Crystal structures of the T4 phage beta-glucosyltransferase and the D100A mutant in complex with UDP-glucose: glucose binding and identification of the catalytic base for a direct displacement mechanism

T4 phage beta-glucosyltransferase (BGT) is an inverting glycosyltransferase (GT) that transfers glucose from uridine diphospho-glucose (UDP-glucose) to an acceptor modified DNA. BGT belongs to the GT-B structural superfamily, represented, so far, by five different inverting or retaining GT families. Here, we report three high-resolution X-ray structures of BGT and a point mutant solved in the presence of UDP-glucose. The two co-crystal structures of the D100A mutant show that, unlike the wild-type enzyme, this mutation prevents glucose hydrolysis. This strongly indicates that Asp100 is the catalytic base. We obtained the wild-type BGT-UDP-glucose complex by soaking substrate-free BGT crystals. Comparison with a previous structure of BGT solved in the presence of the donor product UDP and an acceptor analogue provides the first model of an inverting GT-B enzyme in which both the donor and acceptor substrates are bound to the active site. The structural analyses support the in-line displacement reaction mechanism previously proposed, locate residues involved in donor substrate specificity and identify the catalytic base.     

Open and closed structures of the UDP-glucose pyrophosphorylase from Leishmania major

Uridine diphosphate-glucose pyrophosphorylase (UGPase) represents a ubiquitous enzyme, which catalyzes the formation of UDP-glucose, a key metabolite of the carbohydrate pathways of all organisms. In the protozoan parasite Leishmania major, which causes a broad spectrum of diseases and is transmitted to humans by sand fly vectors, UGPase represents a virulence factor because of its requirement for the synthesis of cell surface glycoconjugates. Here we present the crystal structures of the L. major UGPase in its uncomplexed apo form (open conformation) and in complex with UDP-glucose (closed conformation). The UGPase consists of three distinct domains. The N-terminal domain exhibits species-specific differences in length, which might permit distinct regulation mechanisms. The central catalytic domain resembles a Rossmann-fold and contains key residues that are conserved in many nucleotidyltransferases. The C-terminal domain forms a left-handed parallel beta-helix (LbetaH), which represents a rarely observed structural element. The presented structures together with mutagenesis analyses provide a basis for a detailed analysis of the catalytic mechanism and for the design of species-specific UGPase inhibitors.     

Structural basis of cooperativity in human UDP-glucose dehydrogenase

Background

UDP-glucose dehydrogenase (UGDH) is the sole enzyme that catalyzes the conversion of UDP-glucose to UDP-glucuronic acid. The product is used in xenobiotic glucuronidation in hepatocytes and in the production of proteoglycans that are involved in promoting normal cellular growth and migration. Overproduction of proteoglycans has been implicated in the progression of certain epithelial cancers, while inhibition of UGDH diminished tumor angiogenesis in vivo. A better understanding of the conformational changes occurring during the UGDH reaction cycle will pave the way for inhibitor design and potential cancer therapeutics.

Methodology

Previously, the substrate-bound of UGDH was determined to be a symmetrical hexamer and this regular symmetry is disrupted on binding the inhibitor, UDP-α-D-xylose. Here, we have solved an alternate crystal structure of human UGDH (hUGDH) in complex with UDP-glucose at 2.8 Å resolution. Surprisingly, the quaternary structure of this substrate-bound protein complex consists of the open homohexamer that was previously observed for inhibitor-bound hUGDH, indicating that this conformation is relevant for deciphering elements of the normal reaction cycle.

Conclusion

In all subunits of the present open structure, Thr131 has translocated into the active site occupying the volume vacated by the absent active water and partially disordered NAD⁺ molecule. This conformation suggests a mechanism by which the enzyme may exchange NADH for NAD⁺ and repolarize the catalytic water bound to Asp280 while protecting the reaction intermediates. The structure also indicates how the subunits may communicate with each other through two reaction state sensors in this highly cooperative enzyme.     

