Recombinant Candida albicans mRNA export factor MEX67 (MEX67), partial

What is the structural composition of C. albicans MEX67 and how does it compare to homologs in other fungal species?

Methodologically, researchers investigating MEX67 structure should employ comparative structural analyses between C. albicans and other model organisms. X-ray crystallography has proven effective for resolving the MTR2-MEX67 domain complex . For regions not amenable to crystallization, alternative approaches like NMR spectroscopy or cryo-electron microscopy may be necessary to complete the structural characterization.

What are the established functional domains of MEX67 and their roles in mRNA export?

Based on studies in yeast systems, MEX67 contains several functional domains with specific roles:

• The NTF2-like domain mediates heterodimerization with MTR2, which is essential for nuclear export function .

• The ubiquitin-associated (UBA) domain plays a crucial role in proper nuclear export of mRNA and contributes significantly to the recruitment of MEX67 to transcribing genes .

• The UBA domain directly interacts with polyubiquitin chains and with Hpr1, a component of the THO/TREX complex that couples transcription to mRNA export .

To experimentally characterize these domains in C. albicans MEX67, researchers should create domain deletion or mutation constructs and assess their impacts on protein localization, interaction networks, and mRNA export efficiency. Chromatin immunoprecipitation (ChIP) experiments have successfully demonstrated MEX67 recruitment to actively transcribed genes in other systems .

How does the recruitment of MEX67 to transcribing genes occur in fungal cells?

MEX67 recruitment to actively transcribed genes occurs in a transcription-dependent manner, as demonstrated by ChIP experiments on galactose-inducible GAL10 and constitutively expressed PMA1 genes . The UBA domain of MEX67 plays a critical role in this process - deletion of this domain results in decreased cotranscriptional recruitment of MEX67 along transcribed genes .

The mechanism involves interaction between the UBA domain of MEX67 and Hpr1, a component of the THO complex . This interaction has been confirmed through multiple experimental approaches:

• Two-hybrid screening identified a 203-amino acid C-terminal fragment of Hpr1 that interacts with UBA-MEX67

• In vitro binding assays with recombinant proteins confirmed direct interaction

• Coimmunoprecipitation verified the interaction occurs in intact cells at physiological expression levels

This interaction transiently protects Hpr1 from ubiquitin/proteasome-mediated degradation, thereby coordinating recruitment of the mRNA export machinery with transcription and early mRNP assembly .

What expression systems are most effective for producing recombinant C. albicans MEX67?

For recombinant production of C. albicans MEX67, several expression systems have proven effective in related research:

• E. coli expression systems using BL21(DE3) strains with pET-derived vectors have been successfully employed for MEX67 and related proteins .

• Co-expression with MTR2 is recommended for proper folding and function, as these proteins form a heterodimeric complex that is essential for activity .

• For purification strategies, both His-tagged and GST-tagged fusion constructs have demonstrated success in pull-down experiments and structural studies .

When designing expression constructs, researchers should consider whether to express the full-length protein or specific domains (such as the NTF2-like domain or UBA domain) depending on the specific research question. For functional studies of MTR2-MEX67 interactions, recombinant GST-fusion proteins immobilized on glutathione beads have successfully demonstrated near-stoichiometric binding of partner proteins .

How does the interaction between MEX67 and the nuclear pore complex differ between fungal species, and what implications does this have for antifungal development?

The interaction between MEX67 and the nuclear pore complex (NPC) involves multiple contact points rather than a single predominant interaction site. In S. cerevisiae, MEX67 associates with the NPC through interactions with FG-repeat nucleoporins . Quantitative image analysis has shown that deletion of GLFG repeats in Nup116 has particularly significant effects on MEX67 localization .

Remarkably, MEX67 can perform its essential function even when permanently tethered to the NPC via Nup116, as demonstrated by fusion protein experiments . This finding has significant implications for antifungal development, suggesting that:

• The dynamic association-dissociation of MEX67 with the NPC might not be essential for its function

• Compounds that alter MEX67 mobility rather than completely inhibiting its activity could be effective

• Species-specific differences in MEX67-nucleoporin interactions could be exploited for selective targeting

Methodologically, researchers investigating these interactions should employ quantitative imaging approaches like nuclear rim intensity measurement (NuRIM) to precisely measure MEX67 localization at the nuclear envelope . Comparative studies between C. albicans and human systems could identify fungi-specific interactions as potential therapeutic targets.

