Effect of the manganese ion on human alpha3/4 fucosyltransferase III activity

Crystal structure of mango α1,3/α1,4-fucosyltransferase elucidates unique elements that regulate Lewis A-dominant oligosaccharide assembly

In various organisms, α1,3/α1,4-fucosyltransferases (CAZy GT10 family enzymes) mediate the assembly of type I (Galβ1,3GlcNAc) and/or type II (Galβ1,4GlcNAc)-based Lewis structures that are widely distributed in glycoconjugates. Unlike enzymes of other species, plant orthologues show little fucosyltransferase activity for type II-based glycans and predominantly catalyze the assembly of the Lewis A structure [Galβ1,3(Fucα1,4)GlcNAc] on the type I disaccharide unit of their substrates. However, the structural basis underlying this unique substrate selectivity remains elusive. In this study, we investigated the structure-function relationship of MiFUT13A, a mango α1,3/α1,4-fucosyltransferase. The prepared MiFUT13A displayed distinct α1,4-fucosyltransferase activity. Consistent with the enzymatic properties of this molecule, X-ray crystallography revealed that this enzyme has a typical GT-B fold-type structure containing a set of residues that are responsible for its SN2-like catalysis. Site-directed mutagenesis and molecular docking analyses proposed a rational binding mechanism for type I oligosaccharides. Within the catalytic cleft, the pocket surrounding Trp121 serves as a binding site, anchoring the non-reducing terminal β1,3-galactose that belongs to the type I disaccharide unit. Furthermore, Glu177 was postulated to function as a general base catalyst through its interaction with the 4-hydroxy group of the acceptor N-acetylglucosamine residue. Adjacent residues, specifically Thr120, Thr157 and Asp175 were speculated to assist in binding of the reducing terminal residues. Intriguingly, these structural elements were not fully conserved in mammalian orthologue which also shows predominant α1,4-fucosyltransferase activity. In conclusion, we have proposed that MiFUT13A generates the Lewis A structure on type I glycans through a distinct mechanism, divergent from that of mammalian enzymes.

The yeast Gdt1 protein mediates the exchange of H+ for Ca2+ and Mn2+ influencing the Golgi pH

The GDT1 family is broadly spread and highly conserved among living organisms. GDT1 members have functions in key processes like glycosylation in humans and yeasts, and photosynthesis in plants. These functions are mediated by their ability to transport ions. While transport of Ca²⁺ or Mn²⁺ is well established for several GDT1 members, their transport mechanism is poorly understood. Here, we demonstrate that H⁺ ions are transported in exchange for Ca²⁺ and Mn²⁺ cations by the Golgi-localized yeast Gdt1 protein. We performed direct transport measurement across a biological membrane by expressing Gdt1p in Lactococcus lactis bacterial cells and by recording either the extracellular pH or the intracellular pH during the application of Ca²⁺, Mn²⁺ or H⁺ gradients. Besides, in vivo cytosolic and Golgi pH measurements were performed in Saccharomyces cerevisiae with genetically encoded pH probes targeted to those subcellular compartments. These data point out that the flow of H⁺ ions carried by Gdt1p could be reversed according to the physiological conditions. Together, our experiments unravel the influence of the relative concentration gradients for Gdt1p-mediated H⁺ transport and pave the way to decipher the regulatory mechanisms driving the activity of GDT1 orthologs in various biological contexts.

Golgi Metal Ion Homeostasis in Human Health and Diseases

The Golgi apparatus is a membrane organelle located in the center of the protein processing and trafficking pathway. It consists of sub-compartments with distinct biochemical compositions and functions. Main functions of the Golgi, including membrane trafficking, protein glycosylation, and sorting, require a well-maintained stable microenvironment in the sub-compartments of the Golgi, along with metal ion homeostasis. Metal ions, such as Ca²⁺, Mn²⁺, Zn²⁺, and Cu²⁺, are important cofactors of many Golgi resident glycosylation enzymes. The homeostasis of metal ions in the secretory pathway, which is required for proper function and stress response of the Golgi, is tightly regulated and maintained by transporters. Mutations in the transporters cause human diseases. Here we provide a review specifically focusing on the transporters that maintain Golgi metal ion homeostasis under physiological conditions and their alterations in diseases.