Conformational change upon product binding to Klebsiella pneumoniae UDP-glucose dehydrogenase: a possible inhibition mechanism for the key enzyme in polymyxin resistance

Cationic modification of lipid A with 4-amino-4-deoxy-L-arabinopyranose (L-Ara4N) allows the pathogen Klebsiella pneumoniae to resist the antibiotic polymyxin and other cationic antimicrobial peptides. UDP-glucose dehydrogenase (Ugd) catalyzes the NAD?-dependent twofold oxidation of UDP-glucose (UPG) to produce UDP-glucuronic acid (UGA), a requisite precursor in the biosynthesis of L-Ara4N and bacterial exopolysaccharides. Here we report five crystal structures of K. pneumoniae Ugd (KpUgd) in its apo form, in complex with UPG, UPG/NADH, two UGA molecules, and finally with a C-terminal His?-tag. The UGA-complex structure differs from the others by a 14° rotation of the N-terminal domain toward the C-terminal domain, and represents a closed enzyme conformation. It also reveals that the second UGA molecule binds to a pre-existing positively charged surface patch away from the active site. The enzyme is thus inactivated by moving the catalytically important residues C253, K256 and D257 from their original positions. Kinetic data also suggest that KpUgd has multiple binding sites for UPG, and that UGA is a competitive inhibitor. The conformational changes triggered by UGA binding to the allosteric site can be exploited in designing potent inhibitors.     

Structure and mechanism of human UDP-glucose 6-dehydrogenase

Elevated production of the matrix glycosaminoglycan hyaluronan is strongly implicated in epithelial tumor progression. Inhibition of synthesis of the hyaluronan precursor UDP-glucuronic acid (UDP-GlcUA) therefore presents an emerging target for cancer therapy. Human UDP-glucose 6-dehydrogenase (hUGDH) catalyzes, in two NAD(+)-dependent steps without release of intermediate aldehyde, the biosynthetic oxidation of UDP-glucose (UDP-Glc) to UDP-GlcUA. Here, we present a structural characterization of the hUGDH reaction coordinate using crystal structures of the apoenzyme and ternary complexes of the enzyme bound with UDP-Glc/NADH and UDP-GlcUA/NAD(+). The quaternary structure of hUGDH is a disc-shaped trimer of homodimers whose subunits consist of two discrete α/β domains with the active site located in the interdomain cleft. Ternary complex formation is accompanied by rigid-body and restrained movement of the N-terminal NAD(+) binding domain, sequestering substrate and coenzyme in their reactive positions through interdomain closure. By alternating between conformations in and out of the active site during domain motion, Tyr(14), Glu(161), and Glu(165) participate in control of coenzyme binding and release during 2-fold oxidation. The proposed mechanism of hUGDH involves formation and breakdown of thiohemiacetal and thioester intermediates whereby Cys(276) functions as the catalytic nucleophile. Stopped-flow kinetic data capture the essential deprotonation of Cys(276) in the course of the first oxidation step, allowing the thiolate side chain to act as a trap of the incipient aldehyde. Because thiohemiacetal intermediate accumulates at steady state under physiological reaction conditions, hUGDH inhibition might best explore ligand binding to the NAD(+) binding domain.     

The structure of Clostridium difficile toxin A glucosyltransferase domain bound to Mn(2+) and UDP provides insights into glucosyltransferase activity and product release

Clostridium?difficile toxin?A (TcdA) is a member of the large clostridial toxin family, and is responsible, together with C.?difficile toxin?B (TcdB), for many clinical symptoms during human infections. Like other large clostridial toxins, TcdA catalyzes the glucosylation of GTPases, and is able to inactivate small GTPases within the host cell. Here, we report the crystal structures of the TcdA glucosyltransferase domain (TcdA-GT) in the apo form and in the presence of Mn(2+) and hydrolyzed UDP-glucose. These structures, together with the recently reported crystal structure of TcdA-GT bound to UDP-glucose, provide a detailed understanding of the conformational changes of TcdA that occur during the catalytic cycle. Indeed, we present a new intermediate conformation of a so-called 'lid' loop (residues?510-522 in TcdA), concomitant with the absence of glucose in the catalytic domain. The recombinant TcdA was expressed in Brevibacillus in the inactive apo form. High thermal stability of wild-type TcdA was observed only after the addition of both Mn(2+) and UDP-glucose. The glucosylhydrolase activity, which is readily restored after reconstitution with both these cofactors, was similar to that reported for TcdB. Interestingly, we found that ammonium, like K(+) , is able to activate the UDP-glucose hydrolase activities of TcdA. Consequently, the presence of ammonium in the crystallization buffer enabled us to obtain the first crystal structure of TcdA-GT bound to the hydrolysis product UDP. Database ??Coordinates of apo-TcdA-GT and Mn(2+) -UDP-TcdA-GT are available in the Protein Data Bank under the accession numbers 4DMV and 4DMW, respectively.     

