Recombinant Saccharomyces cerevisiae Cell division control protein 50 (CDC50)

What is the molecular structure and function of CDC50 in S. cerevisiae?

CDC50 (YCR094W) is an endosomal transmembrane protein containing 391 amino acids (1173 nucleotides) located on chromosome III of S. cerevisiae. It functions primarily as a regulatory subunit for the phospholipid flippase Drs2p, where the interaction is essential for Drs2p catalytic activity . The protein contains multiple transmembrane domains and appears to be solubilized from membranes only through detergent treatment, confirming its strong membrane association .

Structurally, CDC50 belongs to a family with strong similarity to two other S. cerevisiae proteins encoded by YNR048w and YNL323w. These proteins demonstrate functional redundancy, as overexpression of either can suppress the cdc50-1 mutation phenotype .

How does CDC50 mutation affect cellular phenotypes in yeast?

The cdc50Δ null mutant exhibits several distinct phenotypes:

• Cold-sensitive growth arrest at 14°C with a small bud phenotype

• G1 phase arrest at START at non-permissive temperatures

• Depolarization of cortical actin patches and mislocalization of Myo5p

• Disappearance of actin cables

• Mislocalization of Bni1p and Gic1p (effectors of Cdc42p small GTPase)

• Defects in late-stage endocytosis (but not in the internalization step)

• Modest defects in vacuolar protein sorting

• Abnormal large membranous structures and aggregations of small 30-40nm vesicles

• Aberrant vacuolar morphology

These phenotypes collectively suggest CDC50's critical role in maintaining proper cell polarity, membrane organization, and vesicular trafficking.

What are the established protein interactions of CDC50?

CDC50 participates in several important protein interactions:

These interactions place CDC50 at the intersection of membrane organization, vesicular trafficking, and cytoskeletal regulation pathways .

What methodologies are most effective for studying CDC50's role in phospholipid flippase activity?

To effectively study CDC50's role in phospholipid flippase activity, researchers should consider these methodological approaches:

• Subcellular Fractionation and Extraction:

  • Prepare crude extracts from midlog cells expressing tagged CDC50 (e.g., CDC50-HA)

  • Lyse cells by agitation with glass beads in appropriate lysis buffer containing protease inhibitors

  • Separate extracts into P13/S13 fractions by centrifugation at 13,000 × g

  • Further separate S13 into P100/S100 fractions at 100,000 × g

  • For extraction experiments, treat precleared extracts with 0.5M NaCl, 2.5M urea, 0.1M sodium carbonate, 1% Triton X-100, or lysis buffer

  • Analyze fractions by immunoblotting

• Phospholipid Translocation Assays:

  • Use fluorescently labeled phospholipid analogs (e.g., NBD-PS, NBD-PE) to track flipping activity

  • Compare wild-type, cdc50Δ, and complemented strains for differences in phospholipid asymmetry

  • Analyze by flow cytometry to quantify differences in membrane composition

• Protein-Protein Interaction Analysis:

  • Perform co-immunoprecipitation experiments to identify CDC50-Drs2p complex formation

  • Use yeast two-hybrid assays to map interaction domains

  • Apply proximity-dependent biotin identification (BioID) to identify transient interactions

These approaches provide complementary data on CDC50's molecular function in phospholipid translocation and membrane organization.

How can researchers differentiate between direct and indirect effects of CDC50 on the actin cytoskeleton?

Distinguishing direct from indirect effects of CDC50 on actin organization requires a multi-faceted approach:

• Temporal Analysis of Actin Defects:

  • Employ a temperature-sensitive CDC50 allele for rapid inactivation

  • Track actin changes at short time intervals post-shift (5, 10, 15, 30 min)

  • Primary (direct) effects should manifest before secondary consequences

• Protein Domain Analysis:

  • Generate truncation or point mutations in specific CDC50 domains

  • Identify mutations that separate membrane trafficking functions from actin organization functions

  • Complement with domains from other CDC50 family members to identify functional regions

• Bypass Suppression Experiments:

  • Test whether constitutively active versions of actin regulatory proteins (e.g., Arp2/3 activators) can bypass CDC50 function

  • Determine if artificial tethering of actin regulators to endosomal membranes rescues cdc50Δ phenotypes

• Biochemical Interaction Studies:

  • Perform direct binding assays between purified CDC50-Drs2p complex and actin/actin regulatory proteins

  • Investigate if lipid asymmetry changes directly affect actin binding proteins

These methodologies can help dissect the complex relationship between CDC50, membrane organization, and actin cytoskeleton regulation .

What experimental approaches resolve contradictory data regarding CDC50's role in transcriptional regulation versus membrane trafficking?

