Recombinant Arabidopsis thaliana DUF21 domain-containing protein At1g55930, chloroplastic (CBSDUFCH2)

What is the DUF21 domain-containing protein At1g55930 and where is it localized in Arabidopsis thaliana?

The DUF21 domain-containing protein At1g55930 (CBSDUFCH2) is a chloroplastic precursor protein identified in the model plant Arabidopsis thaliana. As indicated by its name, this protein contains a Domain of Unknown Function 21 (DUF21), which represents a conserved protein domain whose specific biochemical function has not been fully characterized. The protein is encoded by the gene located at locus At1g55930 (ORF name: F14J16.20) in the Arabidopsis genome .

The protein is localized primarily in chloroplasts, as indicated by its chloroplastic precursor designation. This localization is significant because it suggests the protein may be involved in chloroplast-specific functions such as photosynthesis, plastid development, or chloroplast signaling pathways. The chloroplastic targeting is likely facilitated by an N-terminal transit peptide that directs the protein to this organelle after synthesis in the cytoplasm.

How are DUF21 domain-containing proteins classified and what protein families are they associated with?

DUF21 domain-containing proteins are classified based on their conserved domain architecture. In addition to the DUF21 domain, the CBSDUFCH2 protein also contains CBS domains (Cystathionine Beta-Synthase domains), as indicated by its name. CBS domains are known to bind adenosine-containing molecules such as AMP, ATP, and S-adenosylmethionine, suggesting a potential regulatory role.

The protein classification can be summarized in the following table:

The association of DUF21 and CBS domains suggests that this protein may function in metabolic regulation or stress response pathways, potentially responding to cellular energy status through the CBS domains while executing specific functions via the DUF21 domain.

What evolutionary conservation patterns are observed in DUF21 domain-containing proteins across plant species?

DUF21 domain-containing proteins show significant evolutionary conservation across plant species, particularly among angiosperms. Recent research on homologous proteins in watermelon (Citrullus lanatus) and cucumber (Cucumis sativus) demonstrates functional conservation of these proteins across different plant families .

Comparative genomic analyses indicate that DUF21 domain-containing proteins emerged early in plant evolution and have been maintained through selective pressure, suggesting important biological functions. The conservation extends beyond sequence similarity to functional conservation, as demonstrated by similar phenotypic effects observed when homologous genes are disrupted in different plant species.

For instance, studies in watermelon identified ClDUF21 as regulating plant height by modulating the brassinosteroid synthesis pathway through interaction with ClDWF1 . Similar functionality was observed with the homologous gene CsDUF21 in cucumber, suggesting conserved mechanistic pathways across Cucurbitaceae family members . This functional homology provides valuable insights for understanding the potential roles of the Arabidopsis CBSDUFCH2 protein.

What are the recommended methods for recombinant expression and purification of CBSDUFCH2?

For successful recombinant expression and purification of CBSDUFCH2, a multi-step approach is recommended based on established protocols for chloroplastic proteins:

• Expression System Selection: For plant chloroplastic proteins like CBSDUFCH2, Escherichia coli expression systems typically offer good yields. The BL21(DE3) strain is recommended due to its deficiency in lon and ompT proteases, reducing degradation of the recombinant protein. Alternative expression systems include insect cells (Sf9, High Five) or yeast (Pichia pastoris) if post-translational modifications are crucial.

• Vector Design Considerations:

  • Remove the chloroplast transit peptide sequence from the expression construct to improve solubility

  • Include a cleavable affinity tag (His6, GST, or MBP) to facilitate purification

  • Consider codon optimization for the expression host

  • Design constructs with and without the CBS domains to assess their contribution to function and solubility

• Expression Optimization:

  • Test multiple induction temperatures (16°C, 25°C, and 37°C)

  • Vary IPTG concentrations (0.1 mM to 1.0 mM)

  • Consider auto-induction media for higher yields

  • Include molecular chaperones (GroEL/GroES, DnaK) to improve folding

• Purification Strategy:

  • Initial capture via affinity chromatography (IMAC for His-tagged proteins)

  • Intermediate purification using ion exchange chromatography

  • Final polishing with size exclusion chromatography

  • Consider on-column refolding if the protein forms inclusion bodies

The methodological approach should be tailored to the specific research questions. For structural studies, higher purity is essential, while for functional assays, higher yields with moderate purity may be sufficient. All purification steps should be performed at 4°C with appropriate protease inhibitors to minimize degradation of this plant protein.

What experimental design approaches are most effective for investigating protein-protein interactions involving CBSDUFCH2?

Investigating protein-protein interactions involving CBSDUFCH2 requires a multi-technique approach to generate robust and reliable data. Based on recent advances in protein interaction studies, the following experimental design is recommended:

• Primary Screening Methods:

  • Yeast Two-Hybrid (Y2H): Effective for initial screening of potential interactors. Construct CBSDUFCH2 as both bait (DNA-binding domain fusion) and prey (activation domain fusion) to capture different interaction contexts. Screen against Arabidopsis cDNA libraries or specific candidate proteins .