Structural basis for subunit assembly in UDP-glucose pyrophosphorylase from Saccharomyces cerevisiae

UDP-glucose is the universal activated form of glucose, employed in all organisms for glucosyl transfer reactions and as precursor for various activated carbohydrates. In animal and fungal metabolism, UDP-glucose is required for utilization of galactose and for the synthesis of glycogen, the major carbohydrate storage polymer. The formation of UDP-glucose is catalyzed by UDP-glucose pyrophosphorylase (UGPase), which is highly conserved among eukaryotes. Here, we present the crystal structure of yeast UGPase, Ugp1p. Both in solution and in the crystal, Ugp1p forms homooctamers, which represent the enzymatically active form of the protein. Ugp1p subunits consist of three domains, with the active site presumably located in the central SpsA GnT I core (SGC) domain. The association in the octamer is mediated by contacts between left-handed beta-helices in the C-terminal domains, forming a toroidal solenoid structure in the core of the complex. The catalytic domains attached to this scaffold core do not directly contact each other, consistent with simple Michaelis-Menten kinetics found for Ugp1p. Conservation of hydrophobic residues at the subunit interfaces suggests that all fungal and animal homologs form this quarternary structure arrangement in contrast to monomeric plant UGPases, which have charged residues at these positions. Implications of this oligomeric arrangement for regulation of UGPase activity in fungi and animals are discussed.     

2.0A resolution crystal structures of the ternary complexes of human phenylalanine hydroxylase catalytic domain with tetrahydrobiopterin and 3-(2-thienyl)-L-alanine or L-norleucine: substrate specificity and molecular motions related to substrate binding

The crystal structures of the catalytic domain of human phenylalanine hydroxylase (hPheOH) in complex with the physiological cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) and the substrate analogues 3-(2-thienyl)-L-alanine (THA) or L-norleucine (NLE) have been determined at 2.0A resolution. The ternary THA complex confirms a previous 2.5A structure, and the ternary NLE complex shows that similar large conformational changes occur on binding of NLE as those observed for THA. Both structures demonstrate that substrate binding triggers structural changes throughout the entire protomer, including the displacement of Tyr138 from a surface position to a buried position at the active site, with a maximum displacement of 20.7A for its hydroxyl group. Two hinge-bending regions, centred at Leu197 and Asn223, act in consort upon substrate binding to create further large structural changes for parts of the C terminus. Thus, THA/L-Phe binding to the active site is likely to represent the epicentre of the global conformational changes observed in the full-length tetrameric enzyme. The carboxyl and amino groups of THA and NLE are positioned identically in the two structures, supporting the conclusion that these groups are of key importance in substrate binding, thus explaining the broad non-physiological substrate specificity observed for artificially activated forms of the enzyme. However, the specific activity with NLE as the substrate was only about 5% of that with THA, which is explained by the different affinities of binding and different catalytic turnover.     

Crystal structure of the tandem phosphatase domains of RPTP LAR.

