Recombinant Candida albicans Chitin synthase export chaperone (CHS7)

What is the primary function of Candida albicans CHS7?

CHS7 in Candida albicans functions as a specialized chaperone that facilitates the proper folding and transport of chitin synthase 3 (CHS3) from the endoplasmic reticulum (ER) to the Golgi apparatus. Unlike general chaperones, CHS7 belongs to the Shr3-like family of specialized chaperones that prevent the aggregation of specific plasma membrane proteins at the ER level. CHS7 does not remain in the ER after CHS3 exit but forms a complex with CHS3 that enables its proper folding and subsequent activity in the plasma membrane . The deletion of CHS7 results in CHS3 accumulation in the ER, demonstrating its essential role in chitin synthase transport through the secretory pathway .

How does CHS7 differ from other chaperone proteins in Candida albicans?

CHS7 belongs to a specialized group of ER chaperone-like transmembrane proteins known as "Shr3-like" proteins, which in Saccharomyces cerevisiae includes Shr3, Pho86, Gsf2, and Chs7 . Unlike general chaperones such as heat shock proteins (HSPs), CHS7 does not interact with a broad range of cargo proteins and lacks conserved domains common to other chaperone families . Its specificity is demonstrated by its selective interaction with CHS3, facilitating its transport through the secretory pathway. While HSPs like the 70-kDa CaHsp70 in C. albicans function as immunodominant proteins with roles in stress response and immunomodulation , CHS7 has a much more specialized function in cell wall biogenesis through chitin synthase trafficking.

What protein domains characterize CHS7, and how do they relate to its function?

CHS7 is characterized by multiple transmembrane domains that anchor it to the ER membrane. Similar to other Shr3-like proteins, CHS7 likely utilizes its hydrophilic C-terminal domain to associate with COPII coatomer subunits, facilitating CHS3 transport through the secretory pathway . This mechanism resembles that of Shr3, which uses its C-terminal domain to interact with COPII components for amino acid permease transport. The transmembrane regions of CHS7 are crucial for its interaction with the transmembrane domains of CHS3, enabling proper folding and preventing aggregation of CHS3 in the ER. Unlike heat shock proteins that contain ATPase domains for their chaperone activity, CHS7's chaperone function appears to be independent of ATP hydrolysis, relying instead on direct protein-protein interactions with its specific cargo.

What are the optimal approaches for expressing and purifying recombinant CHS7?

For expressing recombinant CHS7, a bacterial expression system using E. coli may not be optimal due to the multiple transmembrane domains of CHS7, which often lead to protein aggregation and inclusion body formation. Instead, eukaryotic expression systems such as Pichia pastoris or S. cerevisiae are recommended, as they provide the necessary ER machinery for proper protein folding. When using yeast expression systems, consider employing strong inducible promoters like GAL1 or AOX1, coupled with a C-terminal epitope tag (6xHis or FLAG) for purification that doesn't interfere with the N-terminal signal sequence.

For purification, a two-step approach is most effective: first, isolate membrane fractions using ultracentrifugation after cell lysis, then solubilize the membrane proteins using mild detergents such as n-dodecyl-β-D-maltoside (DDM) or digitonin, which preserve protein structure and function. Affinity chromatography using the C-terminal tag can then be performed, followed by size exclusion chromatography to enhance purity. To verify proper folding and function, co-immunoprecipitation assays with CHS3 can confirm whether the recombinant CHS7 retains its native binding capacity.

How can researchers effectively generate and characterize CHS7 deletion mutants?

Generation of CHS7 deletion mutants in C. albicans requires careful consideration of this organism's diploid nature. The recommended approach involves homologous recombination using a disruption cassette containing a selectable marker (such as URA3 or NAT1) flanked by sequences homologous to regions upstream and downstream of the CHS7 gene. Due to C. albicans' diploid nature, sequential deletion of both alleles is necessary, often using different selectable markers.

Characterization of Δchs7 mutants should involve multiple approaches:

• Molecular verification: PCR confirmation of gene deletion and Southern blotting to verify correct integration.

• Chitin content analysis: Quantification using calcofluor white staining and fluorescence microscopy, or biochemical determination of chitin levels.

• Cell wall integrity assessment: Growth assays in the presence of cell wall stressors (Congo red, calcofluor white, SDS).

• Subcellular localization studies: Immunofluorescence or GFP-tagging of CHS3 to determine its localization in the Δchs7 background.

• Morphological analysis: Examination of hyphal formation, cellular morphology, and growth rate.

Complementation with the wild-type CHS7 gene should restore the wild-type phenotype, confirming the specificity of the observed effects .