Transport, functions, and interaction of calcium and manganese in plant organellar compartments

Calcium (Ca2+) and manganese (Mn2+) are essential elements for plants and have similar ionic radii and binding coordination. They are assigned specific functions within organelles, but share many transport mechanisms to cross organellar membranes. Despite their points of interaction, those elements are usually investigated and reviewed separately. This review takes them out of this isolation. It highlights our current mechanistic understanding and points to open questions of their functions, their transport, and their interplay in the endoplasmic reticulum (ER), vesicular compartments (Golgi apparatus, trans-Golgi network, pre-vacuolar compartment), vacuoles, chloroplasts, mitochondria, and peroxisomes. Complex processes demanding these cations, such as Mn2+-dependent glycosylation or systemic Ca2+ signaling, are covered in some detail if they have not been reviewed recently or if recent findings add to current models. The function of Ca2+ as signaling agent released from organelles into the cytosol and within the organelles themselves is a recurrent theme of this review, again keeping the interference by Mn2+ in mind. The involvement of organellar channels [e.g. glutamate receptor-likes (GLR), cyclic nucleotide-gated channels (CNGC), mitochondrial conductivity units (MCU), and two-pore channel1 (TPC1)], transporters (e.g. natural resistance-associated macrophage proteins (NRAMP), Ca2+ exchangers (CAX), metal tolerance proteins (MTP), and bivalent cation transporters (BICAT)], and pumps [autoinhibited Ca2+-ATPases (ACA) and ER Ca2+-ATPases (ECA)] in the import and export of organellar Ca2+ and Mn2+ is scrutinized, whereby current controversial issues are pointed out. Mechanisms in animals and yeast are taken into account where they may provide a blueprint for processes in plants, in particular, with respect to tunable molecular mechanisms of Ca2+ versus Mn2+ selectivity.

Biometals and glycosylation in humans: Congenital disorders of glycosylation shed lights into the crucial role of Golgi manganese homeostasis

About half of the eukaryotic proteins bind biometals that participate in their structure and functions in virtually all physiological processes, including glycosylation. After reviewing the biological roles and transport mechanisms of calcium, magnesium, manganese, zinc and cobalt acting as cofactors of the metalloproteins involved in sugar metabolism and/or glycosylation, the paper will outline the pathologies resulting from a dysregulation of these metals homeostasis and more particularly Congenital Disorders of Glycosylation (CDGs) caused by ion transporter defects. Highlighting of CDGs due to defects in SLC39A8 (ZIP8) and TMEM165, two proteins transporting manganese from the extracellular space to cytosol and from cytosol to the Golgi lumen, respectively, has emphasized the importance of manganese homeostasis for glycosylation. Based on our current knowledge of TMEM165 structure and functions, this review will draw a picture of known and putative mechanisms regulating manganese homeostasis in the secretory pathway.

Biochemical characterization of Helicobacter pylori α-1,4 fucosyltransferase: metal ion requirement, donor substrate specificity and organic solvent stability

The effect of metal ions on the activity, the donor substrate specificity, and the stability in organic solvents of Helicobacter pylori α-1,4 fucosyltransferase were studied. The recombinant enzyme was expressed as soluble form in E. coli strain AD494 and purified in a one step affinity chromatography. Its activity was highest in cacodylate buffer at pH 6.5 in the presence of 20 mM Mn2+ ions at 37°C. Mn2+ ions could be substituted by other metal ions. In all cases, Mn2+ ions proofed to be the most effective (Mn2+ > Co2+ > Ca2+ > Mg2+ > Cu2+ > Ni2+ > EDTA). The enzyme shows substrate specificity for Type I disaccharide (1) with a KM of 114 μM. In addition, the H. pylori α-1,4 fucosyltransferase efficiently transfers GDP-activated L-fucose derivatives to Galβ1-3GlcNAc-OR (1). Interestingly, the presence of organic solvents such as DMSO and methanol up to 20% in the reaction medium does not affect significantly the enzyme activity. However, at the same concentration of dioxane, activity is totally abolished.

Distantly related plant and nematode core α1,3-fucosyltransferases display similar trends in structure-function relationships

Here, we present a comparative structure-function study of a nematode and a plant core α1,3-fucosyltransferase based on deletion and point mutations of the coding regions of Caenorhabditis elegans FUT-1 and Arabidopsis thaliana FucTA (FUT11). In particular, our results reveal a novel "first cluster motif" shared by both core and Lewis-type α1,3-fucosyltransferases of the GT10 family. To evaluate the role of the conserved serine within this motif, this residue was replaced with alanine in FucTA (S218) and FUT-1 (S243). The S218A replacement completely abolished the enzyme activity of FucTA, while the S243A mutant of FUT-1 retained 20% of the "wild-type" activity. Based on the results of homology modeling of FucTA, other residues potentially involved in the donor substrate binding were examined, and mutations of N219 and R226 dramatically affected enzymatic activity. Finally, as both FucTA and FUT-1 were shown to be N-glycosylated, we examined the putative N-glycosylation sites. While alanine replacements at single potential N-glycosylation sites of FucTA resulted in a loss of up to 80% of the activity, a triple glycosylation site mutant still retained 5%, as compared to the control. In summary, our data indicate similar trends in structure-function relationships of distantly related enzymes which perform similar biochemical reactions and form the basis for future work aimed at understanding the structure of α1,3-fucosyltransferases in general.

