Purification and characterization of encystment-induced glucosamine 6-phosphate isomerase in Giardia

Allosteric Activation of Escherichia coli Glucosamine-6-Phosphate Deaminase (NagB) In Vivo Justified by Intracellular Amino Sugar Metabolite Concentrations

We have investigated the impact of growth on glucosamine (GlcN) and N-acetylglucosamine (GlcNAc) on cellular metabolism by quantifying glycolytic metabolites in Escherichia coli Growth on GlcNAc increased intracellular pools of both GlcNAc6P and GlcN6P 10- to 20-fold compared to growth on glucose. Growth on GlcN produced a 100-fold increase in GlcN6P but only a slight increase in GlcNAc6P. Changes to the amounts of downstream glycolytic intermediates were minor compared to growth on glucose. The enzyme glucosamine-6P deaminase (NagB) is required for growth on both GlcN and GlcNAc. It is an allosteric enzyme in E. coli, displaying sigmoid kinetics with respect to its substrate, GlcN6P, and is allosterically activated by GlcNAc6P. The high concentration of GlcN6P, accompanied by the small increase in GlcNAc6P, drives E. coli NagB (NagBEc) into its high activity state, as observed during growth on GlcN (L. I. Álvarez-Añorve, I. Bustos-Jaimes, M. L. Calcagno, and J. Plumbridge, J Bacteriol 191:6401-6407, 2009, http://dx.doi.org/10.1128/JB.00633-09). The slight increase in GlcNAc6P during growth on GlcN is insufficient to displace NagC, the GlcNAc6P-responsive repressor of the nag genes, from its binding sites, so there is only a small increase in nagB expression. We replaced the gene for the allosteric NagBEc enzyme with that of the nonallosteric, B. subtilis homologue, NagBBs We detected no effects on growth rates or competitive fitness on glucose or the amino sugars, nor did we detect any effect on the concentrations of central metabolites, thus demonstrating the robustness of amino sugar metabolism and leaving open the question of the role of allostery in the regulation of NagB.

IMPORTANCE

Chitin, the polymer of N-acetylglucosamine, is an abundant biomaterial, and both glucosamine and N-acetylglucosamine are valuable nutrients for bacteria. The amino sugars are components of numerous essential macromolecules, including bacterial peptidoglycan and mammalian glycosaminoglycans. Controlling the biosynthetic and degradative pathways of amino sugar metabolism is important in all organisms to avoid loss of nitrogen and energy via a futile cycle of synthesis and breakdown. The enzyme glucosamine-6P deaminase (NagB) is central to this control, and N-acetylglucosamine-6P is the key signaling molecule regulating amino sugar utilization in Escherichia coli Here, we investigate how the metabolic status of the bacteria impacts on the activity of NagBEc and the N-acetylglucosamine-6P-sensitive transcriptional repressor, NagC.

Heterogeneity of quaternary structure of glucosamine-6-phosphate deaminase from Giardia lamblia

The oligoHis-tagged versions of glucosamine-6-phosphate deaminase from Giardia lamblia (GlmNagB-HisN, GlmNagB-HisC) were constructed and purified to hear homogeneity, and their kinetic and structural properties were compared to those of the wild-type enzyme (GlmNagB). Introduction of the oligoHis tag at the GlmNagB C-terminus resulted in almost complete loss of the catalytic activity, while the catalytic properties of GlmNagB-HisN and GlmNagB were very similar. The recombinant and wild-type enzyme exhibits heterogeneity of the quaternary structure and in solution exists in three interconvertible forms, namely, monomeric, homodimeric, and homotetrameric. Although the monomeric form is prevalent, the monomer/dimer/tetramer ratios depended on protein concentration and fell within the range from 72:27:1 to 39:23:38. The enzyme is fully active in each of the oligomeric structures, efficiently catalyzes synthesis of D-glucosamine-6-phosphate from D-fructose-6-phosphate and ammonia, and its activity is not modified by GlcNAc6P, UDP-GlcNAc, or UDP-GalNAc. GlcN6P deaminase of G. lamblia represents a novel structural and functional type of enzyme of the NagB subfamily.

Cyst and encystment in protozoan parasites: optimal targets for new life-cycle interrupting strategies?

Certain protozoan parasites use survival strategies to reside outside the host such as the formation of cysts. This dormant and resistant stage results from the complex process of encystment that involves diverse molecular and cellular modifications. The stimuli and changes associated with cyst biogenesis are a matter of ongoing studies in human and animal protozoan parasites such as amoeba and Giardia species because blocking every step in the encystment pathway should, in theory, interrupt their life cycles. The present review thoroughly examines this essential process in those protozoan parasites and discusses the possibility of using that information to develop new kinds of anti-parasite specific and life cycle-interrupting drugs, aimed at holding back the dissemination of these infections.