What role does the ubiquitin-associated (UBA) domain play in MEX67 function, and how can this be experimentally investigated?

The UBA domain of MEX67 serves multiple critical functions beyond conventional mRNA export:

• It directly interacts with polyubiquitin chains and with Hpr1, a component of the THO/TREX complex .

• It contributes significantly to the recruitment of MEX67 to transcribing genes, as demonstrated by chromatin immunoprecipitation (ChIP) experiments .

• The interaction between UBA-MEX67 and Hpr1 transiently protects Hpr1 from ubiquitin/proteasome-mediated degradation .

• This mechanism coordinates recruitment of the mRNA export machinery with transcription and early mRNP assembly .

To experimentally investigate these functions in C. albicans MEX67, researchers should:

• Generate UBA domain deletion mutants and assess their impact on mRNA export through poly(A)+ RNA in situ hybridization

• Perform ChIP experiments to quantify recruitment to actively transcribed genes

• Conduct in vitro binding assays with recombinant UBA domain and C. albicans Hpr1

• Investigate ubiquitylation patterns of Hpr1 in the presence of wild-type versus UBA-deleted MEX67

• Assess whether the partial block in Hpr1 ubiquitylation results in defective mRNA nuclear export, as observed in other systems

How does Dbp5 interact with MEX67 to facilitate mRNA export, and how might this interaction be targeted therapeutically?

Dbp5, a DEAD-box RNA helicase, associates with RNA-bound MEX67 and Nab2 to facilitate mRNA export. This association occurs in an RNase-sensitive manner, indicating that RNA plays a role in mediating or stabilizing the interaction . Interestingly, the RNA-binding capacity of Dbp5 itself is not required for its association with MEX67, as demonstrated by experiments with the RNA-binding deficient dbp5 R369G variant .

This suggests a model where Dbp5 is recruited to MEX67-containing mRNPs through protein-protein interactions, positioning it to remodel these complexes during mRNA export. The interaction likely involves:

• Association of Dbp5 with MEX67 in RNA-bound complexes

• Remodeling of MEX67-mRNP interactions to facilitate directional transport

• Potential coordination with other export factors at the nuclear pore

To target this interaction therapeutically, researchers could:

• Identify the specific interaction interfaces between Dbp5 and MEX67

• Screen for small molecules that disrupt this interaction

• Develop compounds that alter the remodeling activity rather than blocking binding entirely

• Compare the interaction surfaces between fungal and human homologs to identify selective targeting opportunities

What experimental approaches can distinguish between direct and indirect effects when analyzing MEX67 mutant phenotypes?

Distinguishing direct from indirect effects in MEX67 functional studies presents a significant challenge. Several methodological approaches can help address this:

• Temporal analysis of phenotypes:

  • Use rapid depletion systems (e.g., auxin-inducible degron tags) to identify immediate versus secondary effects

  • Perform time-course experiments after MEX67 depletion to establish causality chains

  • Use metabolic labeling of newly synthesized RNA to track export defects with temporal precision

• Rescue experiments with domain-specific variants:

  • Complement MEX67 mutations with wild-type or domain-mutant alleles

  • Use the MEX67-Nup116 fusion system to test whether tethering to the NPC rescues specific phenotypes

  • Implement orthogonal systems (e.g., anchor-away) to confirm direct effects

• Targeted molecular approaches:

  • ChIP experiments can directly assess MEX67 recruitment to specific genes

  • RNA immunoprecipitation can identify directly bound transcripts

  • In situ hybridization for specific mRNAs can distinguish transcript-specific from global export defects

• Comparative analysis with partner protein mutants:

  • Compare phenotypes with MTR2 mutations, which should affect the same direct processes

  • Analyze effects of mutations in adaptor proteins like Yra1

  • Use partial loss-of-function alleles to establish dose-response relationships

What are the optimal purification strategies for recombinant C. albicans MEX67 that maintain its structural integrity and function?

