Recombinant Saccharomyces cerevisiae Dolichol-phosphate mannosyltransferase (DPM1)

What is the basic function of Saccharomyces cerevisiae DPM1?

DPM1 is the structural gene encoding dolichylphosphate mannose synthase (DPMS) in Saccharomyces cerevisiae. The enzyme catalyzes the essential reaction whereby mannose is transferred from GDP-mannose to dolichol phosphate (Dol-P) to yield dolichyl-phosphate mannose (Dol-P-Man) . This 31-kDa protein plays a critical role in glycosylation pathways as Dol-P-Man serves as the mannosyl donor for multiple glycosylation processes including N-linked glycosylation, O-mannosylation, and glycosylphosphatidylinositol (GPI) anchor biosynthesis . These processes are essential for proper cell wall formation and protein modifications in yeast.

How does DPM1 differ between yeast and human systems?

S. cerevisiae DPM1 functions as a single protein, while human DPM1 lacks a carboxy-terminal transmembrane domain and signal sequence, requiring regulation by additional proteins such as DPM2 . The yeast enzyme (31 kDa) can function independently, whereas human DPM1 operates as part of a complex. These differences are important considerations when using S. cerevisiae DPM1 as a model for human disorders or when expressing the yeast enzyme in mammalian systems .

What are the most effective methods for recombinant expression of S. cerevisiae DPM1?

Multiple expression systems have been successfully employed for recombinant production of S. cerevisiae DPM1:

Expression in E. coli:

• Create expression constructs using standard molecular cloning techniques with appropriate tags (GST, MBP, or His-tag) to facilitate purification

• Transform into competent E. coli cells

• Induce expression with IPTG under optimized conditions (typically 18-25°C to enhance solubility)

• Lyse cells and purify using appropriate affinity chromatography

Expression in native S. cerevisiae:

• Subclone DPM1 into a yeast expression vector, such as a modified pEG(KT) backbone

• Create fusion constructs with tags such as MBP separated by a TEV protease cleavage site

• Transform into protease-deficient yeast strains (e.g., DDY1810: MATa, leu2Δ, trp1Δ, ura3-52, prb1-1122, pep4-3, pre1-451)

• Grow cells in selective medium with appropriate carbon sources (e.g., raffinose)

• Induce expression with galactose

• Harvest cells, lyse mechanically (e.g., using an EmulsiFlex homogenizer), and purify using affinity chromatography

Research has shown that yeast-expressed DPM1 may exhibit different catalytic properties compared to bacterially expressed protein, potentially due to differences in post-translational modifications .

What purification methods yield the most active DPM1 enzyme?

A methodical purification workflow for obtaining highly active S. cerevisiae DPM1 includes:

• Affinity Chromatography: Use an appropriate affinity tag (MBP or GST) for initial capture

• Tag Removal: If desired, cleave affinity tags using specific proteases (e.g., TEV protease for MBP-TEV-DPM1 constructs)

• Ion Exchange Chromatography: Further purify using anion or cation exchange depending on the protein's isoelectric point

• Size Exclusion Chromatography: Final polishing step to obtain homogeneous protein

Key considerations for maintaining enzyme activity:

• Maintain cold temperature (4°C) throughout purification

• Include protease inhibitors (EDTA-free tablet, PMSF) in buffers

• Use glycerol (10-20%) in storage buffers to stabilize the enzyme

• Add reducing agents (DTT or β-mercaptoethanol) to prevent oxidation

• Include appropriate detergents if needed for stability

• Store in small aliquots at -80°C to avoid freeze-thaw cycles

How can I assess the purity and activity of purified DPM1?