What methods are most effective for analyzing CHS7-CHS3 interactions in vivo and in vitro?

For analyzing CHS7-CHS3 interactions, a multi-faceted approach yields the most comprehensive results:

In vivo methods:

• Bimolecular Fluorescence Complementation (BiFC): By tagging CHS7 and CHS3 with complementary fragments of a fluorescent protein, their interaction in living cells can be visualized.

• Co-immunoprecipitation (Co-IP): Using epitope-tagged versions of CHS7 and CHS3, their interaction can be detected after cell lysis under conditions that preserve protein-protein interactions.

• Fluorescence Resonance Energy Transfer (FRET): By tagging CHS7 and CHS3 with appropriate fluorophore pairs, their proximity and interaction can be monitored in living cells.

In vitro methods:

• Pull-down assays: Using recombinant versions of both proteins, one with an affinity tag, to capture and detect the interaction partner.

• Surface Plasmon Resonance (SPR): To determine binding kinetics and affinity constants between purified CHS7 and CHS3.

• Isothermal Titration Calorimetry (ITC): For quantitative analysis of binding thermodynamics.

For mapping the specific interaction domains, a series of truncation or point mutants of both proteins should be generated and tested using the above methods. To determine if the interaction is direct or mediated by other factors, in vitro binding assays with purified components are essential. Additionally, crosslinking studies followed by mass spectrometry can identify specific residues involved in the interaction interface.

How does phosphorylation or other post-translational modifications regulate CHS7 activity?

Post-translational modifications (PTMs) of CHS7 likely play crucial roles in regulating its chaperone activity and interaction with CHS3. Mass spectrometry analysis of purified CHS7 can identify phosphorylation sites and other modifications such as glycosylation or ubiquitination. Based on studies of related chaperones, phosphorylation of CHS7 may regulate its binding affinity for CHS3 or its interaction with COPII components.

To investigate the functional significance of identified phosphorylation sites, site-directed mutagenesis can be employed to generate phosphomimetic (serine/threonine to glutamate) or phosphodeficient (serine/threonine to alanine) mutants. These mutants can then be assessed for their ability to facilitate CHS3 transport and chitin synthesis. Kinase inhibitor studies or kinase deletion mutants can help identify the specific kinases responsible for CHS7 phosphorylation.

For other PTMs, similar approaches can be used: mutation of modified residues followed by functional assessment. Additionally, studying CHS7 PTMs under different stress conditions (cell wall stress, heat shock, hyphal induction) may reveal condition-specific regulatory mechanisms that control chitin synthesis in response to environmental cues.

What is the relationship between CHS7 and the unfolded protein response in Candida albicans?

The unfolded protein response (UPR) is likely intimately connected with CHS7 function, as ER stress affects protein folding and transport. Research in related fungal systems suggests that the UPR regulates chaperone expression and function. To investigate this relationship, researchers should:

• Analyze CHS7 expression levels under UPR-inducing conditions (treatment with tunicamycin, dithiothreitol, or deletion of key UPR components like IRE1 or HAC1).

• Determine if CHS7 is transcriptionally regulated by the UPR using promoter reporter assays and chromatin immunoprecipitation to detect Hac1 binding to the CHS7 promoter.

• Examine whether UPR activation affects CHS7-mediated transport of CHS3 by monitoring CHS3 localization under ER stress conditions.

Evidence from Neurospora crassa indicates that ER stress induced by tunicamycin and dithiothreitol causes changes in the subcellular distribution of chitin synthase export proteins and their cargo, supporting the hypothesis that these chaperones function as ER proteins integrated with the UPR pathway . In C. albicans, this relationship may be particularly relevant during host-pathogen interactions, where the fungus experiences various stresses that might trigger the UPR and consequently affect cell wall remodeling through CHS7-mediated processes.

How does CHS7 coordinate with other chitin synthase export chaperones in fungal cell wall biogenesis?

The coordination of CHS7 with other chitin synthase export chaperones represents a complex network controlling fungal cell wall architecture. While S. cerevisiae possesses a single CHS export receptor, filamentous fungi like Neurospora crassa have evolved multiple copies, such as CSE-7 and CSE-8 . This evolutionary divergence suggests specialized functions for different chitin synthase-chaperone pairs.

To investigate this coordination, researchers should:

• Generate double or triple knockout mutants of different chitin synthase export chaperones to identify compensatory mechanisms or synthetic phenotypes.

• Perform transcriptomic and proteomic analyses to determine if the expression of one chaperone is altered when others are deleted.