Potential drug targets in cyst-wall biosynthesis by intestinal protozoa

Given that resistance to antiprotozoal drugs exists and is likely to increase and given that currently no reliable treatments exist for some of these infections, efforts to find new targets for chemotherapy must be continued. In the case of cyst-forming pathogenic protozoa, one such target might be encystment pathways and cyst-wall assembly. Information is increasing on protozoan encystment and, as it does, we can begin to answer the question of whether targeting it for chemotherapy is a viable drug strategy. Currently, there are significant efforts to understand encystment in Giardia and Entamoeba, and potential targets are being discovered as work on their encystment mechanisms progress. We know with certainty now that Giardia and Entamoeba cyst walls contain unique proteins and polysaccharides which differ from those of their hosts and thus make them potentially interesting targets for a variety of chemotherapeutic attacks. Although we lack detailed information about the other protozoan cyst formers, enough evidence exists for Giardia and Entamoeba that it seems prudent to screen them with some of the antifungal drugs, especially those that target mannoproteins, chitin synthesis, and beta (1, 3) glucan synthesis to ascertain if they target elements in these protozoan pathways that are similar to those found in fungi.

Biology of Giardia lamblia

Giardia lamblia is a common cause of diarrhea in humans and other mammals throughout the world. It can be distinguished from other Giardia species by light or electron microscopy. The two major genotypes of G. lamblia that infect humans are so different genetically and biologically that they may warrant separate species or subspecies designations. Trophozoites have nuclei and a well-developed cytoskeleton but lack mitochondria, peroxisomes, and the components of oxidative phosphorylation. They have an endomembrane system with at least some characteristics of the Golgi complex and encoplasmic reticulum, which becomes more extensive in encysting organisms. The primitive nature of the organelles and metabolism, as well as small-subunit rRNA phylogeny, has led to the proposal that Giardia spp. are among the most primitive eukaryotes. G. lamblia probably has a ploidy of 4 and a genome size of approximately 10 to 12 Mb divided among five chromosomes. Most genes have short 5' and 3' untranslated regions and promoter regions that are near the initiation codon. Trophozoites exhibit antigenic variation of an extensive repertoire of cysteine-rich variant-specific surface proteins. Expression is allele specific, and changes in expression from one vsp gene to another have not been associated with sequence alterations or gene rearrangements. The Giardia genome project promises to greatly increase our understanding of this interesting and enigmatic organism.

Regulation of carbohydrate metabolism during Giardia encystment

Giardia intestinalis trophozoites encyst when they are exposed to bile. During encystment, events related to the inducible synthesis of a novel N-acetyl-D-galactosamine (GalNAc) homopolymer, occur. Within the first 6 h of encystment, mRNA for glucosamine 6-P isomerase (GPI), the first inducible enzyme unique to this pathway appears, oxygen uptake rates double from non-encysting levels, and metronidazole (MTZ) inhibits oxygen uptake. Within 12 h, GPI and its activity are detectable and OU decreases 50% from non-encysting levels; glucose's stimulation and MTZ's inhibition of oxygen uptake cease. In contrast, aspartate uptake remained constant throughout the 40 h monitored. Two genes, gpi 1 and 2 encode for GPI, but only gpi1 is expressed during encystment. Glucosamine 6-P (GlcN6P), the synthetic product of GPI, activates UDP-N-acetylglucosamine (UDP-GlcNAc) pyrophosphorylase, a downstream enzyme, 3 to 5-fold in the direction of UDP-GlcNAc synthesis. UDP-GlcNAc is epimerized to UDP-GalNAc and UDP-GalNAc is polymerized by "cyst wall synthase" (beta 1 --> 3 GalNAc transferase) into a highly insoluble beta 1,3-linked homopolymer. This GalNAc polysaccharide, the major component of cyst wall filaments, forms, in conjunction with polypeptides, the outer cyst wall of Giardia.

UDP-N-acetylglucosamine pyrophosphorylase, a key enzyme in encysting Giardia, is allosterically regulated