Purity Assessment:

• SDS-PAGE with Coomassie or silver staining (>95% homogeneity is desirable)

• Western blotting using anti-DPM1 antibodies

• Mass spectrometry for identity confirmation

Activity Assays:

• Radiometric assay: Measure transfer of [14C]-mannose from GDP-[14C]-mannose to Dol-P

• Coupled enzymatic assay: Measure the release of GDP during the reaction using coupling enzymes

• HPLC-based assay: Quantify the formation of Dol-P-Man or consumption of GDP-mannose

A standard DPM1 activity assay involves:

• Reaction buffer containing 50 mM HEPES (pH 7.5), 10 mM MgCl2, 0.1% Triton X-100

• GDP-mannose (typically 50-200 μM)

• Dolichyl-phosphate (Dol-P) substrate (10-50 μM)

• Purified DPM1 enzyme (0.1-1 μg)

• Incubation at 30°C for 15-30 minutes

• Termination of reaction and quantification by appropriate method

What are the key catalytic residues in S. cerevisiae DPM1 and how do they contribute to enzyme function?

While the search results don't provide complete details on all catalytic residues, key functional sites include:

• Serine 141 (S141): Part of the consensus sequence (YRRVIS141) for phosphorylation by cAMP-dependent protein kinase (PKA). Phosphorylation at this residue significantly enhances enzyme activity (6-fold increase in Vmax) without changing Km for GDP-mannose. Mutation of S141 to alanine (S141A) reduces catalytic activity by more than 50%, confirming its importance in regulation .

• ATP-binding site: Critical for enzyme function, though specific residues are not detailed in the search results.

• Lipid-binding domain: Required for interaction with the dolichol phosphate substrate.

The consensus sequence for phosphorylation (YRRVIS141) is not required for substrate recognition but plays a crucial role in catalytic efficiency through regulatory phosphorylation .

How does the structure of DPM1 relate to its membrane association and substrate recognition?

DPM1 is closely associated with the endoplasmic reticulum (ER) membrane and exhibits unique lipid requirements for maximal activity. Based on structural studies of related enzymes:

• Membrane Association: The protein interacts with the cytosolic side of the ER membrane. Unlike human DPM1, S. cerevisiae DPM1 contains a C-terminal transmembrane domain that anchors it to the ER membrane.

• Substrate Recognition: The enzyme must accommodate both a water-soluble substrate (GDP-mannose) and a lipid-soluble substrate (Dol-P). Studies using fluorescent-labeled dolichyl-phosphate derivatives have provided insights into active site structure and substrate binding .

• Conformational Changes: Crystal structures of archaeal DPMS reveal that lipid binding couples to movements of interface helices, metal binding, and acceptor loop dynamics to control critical events leading to Dol-P-Man synthesis .

• Divalent Cation Requirement: The enzyme requires Mg2+ or Mn2+ for activity, which likely assists in coordinating the GDP-mannose substrate.

Understanding these structural features is essential for rational design of inhibitors or enhancers of DPM1 activity .

What experimental approaches can reveal the dynamics of DPM1 during catalysis?

Advanced techniques to study DPM1 dynamics include:

• Fluorescence Resonance Energy Transfer (FRET): Using fluorescent-labeled dolichyl-phosphate derivatives to determine intramolecular distances between amino acid residues near the active site and substrate analogs. This approach has revealed insights into conformational changes during catalysis .

• Site-Directed Mutagenesis: Creating point mutations at putative functional residues (e.g., S141A) to assess their contributions to enzyme activity. This approach confirmed the importance of serine 141 in enzyme regulation .

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS): Although not mentioned in the search results, this technique could provide valuable information about protein dynamics and conformational changes.

• Molecular Dynamics Simulations: Computational approaches to model protein flexibility and substrate interactions.

• Crystallography with different ligands: Capturing different states of the catalytic cycle, as demonstrated with archaeal DPMS structures complexed with nucleotide, donor, and glycolipid product .

How is DPM1 activity regulated in vivo?

DPM1 activity is regulated through several mechanisms:

• Phosphorylation: The enzyme contains a consensus sequence (YRRVIS141) that is phosphorylated by cAMP-dependent protein kinase (PKA). Phosphorylation enhances enzyme activity significantly (6-fold increase in Vmax) without changing the Km for GDP-mannose. This represents a key regulatory mechanism for enzyme function .