  • Split-Ubiquitin System: More suitable than classical Y2H if CBSDUFCH2 exhibits autoactivation or is targeted to membranes within the cell.

• In Vitro Validation:

  • Pull-down Assays: Using recombinant CBSDUFCH2 as bait to capture interactors from plant extracts.

  • Surface Plasmon Resonance (SPR): For quantitative binding kinetics with purified interaction partners.

  • Isothermal Titration Calorimetry (ITC): To determine thermodynamic parameters of interactions.

• In Vivo Confirmation:

  • Bimolecular Fluorescence Complementation (BiFC): To visualize interactions in plant cells and determine subcellular localization.

  • Co-immunoprecipitation (Co-IP): From plant tissues expressing tagged versions of CBSDUFCH2.

  • Förster Resonance Energy Transfer (FRET): For dynamic interaction studies in living cells.

• Systematic Approaches:

  • Tandem Affinity Purification coupled with Mass Spectrometry (TAP-MS): For identifying protein complexes containing CBSDUFCH2 in planta.

  • Proximity-Dependent Biotin Identification (BioID): To identify proteins in the vicinity of CBSDUFCH2 even if interactions are transient.

The experimental design should incorporate proper controls including:

• Non-interacting protein pairs as negative controls

• Known interacting protein pairs as positive controls

• Truncated versions of CBSDUFCH2 to map interaction domains

• Competitive binding assays to validate specificity

Based on findings from homologous proteins, particular attention should be given to potential interactions with brassinosteroid synthesis pathway components like DWF1 . The experimental approach should be systematically documented, with randomization of experimental treatments where appropriate, and replication to ensure statistical validity .

How can CRISPR/Cas9 gene editing be optimized for functional studies of CBSDUFCH2 in Arabidopsis thaliana?

CRISPR/Cas9 gene editing offers powerful approaches for functional characterization of CBSDUFCH2. Based on recent successful applications in plant systems, the following optimization strategy is recommended:

• sgRNA Design and Selection:

  • Design 3-5 sgRNAs targeting different exons of the At1g55930 gene

  • Prioritize targets within functionally critical domains (DUF21 domain, CBS domains)

  • Use plant-optimized scoring algorithms to select sgRNAs with high on-target and low off-target potential

  • Consider the GC content (40-60%) for optimal sgRNA efficiency

• Vector System Selection:

  • Use a plant-optimized CRISPR/Cas9 vector system (e.g., pHEE401E for Arabidopsis)

  • Consider egg cell-specific promoters for the Cas9 expression to increase heritable mutation rates

  • Include a visible marker gene (e.g., GFP, RFP) for easier screening of transformants

• Transformation and Screening Strategy:

  • Apply the floral dip method for Arabidopsis transformation

  • Screen T1 plants for successful transformation using appropriate selection markers

  • Analyze T2 generations for homozygous and heterozygous mutations

  • Use T3 or later generations for phenotypic analyses to ensure stable inheritance

• Mutation Verification Methods:

  • Initial screening by PCR and restriction enzyme digestion (if the target site contains a restriction site)

  • Confirm mutations by Sanger sequencing

  • For complex edits, use next-generation sequencing

  • Validate functional consequences at protein level by western blotting

• Advanced Modification Strategies:

  • Generate precise point mutations in functional domains using base editors

  • Create conditional knockouts using tissue-specific promoters

  • Design knockin constructs for protein tagging at endogenous loci

  • Consider multiplexed editing to target redundant genes simultaneously

Based on research with homologous genes in watermelon, special attention should be paid to phenotypic analysis of growth parameters, particularly plant height and internode elongation, as DUF21 domain-containing proteins have been implicated in plant dwarfism regulation . Additionally, analyze brassinosteroid-related phenotypes and responses to brassinosteroid treatments, as homologous proteins interact with the brassinosteroid synthesis pathway .

What roles do DUF21 domain-containing proteins play in plant growth and development based on current evidence?

Current evidence suggests that DUF21 domain-containing proteins play critical roles in plant growth and development, particularly in the regulation of plant architecture and stature. Recent research has provided significant insights into their functions:

The most compelling evidence comes from studies in watermelon (Citrullus lanatus) where a DUF21 domain-containing protein (ClDUF21) was identified as a key regulator of plant height . Knockout mutants of ClDUF21 generated through CRISPR/Cas9-mediated gene editing displayed pronounced dwarfing phenotypes, characterized by shortened internodes and compact plant architecture . This dwarfing effect was consistently observed across independent mutant lines, confirming the causal relationship between ClDUF21 and plant height regulation.

