Recombinant Pseudomonas syringae pv. tomato Undecaprenyl-diphosphatase (uppP)

What is the biochemical function of Undecaprenyl-diphosphatase (uppP) in Pseudomonas syringae pv. tomato?

Undecaprenyl-diphosphatase (uppP) catalyzes the critical dephosphorylation reaction of undecaprenyl pyrophosphate to undecaprenyl phosphate, releasing inorganic phosphate. The chemical reaction can be represented as:

Undecaprenyl diphosphate + H₂O → Undecaprenyl phosphate + Phosphate

This enzyme belongs to the family of hydrolases, specifically those acting on acid anhydrides in phosphorus-containing anhydrides . The enzymatic activity is enhanced by divalent cations, particularly Ca²⁺ and Mg²⁺, which play essential roles in substrate binding and catalysis . Magnesium or calcium ions coordinate with the pyrophosphate moiety of the substrate, making the inorganic phosphate or pyrophosphate group a better leaving group.

Research shows that the enzyme exhibits optimal activity at pH 6.5-7.0 and maintains high activity across a range of detergent concentrations (0.02-1% DDM), with maximum activity observed at 0.02% DDM .

Why is uppP significant in bacterial pathogenesis and cell wall synthesis?

Undecaprenyl phosphate is an essential carrier lipid in the bacterial cell membrane required for the biosynthesis of peptidoglycan and various carbohydrate polymers, including lipopolysaccharides, teichoic acids, and osmoregulated periplasmic glucans . In the pathway of cell wall synthesis, undecaprenyl phosphate serves as a carrier lipid for the translocation of hydrophilic oligosaccharide precursors (lipid II) across cell membranes via flippases for peptidoglycan assembly in the periplasm.

The significance of uppP extends to bacterial pathogenesis, particularly in plant pathogens like Pseudomonas syringae pv. tomato DC3000, which causes bacterial speck disease in tomato plants . The cell wall integrity affected by uppP activity can influence pathogen survival during infection and host-pathogen interactions.

What conserved domains and motifs are found in the uppP enzyme?

Sequence alignment reveals two highly conserved regions specific to bacterial uppP enzymes:

• Region I (residues 17-30) contains:

  • A glutamate-rich (E/Q)XXXE motif (residues 17-21)

  • Three highly conserved charged amino acids (Glu-17, Glu-21, His-30)

  • Two conserved polar residues (Ser-26, Ser-27)

• Region II (residues 170-178) contains:

  • A strongly conserved PGXSRSXXT motif, which resembles a structural P-loop

  • One proline residue (Pro-170)

  • One positively charged residue (Arg-174)

  • Three polar residues (Ser-173, Ser-175, Thr-178)

  • Two glycine residues (Gly-171, Gly-176)

These conserved motifs are critical for enzyme function. The (E/Q)XXXE motif is functionally similar to the DDXXD motif found in other enzymes that interact with pyrophosphate substrates. The PGXSRSXXT motif resembles structural P-loops commonly found in phosphate-binding enzymes .

What is the two-dimensional topology of uppP?

Based on computational predictions and experimental data, uppP is a highly hydrophobic integral membrane protein with eight transmembrane helices. The predicted topological model shows:

Most of the conserved residues, including the two consensus regions, are localized near the aqueous interface of uppP and oriented toward the periplasmic site, suggesting the catalytic function occurs on the outer side of the plasma membrane .

What methods are effective for expressing and purifying recombinant uppP from Pseudomonas syringae pv. tomato?

The purification of active uppP poses challenges due to its highly hydrophobic nature with eight transmembrane helices. A successful methodology involves:

• Expression vector construction:

  • Use of bacteriorhodopsin as a fusion tag at the N-terminus of uppP

  • Vector harboring the Hmbop1/D94N-uppP gene transformed into E. coli C41(DE3)

• Expression conditions:

  • Growth in LB medium containing 100 mg/ml ampicillin at 37°C

  • Induction with 0.5 mM IPTG and 5-10 mM all-trans-retinal when A600 reaches 0.9

  • Continued growth for 5 hours at 37°C

• Purification protocol:

  • Cell harvesting and resuspension in buffer A (50 mM Tris, pH 7.5, 500 mM NaCl)

  • Cell disruption and membrane collection by ultracentrifugation at 40,000 rpm for 1.5 h

  • Membrane solubilization in buffer A with 1% n-dodecyl-β-d-maltopyranoside at 4°C for 2.5 h

  • Centrifugation at 20,000 rpm for 0.5 h at 4°C

  • Loading supernatant onto nickel-nitrilotriacetic acid column

  • Washing with buffer A containing 75 mM imidazole and 0.05% DDM

  • Treatment with Tobacco etch virus protease during buffer dialysis at 4°C overnight

  • Final elution of native uppP by washing with buffer A containing 0.02% DDM

• Storage:

  • Aliquots of purified protein dropped directly in liquid nitrogen

  • Long-term storage at -80°C

This method has been demonstrated to yield functionally active protein suitable for biochemical and structural studies.