Purification of functional recombinant C. albicans MEX67 requires careful consideration of protein folding, complex formation, and activity preservation:

• Expression system selection:

  • E. coli BL21(DE3) strains with pET vectors have shown success for MEX67 proteins

  • Co-expression with MTR2 is strongly recommended given their heterodimeric functional unit

  • Consider lower temperature induction (18-25°C) to promote proper folding

• Purification workflow:

  • Initial capture via affinity chromatography (Ni-NTA for His-tagged or glutathione for GST-tagged constructs)

  • Ion exchange chromatography to remove nucleic acid contaminants and improperly folded species

  • Size exclusion chromatography to ensure homogeneity and verify complex formation

  • Optional on-column nuclease treatment to remove bound nucleic acids

• Buffer optimization:

  • Include reducing agents (DTT or β-mercaptoethanol) to prevent oxidation of cysteine residues

  • Incorporate stabilizing agents such as glycerol (10-15%) to prevent aggregation

  • Test salt concentration ranges (150-300mM NaCl) to maintain solubility while preserving interactions

  • Consider nucleotide addition (ATP or non-hydrolyzable analogs) for stabilization

• Quality control assessments:

  • Verify homogeneity by dynamic light scattering

  • Assess secondary structure via circular dichroism spectroscopy

  • Confirm partner binding (MTR2, nucleoporins) through pull-down assays

  • Validate RNA-binding activity through electrophoretic mobility shift assays

How can researchers design functional assays to assess the mRNA export activity of recombinant C. albicans MEX67?

Functional assessment of recombinant MEX67 requires assays that reflect its roles in mRNA export:

• In vitro binding assays:

  • RNA binding through electrophoretic mobility shift assays or filter binding

  • Protein partner interactions via pull-down assays with recombinant MTR2, nucleoporins, and RNA adaptors

  • Ubiquitin chain binding assays for the UBA domain

  • Polyubiquitylated Hpr1 protection assays to assess UBA domain function

• Cell-based complementation assays:

  • Rescue of temperature-sensitive mex67 mutants in S. cerevisiae

  • Complementation of C. albicans conditional MEX67 mutants

  • Domain-swapping experiments to identify species-specific functional regions

• mRNA export assays:

  • Fluorescence in situ hybridization (FISH) for poly(A)+ RNA to visualize export defects

  • Nuclear/cytoplasmic fractionation followed by RT-qPCR for specific transcripts

  • Single-molecule RNA tracking in live cells to monitor export kinetics

• Recruitment assays:

  • Chromatin immunoprecipitation to assess recruitment to transcribing genes

  • Live-cell imaging with fluorescently tagged MEX67 to monitor dynamics at nuclear pores

  • Proximity labeling approaches to identify novel interaction partners in the native context

What techniques can be used to study the dynamics of MEX67 during the mRNA export process?

Advanced imaging and biochemical techniques can provide insights into MEX67 dynamics during export:

• Live-cell imaging approaches:

  • Fluorescence recovery after photobleaching (FRAP) to measure mobility at nuclear pores

  • Single-particle tracking of fluorescently labeled MEX67 to follow individual molecules

  • Förster resonance energy transfer (FRET) to detect conformational changes during export

• Quantitative localization studies:

  • Nuclear rim intensity measurement (NuRIM) for precise quantification of NPC association

  • Super-resolution microscopy to resolve sub-NPC localization

  • Single-molecule localization microscopy to determine spatial distribution at nanometer resolution

• Temporal coordination analysis:

  • RNA labeling with MS2 or PP7 systems combined with MEX67 tracking

  • Synchronized expression systems to follow newly synthesized transcripts

  • Multi-color imaging to correlate MEX67 dynamics with other export factors

• Structural dynamics:

  • Hydrogen-deuterium exchange mass spectrometry to identify regions with conformational flexibility

  • Crosslinking mass spectrometry to capture transient interactions

  • Time-resolved structural studies to capture different states of the export complex

How does C. albicans MEX67 differ from its counterparts in non-pathogenic yeasts, and how might these differences be exploited therapeutically?

Understanding the unique features of C. albicans MEX67 compared to non-pathogenic species offers potential for selective targeting:

What insights can be gained from studying MEX67 interactions with RNA adaptors across different fungal species?