Mechanistically, ClDUF21 was found to interact directly with ClDWF1, a key enzyme in the brassinosteroid biosynthesis pathway . Brassinosteroids are essential plant hormones that promote cell elongation and division, and disruption of their biosynthesis typically results in dwarfism. The protein-protein interaction between ClDUF21 and ClDWF1 suggests that DUF21 domain-containing proteins modulate plant growth by regulating brassinosteroid levels or signaling.

This function appears to be evolutionarily conserved, as similar phenotypes and molecular interactions were observed when the homologous gene CsDUF21 was edited in cucumber (Cucumis sativus) . The functional conservation across different plant species suggests that the Arabidopsis CBSDUFCH2 may play analogous roles in growth regulation.

Beyond height regulation, the involvement in hormone pathways suggests that DUF21 domain-containing proteins may influence multiple developmental processes controlled by brassinosteroids, including seed germination, vascular differentiation, reproductive development, and senescence.

How does the chloroplastic localization of CBSDUFCH2 relate to its potential functions?

The chloroplastic localization of CBSDUFCH2 provides important clues about its potential functions and positions this protein at the intersection of several critical plant processes:

The intersection of chloroplastic localization with the growth-regulating functions observed for homologous proteins suggests that CBSDUFCH2 may serve as an integrator that links photosynthetic activity and energy status to growth regulation via hormone pathways. This hypothesis is supported by the increasing recognition of chloroplasts as hubs for integration of environmental signals and growth responses in plants.

What evidence exists for post-translational modifications of CBSDUFCH2 and how might they regulate its function?

According to the iPTMnet database, CBSDUFCH2 (At1g55930) has been identified as having a substrate role in post-translational modifications (PTMs), suggesting that it undergoes one or more types of modifications . This provides important insights into potential regulatory mechanisms controlling this protein's function.

The specific types of PTMs identified for CBSDUFCH2 are not detailed in the provided search results, but typical PTMs in chloroplastic proteins include:

• Phosphorylation: Often occurs on serine, threonine, or tyrosine residues and can activate or inactivate protein function, affect protein-protein interactions, or alter subcellular localization. In chloroplastic proteins, phosphorylation frequently regulates photosynthetic processes and stress responses.

• Acetylation: Modification of lysine residues that can influence protein stability, enzymatic activity, or protein-protein interactions. Chloroplast proteins are increasingly recognized as targets for acetylation, affecting both metabolic enzymes and structural proteins.

• Redox Modifications: Including disulfide bond formation and glutathionylation, which are particularly relevant in the oxidizing environment of the chloroplast during photosynthesis. These modifications can serve as molecular switches in response to changing redox conditions.

• Ubiquitination: May mark chloroplastic proteins for degradation or regulate their activity through non-degradative mechanisms.

The regulatory implications of PTMs on CBSDUFCH2 function likely include:

• Temporal Regulation: PTMs could activate or inactivate the protein in response to diurnal cycles or developmental stages.

• Spatial Regulation: Modifications might affect the sub-organellar localization within the chloroplast.

• Conditional Activation: PTMs may serve as molecular switches that activate the protein only under specific stress conditions.

• Pathway Integration: Modifications could integrate signals from multiple cellular pathways, positioning CBSDUFCH2 as a node in complex regulatory networks.

The presence of CBS domains in CBSDUFCH2 is particularly significant in this context, as these domains are known targets for regulatory phosphorylation in other proteins, affecting their ability to bind adenosine-containing molecules and respond to energy status changes.

How can high-throughput phenotyping approaches be applied to characterize CBSDUFCH2 mutants in Arabidopsis?

High-throughput phenotyping (HTP) offers powerful approaches for comprehensive characterization of CBSDUFCH2 mutants, enabling quantitative assessment of subtle phenotypic changes across multiple parameters simultaneously. The following methodological framework is recommended based on current advances in plant phenomics:

• Automated Growth Monitoring Systems:

  • Deploy time-lapse imaging platforms to track growth dynamics throughout the plant life cycle

  • Implement specialized growth chambers with integrated imaging systems (RGB, fluorescence, hyperspectral) for continuous phenotyping

  • Apply machine learning algorithms for automated image analysis and feature extraction

  • Experimental design should include at least 20-30 biological replicates per genotype arranged in randomized block designs to control for microenvironmental variations

• Morphometric Analysis Protocol:

  • Primary measurements: rosette size, leaf number, leaf shape, internode length, plant height, branching patterns

  • Secondary traits: flowering time, silique development, seed yield

  • Advanced parameters: leaf angle, stem thickness, root architecture

  • Apply principal component analysis to identify the most discriminative morphological features between wild-type and CBSDUFCH2 mutants

• Physiological Phenotyping:

  • Chlorophyll fluorescence imaging to assess photosynthetic efficiency (Fv/Fm, ΦPSII)