Most receptor-like protein tyrosine phosphatases (RPTPs) contain two conserved phosphatase domains (D1 and D2) in their intracellular region. The carboxy-terminal D2 domain has little or no catalytic activity. The crystal structure of the tandem D1 and D2 domains of the human RPTP LAR revealed that the tertiary structures of the LAR D1 and D2 domains are very similar to each other, with the exception of conformational differences at two amino acid positions in the D2 domain. Site-directed mutational changes at these positions (Leu-1644-to-Tyr and Glu-1779-to-Asp) conferred a robust PTPase activity to the D2 domain. The catalytic sites of both domains are accessible, in contrast to the dimeric blocked orientation model previously suggested. The relative orientation of the LAR D1 and D2 domains, constrained by a short linker, is stabilized by extensive interdomain interactions, suggesting that this orientation might be favored in solution.     

Two homologues encoding human UDP-glucose:glycoprotein glucosyltransferase differ in mRNA expression and enzymatic activity

UDP-glucose:glycoprotein glucosyltransferase (UGT) is a soluble protein of the endoplasmic reticulum (ER) that operates as a gatekeeper for quality control by preventing transport of improperly folded glycoproteins out of the ER. We report the isolation of two cDNAs encoding human UDP-glucose:glycoprotein glucosyltransferase homologues. HUGT1 encodes a 1555 amino acid polypeptide that, upon cleavage of an N-terminal signal peptide, is predicted to produce a soluble 173 kDa protein with the ER retrieval signal REEL. HUGT2 encodes a 1516 amino acid polypeptide that also contains a signal peptide and the ER retrieval signal HDEL. HUGT1 shares 55% identity with HUGT2 and 31-45% identity with Drosophila, Caenorhabditis elegans, and Schizosaccharomyces pombe homologues, with most extensive conservation of residues in the carboxy-terminal fifth of the protein, the proposed catalytic domain. HUGT1 is expressed as multiple mRNA species that are induced to similar extents upon disruption of protein folding in the ER. In contrast, HUGT2 is transcribed as a single mRNA species that is not induced under similar conditions. HUGT1 and HUGT2 mRNAs are broadly expressed in multiple tissues and differ slightly in their tissue distribution. The HUGT1 and HUGT2 cDNAs were expressed by transient transfection in COS-1 monkey cells to obtain similar levels of protein localized to the ER. Extracts from HUGT1-transfected cells displayed a 27-fold increase in the transfer of [(14)C]glucose from UDP-[(14)C]glucose to denatured substrates. Despite its high degree of sequence identity with HUGT1, the expressed recombinant HUGT2 protein was not functional under the conditions optimized for HUGT1. Site-directed alanine mutagenesis within a highly conserved region of HUGT1 identified four residues that are essential for catalytic function.     

Crystal and molecular structure and absolute configuration of lincomycin hydrochloride monohydrate

Lincomycin is a broad-spectrum antibiotic synthesized by Streptomyces lincolnensis that is particularly active against Gram-positive bacteria. It is widely used in human and veterinary applications. The crystal structure of lincomycin has been undertaken with a view to obtain the conformational and structural features of the drug in order to afford a comparison of its structural features with other aminoglycoside antibiotics. We report here the details of its structural and conformational features as determined by single-crystal X-ray crystallography. Crystals of lincomycin hydrochloride are orthorhombic, space group P2(1)2(1)2, with the cell dimensions a=18.5294(3) Angstroms, b=20.5980(4) Angstroms, c=6.17380(10) Angstroms, V=2356.35(7) Angstroms3. The structure was solved using X-ray diffraction data and refined to a final R-value of 0.0391 for 2321 reflections (I > or = 2sigma). The absolute configuration was established using the anomalous dispersion of the sulfur and chlorine atoms in the structure. The molecule consists of an amino acid linked by an amide group to a monosaccharide of galactose stereochemistry. A network of hydrogen-bonds stabilizes the crystal structure.     