Directing stem cell trafficking via GPS

The success of stem-cell-based regenerative therapeutics critically hinges on delivering relevant stem/progenitor cells to sites of tissue injury. To achieve adequate parenchymal infiltration following intravascular administration, it is first necessary that circulating cells bind to target tissue endothelium with sufficient strength to overcome the prevailing forces of hemodynamic shear. The principal mediators of these shear-resistant binding interactions consist of a family of C-type lectins known as "selectins" that bind discrete sialofucosylated glycans on their respective ligands. One member of this family, E-selectin, is an endothelial molecule that is inducibly expressed on postcapillary venules at all sites of tissue injury, but is also constitutively expressed on the luminal surface of bone marrow and dermal microvascular endothelium. Most stem/progenitor cells express high levels of CD44, and, in particular, human hematopoietic stem cells express a specialized sialofucosylated glycoform of CD44 known as "hematopoietic cell E-/L-selectin ligand" (HCELL) that functions as a potent E-selectin ligand. This chapter describes a method called "glycosyltransferase-programmed stereosubstitution" (GPS) for custom-modifying CD44 glycans to create HCELL on the surface of living cells that natively lack HCELL. Ex vivo glycan engineering of HCELL via GPS licenses trafficking of infused cells to endothelial beds that express E-selectin, thereby enabling efficient vascular delivery of stem/progenitor cells to sites where they are needed.

Glycosyltransferase-programmed stereosubstitution (GPS) to create HCELL: engineering a roadmap for cell migration

During evolution of the vertebrate cardiovascular system, the vast endothelial surface area associated with branching vascular networks mandated the development of molecular processes to efficiently and specifically recruit circulating sentinel host defense cells and tissue repair cells at localized sites of inflammation/tissue injury. The forces engendered by high-velocity blood flow commensurately required the evolution of specialized cell surface molecules capable of mediating shear-resistant endothelial adhesive interactions, thus literally capturing relevant cells from the blood stream onto the target endothelial surface and permitting subsequent extravasation. The principal effectors of these shear-resistant binding interactions comprise a family of C-type lectins known as 'selectins' that bind discrete sialofucosylated glycans on their respective ligands. This review explains the 'intelligent design' of requisite reagents to convert native CD44 into the sialofucosylated glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), the most potent E-selectin counter-receptor expressed on human cells, and will describe how ex vivo glycan engineering of HCELL expression may open the 'avenues' for the efficient vascular delivery of cells for a variety of cell therapies.

Exploring specificity of glycosyltransferases: synthesis of new sugar nucleotide related molecules as putative donor substrates

We investigated the specificity of glycosyltransferases toward donor substrates in two complementary directions. First we prepared simple N-acetyl-alpha-D-glucosamine 1-diphosphates: methyl-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-diphosphate, benzyl-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-diphosphate, 4-phenylbutyl-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-diphosphate, by the coupling of the corresponding activated alkyl phosphates with N-acetyl-alpha-D-glucosamine 1-phosphate. These diphosphates as well as 2-acetamido-2-deoxy-alpha-D-glucopyranose 1-diphosphate, tested as donors of N-acetylglucosamine in a reaction catalyzed by Neisseria meningitidis N-acetylglucosaminyltransferase (LgtA), proved to be devoid of activity. Evaluated as inhibitors, only 2-acetamido-2-deoxy-alpha-D-glucopyranose 1-diphosphate showed some inhibitory activity with an IC50 value of 7 mM. In the second approach, we prepared sugar nucleotide mimics having the diphosphate bridge replaced by the oxycarbonylaminosulfonyl linker. The surrogate of GDP-Fuc was synthesized as a 9:1 alpha/beta anomeric mixture, in 40% yield, starting from chlorosulfonyl isocyanate, perbenzylated l-fucopyranose, and a guanosine derivative, protected on the exocyclic amine and secondary hydroxyl functions of ribose. Then two deprotection steps, hydrogenolysis and enzymatic hydrolysis catalyzed by penicillin G amidase afforded the target molecule to be tested as fucose donor with recombinant human alpha-(1-->3/4)-fucosyltransferase (FucT-III). Tested as a 4:1 alpha/beta anomeric mixture, both in the absence and in the presence of cationic cofactors, this new guanosine fucose conjugate proved to be ineffective. Its inhibitory activity toward FucT-III evaluated through a competition fluorescence assay was very poor (IC50 value of 20 mM). The surrogate of UDP-GlcNAc that was already known as its protected acetylated derivative, tested as N-acetylglucosamine donor with LgtA in the presence of Mn(2+) turned out not to be active either.