• Conduct co-localization studies to determine if different chaperones operate in distinct ER subdomains or if they co-localize during specific developmental stages.

In N. crassa, CSE-7 is specifically required for the localization of CHS-4 (a class IV chitin synthase) at the Spitzenkörper and septa, while CSE-8 is needed for CHS-3 (a class I chitin synthase) export . The double mutant Δcse-7;Δcse-8 shows reduced N-acetylglucosamine content and decreased radial growth compared to single mutants, indicating cooperative or additive functions . Similar specialization likely exists in C. albicans, where different chitin synthases contribute to distinct aspects of cell wall formation during yeast, hyphal, and biofilm growth.

How does CHS7 deletion affect Candida albicans virulence in different infection models?

Deletion of CHS7 in C. albicans results in attenuated virulence, as demonstrated in various infection models . This attenuation is likely multifactorial, stemming from altered cell wall composition, morphogenetic defects, and potentially modified host-pathogen interactions. In systemic infection models, Δchs7 mutants typically show reduced fungal burden in kidneys, liver, and brain compared to wild-type strains, correlating with increased survival rates in infected animals.

The impact on virulence can be assessed using:

• Murine models of disseminated candidiasis, tracking fungal burden, inflammatory markers, and survival rates

• In vitro macrophage infection assays, measuring phagocytosis rates and intracellular survival

• Reconstituted human epithelial infection models to assess adhesion, invasion, and tissue damage

• Galleria mellonella larval models as a rapid virulence screening system

The reduced virulence likely stems from multiple factors:

• Decreased chitin content affecting cell wall integrity under host stress conditions

• Altered exposure of β-glucans and mannans, modifying host immune recognition

• Defects in hyphal formation, reducing tissue invasion capacity

• Potential changes in adhesin display affecting host cell attachment

These findings highlight CHS7 as a potential antifungal target, as its inhibition could reduce virulence without directly affecting fungal viability, potentially reducing selective pressure for resistance development.

What is the relationship between CHS7 function and antifungal drug susceptibility?

The relationship between CHS7 function and antifungal susceptibility is complex and clinically relevant. Since CHS7 affects cell wall composition through its role in chitin synthesis, its deletion or inhibition likely influences susceptibility to cell wall-targeting antifungals. Researchers should investigate this relationship through:

• Minimum inhibitory concentration (MIC) determination for Δchs7 mutants against various antifungal classes:

  • Echinocandins (caspofungin, micafungin): May show increased sensitivity due to compromised cell wall integrity

  • Azoles (fluconazole, voriconazole): May show altered sensitivity due to membrane-cell wall interface changes

  • Polyenes (amphotericin B): May show modified sensitivity due to altered membrane exposure

• Time-kill kinetics to assess the rate of fungal killing by antifungals

• Checkerboard assays to identify potential synergistic effects between CHS7 inhibitors and existing antifungals

• Studies on the development of antifungal resistance in wild-type versus Δchs7 backgrounds

The altered chitin content in Δchs7 mutants likely affects the compensatory cell wall remodeling that normally occurs in response to echinocandin treatment, potentially enhancing the efficacy of these drugs. Conversely, reduced chitin content might make the cell more dependent on β-glucan for structural integrity, possibly increasing susceptibility to echinocandins. This relationship underscores the potential of CHS7 as a target for combination therapy approaches in treating resistant Candida infections.

How does CHS7 function differ between commensal and pathogenic states of Candida albicans?

The functional differences in CHS7 between commensal and pathogenic states of C. albicans represent an important aspect of this organism's adaptability. During the commensal-to-pathogen transition, significant remodeling of the cell wall occurs, suggesting potential changes in CHS7 activity or regulation. To investigate these differences:

• Compare CHS7 expression levels between commensal models (gastrointestinal colonization) and infection models (systemic or mucosal infection) using qRT-PCR and immunoblotting.

• Analyze CHS7 subcellular localization in commensal versus pathogenic states using fluorescently tagged proteins, as changes in localization may correlate with functional shifts.

• Examine post-translational modifications of CHS7 under different host conditions, as these may regulate its activity depending on the fungal lifestyle.

• Assess the CHS7-CHS3 interaction strength and dynamics during commensalism versus active infection using co-immunoprecipitation and FRET approaches.

During commensalism, CHS7 likely functions primarily in maintaining normal cell wall architecture for environmental adaptation. In contrast, during pathogenesis, its activity may be upregulated to facilitate rapid cell wall remodeling required for immune evasion, adherence to host tissues, and morphological transitions. Understanding these functional differences could reveal host environmental cues that trigger CHS7 regulatory changes during the transition to pathogenicity, potentially identifying targets for preventing this transition.