Structural studies of duck delta 1 and delta 2 crystallin suggest conformational changes occur during catalysis

Duck delta1 and delta2 crystallin are 94% identical in amino acid sequence, and while delta2 crystallin is the duck orthologue of argininosuccinate lyase (ASL) and catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate, the delta1 isoform is enzymatically inactive. The crystal structures of wild type duck delta1 and delta2 crystallin have been solved at 2.2 and 2.3 A resolution, respectively, and the refinement of the turkey delta1 crystallin has been completed. These structures have been compared with two mutant duck delta2 crystallin structures. Conformational changes were observed in two regions of the N-terminal domain with intraspecies differences between the active and inactive isoforms localized to residues 23-32 and both intra- and interspecies differences localized to the loop of residues 74-89. As the residues implicated in the catalytic mechanism of delta2/ASL are all conserved in delta1, the amino acid substitutions in these two regions are hypothesized to be critical for substrate binding. A sulfate anion was found in the active site of duck delta1 crystallin. This anion, which appears to mimic the fumarate moiety of the argininosuccinate substrate, induces a rigid body movement in domain 3 and a conformational change in the loop of residues 280-290, which together would sequester the substrate from the solvent. The duck delta1 crystallin structure suggests that Ser 281, a residue strictly conserved in all members of the superfamily, could be the catalytic acid in the delta2 crystallin/ASL enzymatic mechanism.     

Probing the pyrophosphate-binding site in potato tuber UDP-glucose pyrophosphorylase with pyridoxal diphosphate.

Potato tuber UDP-glucose pyrophosphorylase (EC 2.7.7.9) catalyzes the reversible uridylyl transfer from UDP-glucose to MgPPi forming glucose 1-phosphate and MgUTP, according to an ordered bi-bi mechanism in which UDP-glucose and MgPPi bind in this order. To probe the active site of this enzyme, we have applied pyridoxal 5'-diphosphate, a reactive PPi analogue. The enzyme was rapidly inactivated when incubated with the reagent in the presence of Mg2+ followed by sodium borohydride reduction. The degree of the inactivation was decreased by MgUTP, MgPPi, and glucose 1-phosphate, but enhanced by UDP-glucose. The enhancement was prevented by co-addition of Pi, the competitive inhibitor with respect to PPi. The complete inactivation corresponded to the incorporation of 0.9-1.1 mol of reagent/mol of enzyme monomer. In the presence of UDP-glucose, labels were almost exclusively incorporated into Lys-329. Thus, this residue may be located near the bound MgPPi and its modification is promoted, probably through conformational changes, by the binding of UDP-glucose to the enzyme. The results of the modification by the same reagent of the mutant enzymes in which Lys-329 and Lys-263 are individually replaced by Gln suggest the roles of these lysyl residues in the binding of MgPPi and in the UDP-glucose-induced conformational changes, respectively.     

X-ray crystal structures of a severely desiccated protein.

Unlike most protein crystals, form IX of bovine pancreatic ribonuclease A diffracts well when severely dehydrated. Crystal structures have been solved after 2.5 and 4 days of desiccation with CaSO4, at 1.9 and 2.0 A resolution, respectively. The two desiccated structures are very similar. An RMS displacement of 1.6 A is observed for main-chain atoms in each structure when compared to the hydrated crystal structure with some large rearrangements observed in loop regions. The structural changes are the result of intermolecular contacts formed by strong electrostatic interactions in the absence of a high dielectric medium. The electron density is very diffuse for some surface loops, consistent with a very disordered structure. This disorder is related to the conformational changes. These results help explain conformational changes during the lyophilization of protein and the associated phenomena of denaturation and molecular memory.     

Crystal structure of ATP sulfurylase from Saccharomyces cerevisiae, a key enzyme in sulfate activation

ATP sulfurylases (ATPSs) are ubiquitous enzymes that catalyse the primary step of intracellular sulfate activation: the reaction of inorganic sulfate with ATP to form adenosine-5'-phosphosulfate (APS) and pyrophosphate (PPi). With the crystal structure of ATPS from the yeast Saccharomyces cerevisiae, we have solved the first structure of a member of the ATP sulfurylase family. We have analysed the crystal structure of the native enzyme at 1.95 Angstroms resolution using multiple isomorphous replacement (MIR) and, subsequently, the ternary enzyme product complex with APS and PPi bound to the active site. The enzyme consists of six identical subunits arranged in two stacked rings in a D:3 symmetric assembly. Nucleotide binding causes significant conformational changes, which lead to a rigid body structural displacement of domains III and IV of the ATPS monomer. Despite having similar folds and active site design, examination of the active site of ATPS and comparison with known structures of related nucleotidylyl transferases reveal a novel ATP binding mode that is peculiar to ATP sulfurylases.     