Several experimental approaches can help resolve the dual roles of CDC50:

• Genetic Separation of Function:

  • Generate a comprehensive library of CDC50 mutants through site-directed mutagenesis

  • Screen for mutants that affect either membrane trafficking or transcriptional regulation but not both

  • Characterize these mutants through detailed phenotypic analysis

• Synthetic Genetic Array Analysis:

  • Cross cdc50Δ with genome-wide deletion collection

  • Identify genetic interactions that cluster with either membrane trafficking or transcriptional regulation networks

  • Perform epistasis analysis with CDC39 and membrane trafficking components

• Chromatin Association Studies:

  • Conduct ChIP-seq to determine if CDC50 associates with chromatin

  • Compare transcriptome changes (RNA-seq) between cdc50Δ and wild-type cells

  • Determine if CDC50's effects on transcription are direct or consequences of altered membrane trafficking

• Domain-Specific Complementation:

  • Create chimeric proteins swapping domains between CDC50 and its paralogs

  • Determine which domains rescue transcriptional versus trafficking defects

  • Engineer CDC50 variants with altered subcellular localization to separate functions

The fact that CDC39 (a component of the CCR4-NOT complex involved in transcription regulation) acts as a multicopy suppressor of cdc50-1 suggests potential transcriptional regulatory functions, which need to be distinguished from CDC50's established membrane trafficking roles .

How can CDC50 be used to study membrane lipid asymmetry in eukaryotic cells?

CDC50 provides an excellent model system for investigating membrane lipid asymmetry:

• Visualization of Phospholipid Distribution:

  • Use fluorescent lipid probes combined with confocal microscopy to compare wild-type and cdc50Δ cells

  • Apply lipid-binding domains fused to fluorescent proteins to track specific phospholipids

  • Quantify differences in phosphatidylserine exposure using Annexin V binding assays

• Mass Spectrometry-Based Lipidomics:

  • Analyze lipid composition changes in purified membrane fractions from wild-type and cdc50Δ cells

  • Use heavy isotope-labeled lipids to track flipping rates in reconstituted systems

  • Compare lipid distributions in inner versus outer leaflets using membrane-impermeable chemical labeling

• Reconstitution Experiments:

  • Purify recombinant CDC50-Drs2p complex for incorporation into artificial liposomes

  • Measure ATP-dependent lipid flipping activity using fluorescence quenching assays

  • Vary lipid composition to determine substrate specificity

• Correlative Phenotype Analysis:

  • Connect specific membrane asymmetry defects to cellular phenotypes through systematic mutation analysis

  • Develop reporter systems for lipid asymmetry disruption in living cells

These approaches would provide comprehensive insights into how CDC50-dependent lipid asymmetry affects cellular functions .

What protocols are most effective for generating and validating recombinant CDC50 protein?

Generating functional recombinant CDC50 protein requires careful consideration of its membrane protein nature:

• Expression System Selection:

  • Evaluate prokaryotic systems (E. coli) for tagged fragments of CDC50

  • Consider eukaryotic expression systems (yeast, insect cells) for full-length protein

  • Use cell-free translation systems with added microsomes for proper membrane insertion

• Purification Strategy:

  • Utilize mild detergents (DDM, LMNG) for membrane protein extraction

  • Apply affinity chromatography (His-tag) followed by size exclusion chromatography

  • Consider amphipol or nanodisc reconstitution for increased stability

• Functional Validation Methods:

  • Test interaction with recombinant Drs2p using pull-down assays

  • Assess incorporation into liposomes and lipid flipping activity

  • Verify proper folding using circular dichroism and limited proteolysis

• Structural Characterization:

  • Apply cryo-electron microscopy for structural analysis of CDC50-Drs2p complex

  • Use hydrogen-deuterium exchange mass spectrometry to map interaction interfaces

  • Perform crosslinking mass spectrometry to determine proximity relationships

Several groups have successfully expressed recombinant CDC50 protein, such as the His-tagged full-length (1-391) S. cerevisiae CDC50 protein in E. coli systems .

How does CDC50 contribute to fungal drug resistance mechanisms?

CDC50 plays significant roles in fungal drug resistance through several mechanisms:

• Membrane Permeability Regulation:

  • The cdc50Δ mutant shows hypersensitivity to azoles (fluconazole) and echinocandins (caspofungin)

  • Increased drug sensitivity correlates with altered plasma membrane integrity

  • Enhanced drug penetration occurs in the absence of CDC50

• Cell Wall Integrity Pathway Modulation:

  • Loss of CDC50 leads to constitutive upregulation of the cell wall integrity (CWI) pathway

  • Cell wall composition changes include increased:

    • Mannan content (14.5% higher)

    • Chitin content (39% higher)

    • β-(1,3)-glucan content (44.3% higher)

  • These alterations correlate with hypersensitivity to cell wall stressors

• Stress Response Regulation:

  • The cdc50Δ mutant shows severe hypersensitivity to Congo Red and Calcofluor White

  • Increased sensitivity to SDS suggests enhanced membrane permeability

  • These phenotypes indicate compromised stress adaptation mechanisms

In Cryptococcus neoformans, CDC50 deletion leads to pronounced caspofungin sensitivity, demonstrating conservation of this drug resistance mechanism across fungal species. These findings suggest CDC50 could be a potential target for sensitizing resistant fungi to existing antifungals .