How do CHS7 homologs differ among pathogenic Candida species and other fungi?

CHS7 homologs show notable diversity among pathogenic Candida species and other fungi, reflecting evolutionary adaptations to different ecological niches and pathogenic strategies. This diversity is particularly evident when comparing unicellular yeasts with filamentous fungi. While Saccharomyces cerevisiae possesses a single CHS export receptor (Chs7), filamentous fungi like Neurospora crassa have multiple orthologs, such as CSE-7 and CSE-8 .

Comparative analysis reveals:

• Sequence conservation is typically higher in the transmembrane domains and C-terminal regions involved in COPII interactions, while greater divergence is seen in other regions.

• Filamentous fungi-specific Shr3-like chaperones like CSE-8 have evolved to facilitate the export of class I chitin synthases, which are absent in S. cerevisiae .

• Pathogenic Candida species show CHS7 adaptations that correlate with their host range and infection strategies.

Phylogenetic analysis indicates that the duplication of chitin synthase export chaperones in filamentous fungi likely occurred early in ascomycete evolution, enabling the development of more complex hyphal growth forms. In N. crassa, CSE-7 is specifically required for CHS-4 (class IV) localization, while CSE-8 facilitates CHS-3 (class I) export , demonstrating functional specialization after gene duplication. This specialization suggests that pathogenic fungi with complex morphologies may have evolved more sophisticated chitin synthase trafficking networks compared to predominantly unicellular yeasts.

What can structural comparisons between fungal CHS7 and related mammalian proteins reveal about potential drug targets?

Structural comparisons between fungal CHS7 and related mammalian proteins can reveal unique features that might be exploited for antifungal development. Although mammals do not synthesize chitin, the Shr3-like family of proteins to which CHS7 belongs has functional analogs in mammals that facilitate protein trafficking through the secretory pathway.

Key structural comparison approaches include:

• Homology modeling of CHS7 based on crystallized structures of related proteins, focusing on transmembrane topology and cargo-binding domains.

• Molecular dynamics simulations to identify fungal-specific binding pockets or conformational states absent in mammalian counterparts.

• Analysis of the CHS7-CHS3 interaction interface to identify fungal-specific contacts that could be targeted without affecting host proteins.

Potential drug target characteristics from these comparisons include:

• Fungal-specific transmembrane arrangements or lipid interaction sites

• Unique cargo recognition domains that have no counterpart in mammalian trafficking chaperones

• COPII interaction motifs with fungal-specific features

• Post-translational modification sites that regulate CHS7 activity in a fungal-specific manner

These structural differences could guide the design of small molecules that selectively disrupt CHS7 function without affecting host protein trafficking. The ideal candidates would interfere with the CHS7-CHS3 interaction or prevent CHS7-mediated recruitment of COPII components, thereby inhibiting chitin synthesis without directly targeting chitin synthase enzymes, potentially circumventing existing resistance mechanisms.

How has the evolution of CHS7 contributed to fungal adaptation to different ecological niches?

The evolution of CHS7 has played a significant role in fungal adaptation to diverse ecological niches, particularly through its impact on cell wall architecture and plasticity. Phylogenetic analyses reveal that chitin synthase export chaperones have undergone substantial diversification, especially in filamentous fungi that occupy complex environmental niches.

Several evolutionary patterns are noteworthy:

• Gene duplication events have led to specialized chaperones for different chitin synthase classes, as seen in Neurospora crassa with CSE-7 and CSE-8 . This specialization allows for more precise regulation of cell wall composition in response to environmental challenges.

• Sequence divergence in cargo-binding domains reflects adaptation to specific chitin synthases that have evolved for particular functions (e.g., hyphal tip growth, septation, stress response).

• Regulatory elements in CHS7 promoters show niche-specific patterns, suggesting adaptation to different environmental triggers.

The functional consequences of these evolutionary changes include:

• Enhanced ability to remodel the cell wall during host-pathogen interactions

• Specialized cell wall structures for different morphological states

• Optimized chitin deposition patterns for particular growth habits (yeast, pseudohyphal, or true hyphal forms)

In N. crassa, the Δcse-8 knockout strain showed disrupted sexual development, with 20% of perithecia from homozygous crosses exhibiting two ostioles . This phenotype illustrates how chitin synthase trafficking has been integrated into complex developmental processes during fungal evolution. Similarly, the reduced N-acetylglucosamine content and decreased radial growth in the Δcse-7;Δcse-8 double mutant demonstrate the cumulative importance of specialized chitin synthase export pathways in filamentous growth .