Crystal structure of Anaerobiospirillum succiniciproducens PEP carboxykinase reveals an important active site loop

The 2.2 Angstroms resolution crystal structure of the enzyme phosphoenolpyruvate carboxykinase (PCK) from the bacterium Anaerobiospirillum succiniciproducens complexed with ATP, Mg(2+), Mn(2+) and the transition state analogue oxalate has been solved. The 2.4 Angstroms resolution native structure of A. succiniciproducens PCK has also been determined. It has been found that upon binding of substrate, PCK undergoes a conformational change. Two domains of the molecule fold towards each other, with the substrates and metal ions held in a cleft formed between the two domains. This domain movement is believed to accelerate the reaction PCK catalyzes by forcing bulk solvent molecules out of the active site. Although the crystal structure of A. succiniciproducens PCK with bound substrate and metal ions is related to the structures of PCK from Escherichia coli and Trypanosoma cruzi, it is the first crystal structure from this class of enzymes that clearly shows an important surface loop (residues 383-397) from the C-terminal domain, hydrogen bonding with the peptide backbone of the active site residue Arg60. The interaction between the surface loop and the active site backbone, which is a parallel beta-sheet, seems to be a feature unique of A. succiniciproducens PCK. The association between the loop and the active site is the third type of interaction found in PCK that is thought to play a part in the domain closure. This loop also appears to help accelerate catalysis by functioning as a 'lid' that shields water molecules from the active site.     

Uridine(5')diphospho(1)-alpha-D-glucose. A binding study to glycogen phosphorylase b in the crystal

UDP-glucose is an R-state inhibitor of glycogen phosphorylase b, competitive with the substrate, glucose 1-phosphate and noncompetitive with the allosteric activator, AMP. Diffusion of 100 mM UDP-glucose into crystals of phosphorylase b resulted in a difference Fourier synthesis at 0.3-nm resolution that showed two peaks: (a) binding at the allosteric site and (b) binding at the catalytic site. At the allosteric site the whole of the UDP-glucose molecule can be located. It is in a well defined folded conformation with its uracil portion in a similar position to that observed for the adenine of AMP. The uracil and the glucose moieties stack against the aromatic side chains of Tyr-75 and Phe-196, respectively. The phosphates of the pyrophosphate component interact with Arg-242, Arg-309 and Arg-310. At the catalytic site, the glucose-1-P component of UDP-glucose is firmly bound in a position similar to that observed for glucose 1-phosphate. The pyrophosphate is also well located with the glucose phosphate interacting with the main-chain NH groups at the start of the glycine-loop alpha helix and the uridine phosphate interacting through a water molecule with the 5'-phosphate of the cofactor pyridoxal phosphate and with the side chains of residues Tyr-573, Lys-574 and probably Arg-569. However the position of the uridine cannot be located although analysis by thin-layer chromatography showed that no degradation had taken place. Binding of UDP-glucose to the catalytic site promotes extensive conformational changes. The loop 279-288 which links the catalytic site to the nucleoside inhibitor site is displaced and becomes mobile. Concomitant movements of residues His-571, Arg-569, and the loop 378-383, together with the major loop displacement, result in an open channel to the catalytic site. Comparison with other structural results shows that these changes form an essential feature of the T to R transition. They allow formation of the phosphate recognition site at the catalytic site and destroy the nucleoside inhibitor site. Kinetic experiments demonstrate that UDP-glucose activates the enzyme in the presence of high concentrations of the weak activator IMP, because of its ability to decrease the affinity of IMP for the inhibitor site.     

