Interestingly, GlcAT-I overexpression significantly enhanced GAG synthesis and deposition as evidenced by (35)S-sulfate incorporation, histology, estimation of GAG content, and fluorophore-assisted carbohydrate electrophoresis analysis. Metabolic labeling and Western blot analyses further suggested that GlcAT-I expression led to an increase in the abundance rather than in the length of GAG chains. Importantly, GlcAT-I delivery was able to overcome IL-1beta-induced PG depletion and maintain the anabolic activity of chondrocytes. Moreover, GlcAT-I also restored PG synthesis to a normal level in cartilage explants previously depleted from endogenous PGs by IL-1beta-treatment. In concert, our investigations strongly indicated that GlcAT-I was able to control and reverse articular cartilage defects in terms of PG anabolism and GAG content associated with IL-1beta. This study provides a basis for a gene therapy approach to promote cartilage repair in degenerative joint diseases.

Site-directed mutagenesis and protein 3D-homology modelling suggest a catalytic mechanism for UDP-glucose-dependent betanidin 5-O-glucosyltransferase from Dorotheanthus bellidiformis

In livingstone daisy (Dorotheanthus bellidiformis), betanidin 5-O-glucosyltransferase (UGT73A5) is involved in the regiospecific glucosylation of betanidin and various flavonols. Based on sequence alignments several amino acid candidates which might be essential for catalysis were identified. The selected amino acids of the functionally expressed protein, suggested to be involved in substrate binding and turnover, were substituted via site-directed mutagenesis. The substitution of two highly conserved amino acids, Glu378, located in the proposed UDP-glucose binding site, and His22, located close to the N-terminus, led to the complete loss of enzyme activity. A 3D model of this regiospecific betanidin and flavonoid glucosyltransferase was constructed and the active site modelled. This model was based on the crystallographic structure of a bacterial UDP-glucose-dependent glucosyltransferase from Amycolatopsis orientalis used as a template and the generated null mutations. To explain the observed inversion in the configuration of the bound sugar, semiempirical calculations favour an SN-1 reaction, as one plausible alternative to the generally proposed SN-2 mechanism discussed for plant natural product glucosyltransferases. The calculated structural data do not only explain the abstraction of a proton from the acceptor betanidin, but further imply that the reaction mechanism might also involve a catalytic triad, with similarities described for the serine protease family.