Giardia synthesizes UDP-GalNAc during cyst wall formation (encystment) via a pathway of inducible enzymes similar to that used to synthesize chitin or peptidoglycan and that includes the UTP-requiring UDP-N-acetylglucosamine pyrophosphorylase. Although it has never been reported as a regulatory enzyme in any system studied to date, kinetic data including Hill plots demonstrate clearly that UDP-N-acetylglucosamine pyrophosphorylase activity, purified from encysting Giardia, is allosterically activated anabolically by physiological levels of glucosamine 6-phosphate (3 microm). Capillary electrophoresis demonstrates that within 24 h after trophozoites are induced to encyst, the level of glucosamine 6-phosphate increases 3-fold over that of non-encysting cells and that by 48 h into encystment the level of glucosamine 6-phosphate has decreased to non-encysting levels or below. UDP-N-acetylglucosamine pyrophosphorylase protein is present constitutively in encysting as well as non-encysting cells. UDP-N-acetylglucosamine pyrophosphorylase immunoaffinity purified from encysting and non-encysting cells exhibited the same molecular weight, amino acid composition, and circular dichroism spectra. Moreover, regardless of whether the enzyme came from encysting or non-encysting cells, the change in its circular dichroism spectra and up to a 6-fold increase in its specific activity anabolically were due to its activation with glucosamine 6-phosphate. Thus, the data support the idea that UDP-N-acetylglucosamine pyrophosphorylase is a major regulatory point in amino sugar synthesis in encysting Giardia and that its allosteric anabolic activation may shift the equilibrium of this pathway toward UDP-GalNAc synthesis.

Stage-specific expression and targeting of cyst wall protein-green fluorescent protein chimeras in Giardia

In preparation for being shed into the environment as infectious cysts, trophozoites of Giardia spp. synthesize and deposit large amounts of extracellular matrix into a resistant extracellular cyst wall. Functional aspects of this developmentally regulated process were investigated by expressing a series of chimeric cyst wall protein 1 (CWP1)-green fluorescent protein (GFP) reporter proteins. It was demonstrated that a short 110 bp 5' flanking region of the CWP1 gene harbors all necessary cis-DNA elements for strictly encystation-specific expression of a reporter during in vitro encystation, whereas sequences in the 3' flanking region are involved in modulation of steady-state levels of its mRNA during encystation. Encysting Giardia expressing CWP1-GFP chimeras showed formation and maturation of labeled dense granule-like vesicles and subsequent incorporation of GFP-tagged protein into the cyst wall, dependent on which domains of CWP1 were included. The N-terminal domain of CWP1 was required for targeting GFP to regulated compartments of the secretory apparatus, whereas a central domain containing leucine-rich repeats mediated association of the chimera with the extracellular cyst wall. We show that analysis of protein transport using GFP-tagged molecules is feasible in an anaerobic organism and provides a useful tool for investigating the organization of primitive eukaryotic vesicular transport.

Developmental gene regulation in Giardia lamblia: first evidence for an encystation-specific promoter and differential 5' mRNA processing

Giardia lamblia must encyst to survive in the environment and subsequently infect new hosts. We investigated the expression of glucosamine-6-phosphate isomerase (Gln6PI), the first enzyme required for biosynthesis of N-acetylgalactosamine, for the major cyst wall polysaccharide. We isolated two Gln6PI genes that encode proteins with large areas of identity, but distinctive central and terminal regions. Both recombinant enzymes have comparable kinetics. Interestingly, these genes have distinct patterns of expression. Gln6PI-A has a conventional, short 5' untranslated region (UTR), and is expressed at a low level during vegetative growth and encystation. The Gln6PI-B gene has two transcripts - one is expressed constitutively and the second species is highly upregulated during encystation. The non-regulated Gln6PI-B transcript has the longest 5'-UTR known for Giardia and is 5' capped or blocked. In contrast, the Gln6PI-B upregulated transcript has a short, non-capped 5'-UTR. A small promoter region (< 56 bp upstream from the start codon) is sufficient for the regulated expression of Gln6PI-B. Gln6PI-B also has an antisense overlapping transcript that is expressed constitutively. A shorter antisense transcript is detected during encystation. This is the first report of a developmentally regulated promoter in Giardia, as well as evidence for a potential role of 5' RNA processing and antisense RNA in differential gene regulation.

Cloning of two putative Giardia lamblia glucosamine 6-phosphate isomerase genes only one of which is transcriptionally activated during encystment

The biosynthesis of the carbohydrate component of the cyst wall of the protozoan parasite Giardia lamblia, a polymer of N-acetylgalactosamine (GalNac), is by a pathway that is initiated with the conversion of fructose 6-phosphate to glucosamine 6-phosphate by an aminating isomerase, glucose 6-phosphate isomerase. This enzyme appears only after Giardia trophozoites are induced to start the production of cyst wall components after bile is added. To investigate whether induction of glucosamine 6-phosphate isomerase is by protein modification or by transcription activation, its gene was cloned and sequenced. Two genes, gpi1 and gpi2, encoding putative glucosamine 6-phosphate isomerases were identified but one, gpi1 was expressed. The transcript for gpi1 appeared not earlier than 6 h after cells were induced with bile salts. These results show that the first enzyme in the pathway leading to GalNac synthesis in encysting Giardia cyst wall biosynthesis is under transcriptional control.