  Experimental data shows that:

  • In vitro phosphorylated DPM1 exhibits higher enzyme turnover (kcat) and enzyme efficiency (kcat/Km)

  • SDS-PAGE followed by autoradiography of 32P-labeled DPM1 confirms phosphorylation

  • Immunoblotting with anti-phosphoserine antibody establishes the presence of a phosphoserine residue

  • S141A mutant shows reduced catalytic activity after phosphorylation treatment

• Inhibition by Compounds: Aspirin (acetylsalicylic acid) can inhibit DPM1 expression and enzyme activity. Molecular docking results demonstrated that aspirin directly binds to the Ser141 site of DPM1 .

• Substrate Availability: The availability of both GDP-mannose and dolichol-phosphate will naturally regulate the synthesis of Dol-P-Man.

• Membrane Environment: DPM1 function is influenced by the lipid composition of the ER membrane .

What is the role of DPM1 in cell wall integrity and how can it be experimentally assessed?

DPM1 plays a crucial role in cell wall integrity in S. cerevisiae by providing Dol-P-Man for multiple glycosylation pathways. Experimental approaches to assess this function include:

• Scanning Electron Microscopy (SEM): Enables visualization of cell wall ultrastructure changes when DPM1 function is compromised. Studies have shown that inhibition of DPM1 by aspirin causes visible damage to the yeast cell wall .

• Cellular Surface Hydrophobicity (CSH) Assay: Measures changes in the hydrophobicity of the yeast cell surface, which increases when DPM1 function is impaired .

• Drug Sensitivity Assays: Tests cell sensitivity to compounds that target the cell wall (e.g., Calcofluor white, Congo red). Overexpression of DPM1 can reverse cell wall damage induced by inhibitors .

• Lectin Binding Analysis: Differential lectin binding to cell surface glycoproteins can be assessed using fluorescence-activated cell sorting (FACS) .

• Analysis of Lipid-Linked Oligosaccharides: High-performance liquid chromatography (HPLC) fractionation of lipid-linked oligosaccharides synthesized in wild-type and DPM1-deficient cells .

• Endoglycosidase Sensitivity Tests: Restoration of endoglycosidase H sensitivity to oligosaccharides transferred to specific glycoproteins can indicate proper DPM1 function .

What are the genetic interactions of DPM1 with other glycosylation pathway components?

DPM1 functions within a complex network of genes involved in glycosylation pathways. Key genetic interactions include:

• DPAGT1 Pathway: Genome-wide CRISPR screens have identified DPM1 as one of the strongest modifier genes of DPAGT1 function. Surprisingly, inhibition of DPM1 can rescue cell survival under DPAGT1 inhibition and ER stress, despite DPM1 impairment alone typically causing congenital disorders of glycosylation (CDGs) .

• GPI Anchor Biosynthesis Genes: Knockout of multiple GPI anchor biosynthesis genes improves survival and cell surface glycoprotein levels associated with DPAGT1 inhibition and ER stress .

• Fructose Metabolism Pathway: Evidence suggests connections between DPM1, glycolytic pathways, and the hexosamine biosynthetic pathway which provides precursors for glycosylation .

• PMM2: The most common CDG gene, which is involved in GDP-mannose metabolism from fructose-6-phosphate, shows genetic interactions with DPM1 .

These interactions can be studied through:

• Genetic epistasis analysis

• Double knockout/knockdown experiments

• Rescue experiments (e.g., overexpression of one gene in the background of another gene's deficiency)

• Metabolic flux analysis to track changes in pathway activity

How do mutations in DPM1 relate to congenital disorders of glycosylation (CDG)?

While the search results focus primarily on S. cerevisiae DPM1, they mention that mutations in the human DPM1 gene lead to congenital disorders of glycosylation (CDG) . These disorders are characterized by defective glycosylation and can manifest as multi-system disorders affecting various organs, particularly the nervous system.

The relationship between DPM1 and CDGs can be studied using:

• Yeast Complementation Assays: S. cerevisiae DPM1 can complement the glycosylation defect in mammalian cell lines with DPM1 deficiency, as demonstrated by transfection into a mutant Chinese hamster ovary cell line (B4-2-1) .