What methodological approaches can identify CDC50-dependent pathways in cell polarity?

To identify CDC50-dependent pathways in cell polarity, researchers should consider:

• Live Cell Imaging Techniques:

  • Apply time-lapse microscopy to track polarity markers in wild-type versus cdc50Δ cells

  • Use fluorescent fusion proteins to monitor:

    • Actin cytoskeleton components (Abp1-GFP, Lifeact-RFP)

    • Polarity establishment proteins (Cdc42-GFP, Bem1-GFP)

    • Vesicular trafficking markers (FM4-64, GFP-Snc1)

  • Quantify polarization defects through computational image analysis

• Phosphoproteomic Analysis:

  • Compare phosphorylation profiles between wild-type and cdc50Δ cells

  • Focus on polarity-related kinase targets (Cdc28, Cbk1, Gin4)

  • Identify differential phosphorylation events in response to polarity cues

• Synthetic Genetic Interaction Mapping:

  • Generate double mutants between cdc50Δ and polarity genes

  • Screen for synthetic lethality or suppression

  • Map genetic interaction networks to position CDC50 within polarity pathways

• Molecular Bypass Approaches:

  • Test if constitutively active forms of polarity proteins (Cdc42-G12V) rescue cdc50Δ phenotypes

  • Express tethered versions of polarity factors to bypass potential CDC50-dependent localization

These approaches would help uncover how CDC50-dependent membrane asymmetry contributes to the establishment and maintenance of cell polarity, which is disrupted in cdc50 mutants as evidenced by depolarized actin patches and mislocalized polarity factors .

What controls should be included when analyzing CDC50 mutant phenotypes at different temperatures?

Proper experimental design for CDC50 temperature-sensitive phenotype analysis should include:

• Essential Controls:

  • Wild-type strain cultured under identical conditions

  • CDC50 complemented strain (cdc50Δ + CDC50) to confirm phenotype rescue

  • Temperature pre-shift controls to establish baseline phenotypes

  • Time-course analysis after temperature shift (e.g., 0, 2, 4, 8, 24 hours)

• Strain Background Considerations:

  • Use isogenic strains to minimize background effects

  • Include related mutants (e.g., drs2Δ) to distinguish shared versus unique phenotypes

  • Test CDC50 paralogs (YNR048w, YNL323w) deletion mutants for comparison

• Phenotype Documentation Methods:

  • Growth curves at different temperatures (30°C vs. 18°C or 14°C)

  • Microscopic analysis of cell morphology and budding patterns

  • Actin and cell wall staining (Phalloidin, Calcofluor White)

  • Membrane trafficking assays (FM4-64 uptake, CPY sorting)

• Experimental Validation Approaches:

  • Use multiple independent clones of each strain

  • Apply both plate-based and liquid culture growth assays

  • Employ quantitative phenotype measurements rather than qualitative observations

As demonstrated in published studies, cdc50Δ mutants show pronounced growth defects during exponential phase at lower temperatures, but may reach similar optical densities as wild-type after extended incubation periods (24h) .

How can researchers design experiments to study redundant functions between CDC50 and its paralogs?

To effectively study functional redundancy between CDC50 and its paralogs (YNR048w and YNL323w), researchers should:

• Generate and Characterize Multiple Mutant Combinations:

  • Create single, double, and triple deletion mutants

  • Examine phenotypic severity progression across mutant combinations

  • Focus on conditions where single mutants show mild defects but double/triple mutants show severe phenotypes

• Domain Swap Experiments:

  • Create chimeric proteins swapping domains between CDC50 and its paralogs

  • Test complementation efficiency of different domains

  • Identify regions responsible for shared versus unique functions

• Differential Expression Analysis:

  • Compare expression patterns of CDC50 and paralogs across growth conditions

  • Identify conditions where specific family members are upregulated

  • Test if artificial expression of one member can compensate for another's absence

• Specific Functional Readouts:

  • Measure lipid flippase activity in different mutant backgrounds

  • Assess protein-protein interaction profiles of each family member

  • Determine subcellular localization patterns of each protein

Previous research has demonstrated that double disruption of either CDC50 and YNR048w or CDC50 and YNL323w results in severe slow-growth phenotypes compared to single mutants, confirming functional redundancy within this family .