How do specific mutations affect the catalytic activity of uppP?

Site-directed mutagenesis studies reveal the functional significance of key residues in uppP. The table below summarizes the effects of various mutations on enzyme activity:

The data demonstrate that:

• The conserved (E/Q)XXXE motif is crucial for catalysis, with the double mutant E17A/E21A completely losing activity

• His-30 is essential for catalysis

• The P-loop motif (PGXSRSXXT) is critical for function, with most mutations severely reducing activity

• Residues away from the active site have minimal effects on catalytic activity

What are the kinetic parameters of wild-type and mutant uppP enzymes?

The kinetic parameters of wild-type and mutant uppP enzymes have been determined using farnesyl pyrophosphate (Fpp) as a model substrate due to the challenges of using the native substrate (undecaprenyl pyrophosphate) in mixed detergent micelles:

Key observations:

• The E17A mutation increases Km by 4-5 fold, indicating reduced substrate binding affinity

• Both E17A and E21A mutations decrease kcat by approximately 5-fold

• These results suggest that both glutamates are involved in catalytic function, with Glu-17 also playing an important role in substrate binding

The enzyme is inhibited by the polypeptide antibiotic bacitracin, which sequesters the pyrophosphate moiety of the substrate. The IC₅₀ value for bacitracin is approximately 33 μM (with 0.02 μM enzyme and 35 μM Fpp), corresponding to a Ki value of 7.8 μM .

What is the proposed three-dimensional structure and catalytic mechanism of uppP?

The three-dimensional structural model of uppP, constructed using the Rosetta membrane ab initio modeling program and validated by molecular dynamics simulation, reveals:

• Eight transmembrane helices (TM1-8) with a substrate-binding pocket mainly constituted of TM1, TM2, TM4, and TM5

• The pyrophosphate moiety of the substrate sits in an active-site pocket surrounded by charged residues (Glu-17, Glu-21, Arg-174)

• Part of the 55-carbon chain substrate lies on a hydrophobic surface mainly composed of residues in TM2

• The carboxylate groups of Glu-17 and Glu-21 interact with the pyrophosphate moiety through a magnesium ion

• His-30 is positioned in close proximity to the phosphorus center of the substrate

• The guanidinium group of Arg-174 establishes hydrogen bonding with the OH group of the α-phosphate

The proposed catalytic mechanism involves:

• His-30 initiates a nucleophilic attack on the phosphorus center to form a phosphohistidine intermediate

• A water molecule (or OH⁻ ion) makes a second nucleophilic attack on the phosphate of the phosphohistidine intermediate

• Glu-17 and Glu-21 participate in catalysis and substrate binding via a chelated magnesium ion

Molecular dynamics simulations show stability of the model in lipid bilayers, with:

• Transmembrane regions showing small structural changes during simulation (r.m.s.f. of 1.9-2.1 Å)

• Extracellular loops exhibiting higher flexibility (r.m.s.f. of 3.4-3.8 Å)

• Two loops (amino acids 31-43 and 72-85) being flexible in native uppP but stabilized upon substrate binding

How does uppP from Pseudomonas syringae pv. tomato compare with homologs in other bacterial species?

Comparative analysis of uppP sequences from different bacterial species reveals:

• Conservation of catalytic motifs:

  • The (E/Q)XXXE and PGXSRSXXT motifs are highly conserved across bacterial species

  • His-30 is conserved in all analyzed sequences

• Species-specific variations:

  • Pseudomonas syringae pv. tomato (strain DC3000) uppP has unique sequence features compared to homologs in other Pseudomonas species

  • Analysis of Pseudomonas proteomes shows varying levels of sequence identity for uppP-related enzymes

• Differences in uppP-related pathways:

  • The undecaprenyl phosphate recycling pathway shows variations across bacterial species

  • Some species have additional enzymes that can compensate for uppP function

  • In P. syringae pv. tomato DC3000, uppP plays significant roles in both de novo synthesis and recycling pathways

These differences suggest species-specific adaptations in the peptidoglycan synthesis pathway and may contribute to differences in susceptibility to antibiotics that target cell wall synthesis.