• Disease-Causing Mutation Modeling: Creating equivalent mutations in yeast DPM1 to those found in human CDG patients and assessing their effects on enzyme function.

• Glycosylation Profile Analysis: Examining changes in glycosylation patterns in cells with DPM1 deficiency using techniques such as mass spectrometry.

• Cellular Stress Markers: Measuring ER stress responses that may result from defective glycosylation due to DPM1 dysfunction .

Can DPM1 be targeted for antifungal drug development, and what approaches might be most promising?

The search results indicate that DPM1 could be a promising target for antifungal drug development:

• Rationale for Targeting: DPM1 is essential for cell wall integrity in fungi, and its inhibition causes cell wall damage . The difference between fungal and human DPM1 (single protein vs. complex) offers selectivity potential.

• Known Inhibitors: Aspirin has been shown to damage the cell wall of S. cerevisiae by inhibiting the expression and activity of DPM1. Molecular docking results demonstrated that aspirin directly binds to the Ser141 site of DPM1 .

• Approaches for Drug Discovery:

  • Structure-based drug design: Using structural information to design specific inhibitors that target the catalytic site or regulatory regions of fungal DPM1

  • High-throughput screening: Testing compound libraries against purified DPM1 or in cell-based assays

  • Fragment-based drug discovery: Building inhibitors from small molecular fragments that bind to different parts of the enzyme

  • Repositioning of existing drugs: Testing approved drugs (like aspirin) as starting points for developing more potent and selective DPM1 inhibitors

• Validation Methods:

  • Enzymatic assays with purified protein

  • Cell-based assays measuring cell wall integrity

  • Scanning electron microscopy to visualize cell wall defects

  • Cellular surface hydrophobicity measurements

  • In vivo efficacy in fungal infection models

How might DPM1 modulation be harnessed for therapeutic purposes in conditions like DPAGT1-CDG?

Interestingly, genome-wide CRISPR screens have identified DPM1 as one of the strongest modifier genes that can rescue DPAGT1 inhibition and ER stress :

• Paradoxical Rescue Effect: Inhibition of DPM1 vastly improves cell survival under DPAGT1 inhibition and ER stress, despite the fact that DPM1 impairment alone typically causes CDGs. This suggests a complex relationship between different glycosylation pathways.

• Possible Mechanisms:

  • Rebalancing glycosylation precursor pools

  • Reducing competition for limited dolichol phosphate

  • Altering ER stress responses

  • Combined effects on O-mannosylation, N-glycosylation, and GPI anchor biosynthesis pathways

• Potential Therapeutic Approaches:

  • Partial inhibition of DPM1 to rescue DPAGT1-CDG

  • Combined modulation of DPM1 and GPI anchor biosynthesis

  • Targeting specific downstream pathways dependent on Dol-P-Man

• Experimental Validation:

  • Studies in cell culture models of DPAGT1-CDG

  • Investigation in animal models with DPAGT1 deficiency

  • Analysis of glycosylation profiles after DPM1 modulation

  • Examination of specific glycoproteins affected by dual modulation of DPAGT1 and DPM1

This represents a novel potential therapeutic strategy for certain types of CDGs, although careful titration of DPM1 inhibition would be necessary to achieve beneficial effects without causing additional glycosylation defects.

What are the most sensitive methods for measuring DPM1 enzyme kinetics?

Several sophisticated methods can be employed for precise measurement of DPM1 enzyme kinetics:

• Radiometric Assays:

  • Using [14C]- or [3H]-labeled GDP-mannose to track transfer to dolichol phosphate

  • Separating reaction products by thin-layer chromatography or extraction methods

  • Quantifying by scintillation counting

  • Advantages: High sensitivity, direct measurement of product formation

  • Limitations: Requires radioactive materials, special handling and disposal

• Fluorescence-Based Assays:

  • Using fluorescent-labeled dolichyl-phosphate derivatives

  • Monitoring changes in fluorescence intensity or anisotropy during reaction

  • Real-time measurement possible

  • Advantages: No radioactivity, potential for continuous monitoring

  • Applications include determining intramolecular distances between amino acid residues near the active site and substrate analogs