MEX67 interacts with various RNA adaptor proteins to facilitate mRNA export. Comparative analysis of these interactions provides evolutionary and functional insights:

• Known adaptors from model systems:

  • Yra1/REF facilitates binding of MEX67 to mRNP through direct interaction

  • The UBA domain of MEX67 interacts with Hpr1 of the THO/TREX complex

  • Dbp5 associates with RNA-bound MEX67 in an RNase-sensitive manner

• Evolutionary conservation and divergence:

  • Core interactions with adaptors like Yra1 are likely conserved across fungi

  • Species-specific adaptors may exist in pathogenic fungi to facilitate specialized functions

  • Binding affinities and regulatory mechanisms may differ between species

• Functional implications:

  • Different adaptor usage may reflect adaptation to specific gene expression patterns

  • Pathogenic fungi may have evolved specialized adaptor interactions to support virulence gene expression

  • The regulation of adaptor interactions could be tuned to environmental conditions encountered during infection

• Experimental approaches for comparative studies:

  • Yeast two-hybrid screens to identify C. albicans-specific adaptors

  • Protein-protein interaction studies with recombinant proteins from different species

  • Cross-species complementation to test functional conservation

  • Structural studies of adaptor-binding interfaces across species

What methodological challenges exist in studying essential genes like MEX67 in the context of C. albicans pathogenesis?

Studying essential genes in C. albicans presents unique challenges due to its diploid and asexual nature :

• Genetic manipulation barriers:

  • The diploid nature requires modification of both alleles for complete gene deletion

  • Lack of conventional sexual cycles complicates genetic manipulation

  • Essential genes cannot be completely deleted without conditional systems

• Conditional expression strategies:

  • Tetracycline-regulatable promoters for controlled gene repression

  • Temperature-sensitive alleles for conditional inactivation

  • Degron-based systems for rapid protein depletion

  • CRISPR interference approaches for transcriptional repression

• Functional domain analysis approaches:

  • Structure-guided mutagenesis of specific domains while maintaining essential functions

  • Complementation with chimeric proteins containing domains from other species

  • Creation of separation-of-function mutations that affect specific activities

• Methodological considerations for in vivo studies:

  • Development of in vivo conditional systems that function during infection

  • Methods to recover and analyze fungal cells from infected tissues

  • Integration of virulence assays with molecular genetic approaches

What emerging technologies show promise for advancing our understanding of MEX67 function in C. albicans?

Several cutting-edge technologies offer new opportunities for MEX67 research:

• CRISPR-based approaches:

  • CRISPR interference for tunable repression of MEX67 expression

  • Base editing for precise introduction of point mutations

  • Prime editing for flexible gene modification

  • CRISPR screens to identify genetic interactions

• Advanced imaging technologies:

  • Super-resolution microscopy to visualize MEX67 localization at nanometer resolution

  • Single-molecule tracking to follow individual MEX67 molecules during export

  • Lattice light-sheet microscopy for high-speed 3D imaging with reduced phototoxicity

  • Correlative light and electron microscopy to link function with ultrastructural context

• Systems biology approaches:

  • RNA-seq combined with MEX67 depletion to identify affected transcripts

  • Proteomics to map the complete MEX67 interactome

  • Network analysis to position MEX67 in pathogenesis-related pathways

  • Multi-omics integration to understand system-wide effects of MEX67 dysfunction

• Structural biology advances:

  • Cryo-electron microscopy of MEX67 complexes in different functional states

  • Integrative structural biology combining multiple data types

  • Time-resolved structural studies to capture the dynamics of export

How might the understanding of MEX67 function contribute to novel antifungal therapeutic strategies?

MEX67's essential role in C. albicans makes it a promising antifungal target:

• Target validation approaches:

  • Genetic depletion studies to confirm essentiality under infection-relevant conditions

  • Phenotypic analysis of domain-specific mutants to identify critical functional regions

  • Identification of MEX67-dependent virulence pathways

• Drug development strategies:

  • Structure-based design targeting C. albicans-specific MEX67 features

  • High-throughput screening for compounds that disrupt MEX67 interactions

  • Fragment-based approaches to develop inhibitors of specific functional domains

  • Allosteric modulators that alter MEX67 dynamics rather than blocking function

• Therapeutic potential advantages:

  • Essential function makes resistance development less likely

  • Structural differences from human homologs could allow selective targeting

  • Multiple functional domains provide various targeting opportunities

  • Specificity could be achieved by targeting fungal-specific interaction partners

• Combination therapy opportunities:

  • MEX67 inhibitors could sensitize C. albicans to existing antifungals

  • Targeting different steps in mRNA processing and export pathways may provide synergy

  • Combining with host immune modulators could enhance clearance of compromised fungi