What methodologies are effective for studying the role of uppP in bacterial virulence and plant pathogenicity?

Studying the role of uppP in P. syringae pv. tomato virulence requires multidisciplinary approaches:

• Genetic manipulation techniques:

  • Gene knockout/knockdown using homologous recombination or CRISPR-Cas systems

  • Complementation with wild-type or mutant alleles

  • Site-directed mutagenesis to create specific amino acid changes

  • Promoter-reporter fusions to study gene expression under different conditions

• Plant infection assays:

  • Infiltration of bacterial suspensions into plant leaves

  • Spray inoculation to mimic natural infection

  • Measurement of bacterial population dynamics in planta

  • Quantification of disease symptoms (lesion size, bacterial specks)

• Microscopy techniques:

  • Confocal microscopy to visualize bacteria in plant tissues

  • Transmission electron microscopy to examine bacterial cell wall structure

• Biochemical assays:

  • Cell wall composition analysis

  • Antibiotic susceptibility testing

  • Enzyme activity assays using purified components

  • Measurement of peptidoglycan precursor accumulation

• Transcriptomic and proteomic analyses:

  • RNA-seq to examine gene expression changes in response to environmental stimuli

  • Proteome analysis to identify changes in protein abundance and modifications

These methodologies can help elucidate how uppP activity influences bacterial fitness, virulence, and interactions with plant hosts in the context of infection .

How is the expression of uppP regulated in response to environmental conditions and host factors?

Although specific data on uppP regulation in P. syringae pv. tomato is limited, research on related systems suggests several regulatory mechanisms:

• Transcriptional regulation:

  • Two-component regulatory systems (TCS) such as PhoP/PhoQ, which are conserved among Pseudomonas species with sequence identity of at least 83% for PhoP and 64% for PhoQ

  • Environmental signals including pH, magnesium limitation, and antimicrobial peptides may trigger these systems

• Integration with stress responses:

  • Cell envelope stress responses likely influence uppP expression

  • Exposure to plant defense compounds or antibiotics targeting cell wall synthesis may induce expression

• Coordination with other cell wall synthesis genes:

  • Expression is likely coordinated with other peptidoglycan biosynthesis genes

  • May be regulated as part of the cell cycle to ensure proper timing of cell wall synthesis

• Host-specific responses:

  • Plant signals produced during infection may influence uppP expression

  • For example, GABA and L-proline levels significantly increase in tomato plants upon pathogen infection and are involved in regulating plant defense responses

  • These compounds also function as signals for P. syringae pv. tomato, affecting entry and virulence

Understanding the regulation of uppP expression provides insights into potential strategies for pathogen control and the development of novel antibiotics targeting cell wall synthesis.

What are the methodological considerations for studying inhibitors of uppP as potential antimicrobial agents?

Developing and studying inhibitors of uppP requires careful methodological considerations:

• Enzyme assay optimization:

  • Use of appropriate substrates (Fpp as model substrate or native undecaprenyl pyrophosphate)

  • Optimization of detergent conditions (0.02% DDM provides maximum activity)

  • Inclusion of necessary divalent cations (Mg²⁺ or Ca²⁺)

  • Monitoring phosphate release using methods like the Malachite Green assay

  • Adjusting pH to optimal range (6.5-7.0)

• Inhibitor screening approaches:

  • High-throughput screening using colorimetric or fluorometric assays

  • Structure-based virtual screening using 3D models

  • Fragment-based drug discovery

  • Repurposing of known cell wall-targeting antibiotics

• Inhibitor characterization:

  • Determination of IC₅₀ and Ki values

  • Mechanism of inhibition (competitive, non-competitive, uncompetitive)

  • Bacitracin serves as a positive control inhibitor (Ki = 7.8 μM)

• In vivo efficacy testing:

  • Minimum inhibitory concentration (MIC) determination

  • Growth inhibition assays

  • Cell wall integrity assessment

  • Interaction with other antibiotics

• Selectivity assessment:

  • Testing against homologous enzymes from other species

  • Evaluation of effects on host enzymes

  • Toxicity testing in plant and animal models

These methodological considerations are essential for the rational design and evaluation of uppP inhibitors as potential antimicrobial agents for agricultural applications .