• HPLC-Based Methods:

  • Separation and quantification of GDP-mannose consumption or Dol-P-Man production

  • Can be coupled with various detection methods (UV, fluorescence, mass spectrometry)

  • Advantages: High precision, ability to detect multiple reaction components

• Coupled Enzyme Assays:

  • Linking GDP release to reactions that produce measurable signals

  • Continuous spectrophotometric monitoring possible

  • Advantages: Real-time kinetics, no need for product separation

Kinetic parameters that should be determined include:

• Km for both GDP-mannose and dolichol phosphate

• Vmax and kcat

• Effects of pH, temperature, and salt concentration

• Inhibition constants for various inhibitors

For S. cerevisiae DPM1, studies have shown that:

• The yeast-expressed enzyme has a higher Km for substrate (685 ± 47 μM) compared to bacterially expressed enzyme (110 ± 8 μM)

• Vmax values differ between expression systems (5.2 ± 0.1 nmol/min for yeast vs. 4.0 ± 0.1 nmol/min for bacterial expression)

• Enzyme activity is salt-dependent, with activity decreasing as NaCl concentration increases

What are the challenges and solutions for studying membrane-associated properties of DPM1?

DPM1 is a membrane-associated enzyme that presents specific challenges for biochemical and structural studies:

Challenges:

• Maintaining proper folding and activity during solubilization

• Reproducing the native membrane environment for functional studies

• Obtaining sufficient quantities of purified protein

• Crystallizing membrane-associated proteins for structural analysis

Solutions and Approaches:

• Detergent Selection:

  • Systematic screening of detergents for solubilization (e.g., Triton X-100, DDM, CHAPS)

  • Optimization of detergent concentration to maintain activity

  • Use of detergent mixtures to better mimic native membrane environment

• Membrane Mimetics:

  • Reconstitution into liposomes of defined lipid composition

  • Use of nanodiscs to provide a native-like bilayer environment

  • Bicelles or lipid cubic phases for structural studies

• Expression Strategies:

  • Expression in native S. cerevisiae to maintain proper folding and post-translational modifications

  • Fusion constructs with solubility-enhancing tags (MBP, GST)

  • Co-expression with stabilizing partners if needed

• Functional Assays in Membrane Contexts:

  • Measuring activity in crude membrane preparations

  • Reconstitution assays to assess function in artificial membranes

  • Fluorescence-based assays to study protein-lipid interactions

Research has shown that DPM1 isolated from yeast may have different properties compared to bacterially expressed enzyme, highlighting the importance of the expression system and membrane environment for proper function .

How can structural information about DPM1 be leveraged for protein engineering?

Structural information about DPM1 can guide protein engineering efforts for various applications:

• Improving Enzyme Stability:

  • Introducing disulfide bridges at strategic positions

  • Optimizing surface charge distribution

  • Engineering salt bridges to enhance thermostability

  • Rational design based on structural comparisons with thermostable homologs

• Enhancing Catalytic Efficiency:

  • Modifying active site residues to improve substrate binding or catalysis

  • Engineering the consensus sequence region around S141, considering its role in regulating enzyme activity through phosphorylation

  • Altering substrate specificity through mutations in the binding pocket

• Creating Soluble Variants:

  • Removing or modifying membrane-anchoring domains

  • Fusion with solubility-enhancing domains

  • Surface engineering to reduce hydrophobic patches

• Developing Biosensors:

  • Introducing fluorescent protein domains at positions that undergo conformational changes during catalysis

  • Creating FRET-based sensors for detecting substrate binding

  • Engineering allosteric sites for synthetic regulation

• Experimental Approaches:

  • Structure-guided site-directed mutagenesis

  • Directed evolution with appropriate selection strategies

  • Domain swapping with related enzymes

  • Computational design followed by experimental validation

Structural insights from archaeal DPMS crystal structures provide valuable templates for understanding how lipid binding couples to movements of interface helices, metal binding, and acceptor loop dynamics . This information can guide the engineering of DPM1 variants with modified regulatory properties or substrate preferences.