Structure-function relationships in 3-phosphoglycerate kinase: role of the carboxy-terminal peptide

Yeast 3-phosphoglycerate kinase (PGK) is a monomeric enzyme (Mr approximately 45,000) composed of two globular domains. Each domain corresponds approximately to the amino- and carboxy-terminal halves of the polypeptide chain. The carboxy-terminal end extends over the interdomain "hinge" region and packs against the amino-terminal domain. It has been proposed that domain movement, resulting in closure of the active site cleft, is essential for the catalytic function of PGK. Large-scale conformational changes have also been postulated to explain activation of the enzyme by sulfate ions. Using site-specific mutagenesis, we have removed a 15-amino-acid carboxy-terminal fragment, in order to probe its role in the substrate- and sulfate-induced conformational changes. The truncated enzyme exhibited approximately 1% of the activity of native PGK and lost the ability to undergo sulfate-induced activation. The Km for ATP was essentially unchanged (Km = 0.23 mM) in comparison to the native enzyme (Km = 0.30 mM), whereas the Km value for 3-phosphoglycerate was increased about eightfold (Km = 3.85 mM and 0.50 mM, respectively). These results suggest that the carboxy-terminal segment is important for the mechanism of the substrate- and sulfate-induced conformational transitions. CD spectra and sedimentation velocity measurements indicate that the carboxy-terminal peptide is essential for structural integrity of PGK. The increased susceptibility of the truncated enzyme to thermal inactivation implies that the carboxy-terminal peptide also contributes to the stability of PGK.     

The noncatalytic portion of human UDP-glucose: glycoprotein glucosyltransferase I confers UDP-glucose binding and transferase function to the catalytic domain

The eukaryotic cell monitors the fidelity of protein folding in the endoplasmic reticulum and only permits properly folded and/or assembled proteins to transit to the Golgi compartment in a process termed "quality control." An endoplasmic reticulum (ER) lumenal sensor for quality control is the UDP-glucose:glycoprotein glucosyltransferase that targets unfolded glycoproteins for transient, calcium-dependent glucosylation. This modification mediates glycoprotein interaction with the folding machinery comprised of calnexin or calreticulin in conjunction with ERp57. Two human UGT homologues, HUGT1 and HUGT2, exist that share 55% identity. The highest degree of identity resides in the COOH-terminal 20% of these proteins, the putative catalytic domain of HUGT1. However, only HUGT1 displays the expected functional activity. The contribution of the NH2-terminal remainder of HUGT1 to glucosyltransferase function is presently unknown. In this report we demonstrate that HUGT2 is localized to the ER in a manner that overlaps the distribution of HUGT1. Analysis of a series of HUGT1 and HUGT2 chimeric proteins demonstrated that the carboxyl-terminal region of HUGT2 contains a catalytic domain that is functional in place of the analogous portion of HUGT1. Whereas neither catalytic domain displayed detectable activity when expressed alone, co-expression of either catalytic domain with the noncatalytic amino-terminal portion of HUGT1 conferred UDP-Glc binding and transfer of glucose that was specific for unfolded glycoprotein substrates. The results indicate that the amino-terminal 80% of HUGT1 is required for activation of the catalytic domain, whereas the homologous portion of HUGT2 cannot provide this function.     

Toward a blueprint for UDP-glucose pyrophosphorylase structure/function properties: homology-modeling analyses

UDP-glucose pyrophosphorylase (UGPase) is an important enzyme of synthesis of sucrose, cellulose, and several other polysaccharides in all plants. The protein is evolutionarily conserved among eukaryotes, but has little relation, aside from its catalytic reaction, to UGPases of prokaryotic origin. Using protein homology modeling strategy, 3D structures for barley, poplar, and Arabidopsis UGPases have been derived, based on recently published crystal structure of human UDP-N-acetylglucosamine pyrophosphorylase. The derived 3D structures correspond to a bowl-shaped protein with the active site at a central groove, and a C-terminal domain that includes a loop (I-loop) possibly involved in dimerization. Data on a plethora of earlier described UGPase mutants from a variety of eukaryotic organisms have been revisited, and we have, in most cases, verified the role of each mutation in enzyme catalysis/regulation/structural integrity. We have also found that one of two alternatively spliced forms of poplar UGPase has a very short I-loop, suggesting differences in oligomerization ability of the two isozymes. The derivation of the structural model for plant UGPase should serve as a useful blueprint for further function/structure studies on this protein.