How should researchers interpret differences in kinetic parameters between DPM1 expressed in different systems?

The search results indicate significant differences in kinetic parameters between DPM1 expressed in different systems. Researchers should consider several factors when interpreting these differences:

• Post-translational Modifications:

  • S. cerevisiae DPM1 can be phosphorylated at S141, which enhances activity 6-fold

  • Different expression systems (bacterial, yeast, insect cell) have varying capabilities for post-translational modifications

  • Analyze phosphorylation state using phospho-specific antibodies, mass spectrometry, or 32P-labeling

• Protein Folding and Conformation:

  • Native expression in yeast may yield more properly folded protein

  • Compare secondary structure using circular dichroism

  • Assess thermal stability profiles using differential scanning fluorimetry

• Lipid Environment:

  • Native membrane composition differs between expression systems

  • Test enzyme activity with various lipid compositions

  • Consider detergent effects on enzyme properties

• Experimental Data Interpretation:

  • DPM1 from yeast shows higher Km (685 ± 47 μM) versus bacterial expression (110 ± 8 μM)

  • Vmax is higher for yeast-expressed enzyme (5.2 ± 0.1 nmol/min vs. 4.0 ± 0.1 nmol/min)

  • These differences suggest fundamental alterations in substrate binding and catalytic efficiency

• Experimental Validation:

  • Compare enzymes under identical reaction conditions

  • Normalize for protein purity and active site concentration

  • Consider using multiple assay methods to confirm observations

When reporting kinetic parameters, researchers should clearly specify the expression system used and provide detailed methodology to enable proper comparison between studies .

What statistical approaches are most appropriate for analyzing DPM1 activity data?

Appropriate statistical approaches for analyzing DPM1 activity data include:

For DPM1 activity data specifically, consider:

• Accounting for the salt-dependency of enzyme activity

• Analyzing the relationship between phosphorylation state and activity

• Comparing multiple substrates or inhibitors systematically

• Using appropriate controls when studying recombinant variants

What are the key considerations when designing experiments to compare wild-type and mutant DPM1 proteins?

When designing experiments to compare wild-type and mutant DPM1 proteins, researchers should consider:

• Mutation Selection Strategy:

  • Target specific functional domains (e.g., S141A in the phosphorylation consensus sequence)

  • Create mutations equivalent to disease-causing variants in human homologs

  • Use evolutionary conservation analysis to identify critical residues

  • Consider both loss-of-function and gain-of-function mutations

• Expression and Purification Controls:

  • Express and purify wild-type and mutant proteins in parallel

  • Verify protein integrity by SDS-PAGE, Western blot, and mass spectrometry

  • Confirm proper folding using circular dichroism or fluorescence spectroscopy

  • Quantify protein concentration using multiple methods

• Comprehensive Functional Characterization:

  • Determine full kinetic parameters (Km, Vmax, kcat, kcat/Km) for all variants

  • Test multiple substrate concentrations and conditions

  • Assess regulatory properties (e.g., phosphorylation response, salt dependency)

  • Examine membrane association and lipid interactions

• In Vivo Validation:

  • Complementation assays in DPM1-deficient yeast strains

  • Phenotypic analysis (growth, morphology, stress response)

  • Cell wall integrity assessment using microscopy and CSH assays

  • Glycosylation profiling using lectin binding or mass spectrometry

• Structural Analysis:

  • Computational modeling of mutation effects

  • If possible, crystallographic or NMR studies

  • Hydrogen-deuterium exchange mass spectrometry

  • Molecular dynamics simulations

• Control Experiments:

  • Include positive and negative controls for each assay

  • Use catalytically inactive variants as controls

  • Consider testing multiple mutations at each site of interest

Example experimental design from the literature shows that S141A DPM1 mutant exhibited more than half-a-fold reduction in catalytic activity compared with wild-type when both were analyzed after in vitro phosphorylation, confirming that serine 141 is indeed a target for cAMP-dependent protein phosphorylation .