  • Thermal imaging to evaluate transpiration and water use efficiency

  • Gas exchange measurements for photosynthetic capacity and respiration rates

  • Implement automated watering and weighing systems to monitor water consumption

• Stress Response Characterization:

  • Parallel phenotyping under multiple stress conditions (drought, heat, cold, nutrient limitation)

  • Quantify stress resilience through automated survival and recovery assessment

  • Monitor dynamic responses through time-course experiments with high temporal resolution

  • Design factorial experiments to test interactions between genotype and environmental variables

• Data Integration and Analysis:

  • Implement standardized data storage and management systems

  • Apply multivariate statistical analyses to identify correlated phenotypic traits

  • Develop comprehensive phenotypic fingerprints for mutant characterization

  • Correlate phenotypic data with transcriptomic and metabolomic profiles for systems-level understanding

Given the involvement of homologous DUF21 proteins in brassinosteroid pathways and plant architecture regulation , specialized phenotyping assays should focus on brassinosteroid-related phenotypes and responses to brassinosteroid treatments, including hypocotyl elongation in darkness, petiole length, and leaf bending assays.

What transcriptomic approaches can reveal the regulatory networks associated with CBSDUFCH2 function?

To elucidate the regulatory networks associated with CBSDUFCH2 function, a comprehensive transcriptomic investigation using multiple complementary approaches is recommended:

• Differential Expression Analysis in Mutant Lines:

  • Compare gene expression profiles between CBSDUFCH2 knockout/knockdown lines and wild-type plants

  • Implement RNA-seq with at least 3-4 biological replicates per condition

  • Analyze multiple tissues (leaves, roots, stems) and developmental stages

  • Include time-course analyses to capture dynamic responses

  • Apply stringent statistical thresholds (adjusted p-value < 0.05, fold change > 1.5) for identifying differentially expressed genes (DEGs)

• Conditional Transcriptomics:

  • Analyze transcriptional responses under conditions that trigger DUF21 protein function

  • Include treatments with brassinosteroids and brassinosteroid biosynthesis inhibitors

  • Examine responses to different light conditions, considering the chloroplastic localization

  • Investigate stress conditions that might reveal conditional functions

• Cell-Type Specific Expression Profiling:

  • Apply isolation techniques for chloroplast-containing cells (mesophyll, guard cells)

  • Implement INTACT (Isolation of Nuclei TAgged in specific Cell Types) or FANS (Fluorescence-Activated Nuclear Sorting) methods

  • Consider single-cell RNA-seq to resolve cell-type heterogeneity

• Network Analysis Methods:

  • Weighted Gene Co-expression Network Analysis (WGCNA) to identify modules of co-regulated genes

  • Apply causal network inference algorithms to predict regulatory relationships

  • Integrate transcription factor binding site analyses to identify potential direct regulators

  • Compare with known brassinosteroid-responsive gene networks

  • Develop network visualization to map CBSDUFCH2 within the broader regulatory landscape

• Integrative Approaches:

  • Combine transcriptomics with ChIP-seq data if CBSDUFCH2 has potential DNA-binding capabilities

  • Correlate expression changes with metabolomic profiles, particularly brassinosteroid-related metabolites

  • Integrate with protein-protein interaction data to link transcriptional changes with physical interactions

  • Develop multi-omics models to predict system-wide effects of CBSDUFCH2 perturbation

The experimental design should include proper controls for developmental stage, time of day, and environmental conditions to minimize noise from unrelated variables . Analysis should focus particular attention on genes involved in brassinosteroid synthesis and signaling pathways, chloroplast function, and plant architecture regulation, as these represent the most likely functional connections based on studies of homologous proteins .

How can structural biology approaches be applied to understand the function of the DUF21 domain in CBSDUFCH2?

Structural biology approaches offer powerful insights into protein function, particularly for domains like DUF21 whose functions remain poorly characterized. A comprehensive structural investigation of CBSDUFCH2 should include:

The structural information obtained should be integrated with functional data to develop mechanistic models. Based on findings from homologous proteins, particular attention should be paid to structural features that might facilitate interaction with brassinosteroid synthesis enzymes like DWF1 , as well as potential binding sites for adenosine-containing molecules in the CBS domains.

How can knowledge about CBSDUFCH2 contribute to improving crop stress tolerance and yield?

Understanding the functions of CBSDUFCH2 and related DUF21 domain-containing proteins has significant potential for crop improvement strategies, particularly in the areas of stress tolerance and yield enhancement:

• Crop Architecture Optimization:
  Based on the role of DUF21 proteins in regulating plant height through brassinosteroid pathways , targeted modification of homologous genes in crop species could create varieties with optimized architecture. Semi-dwarf varieties with reduced internode length but maintained photosynthetic capacity have historically driven significant yield increases in cereals like rice and wheat. Precise editing of DUF21 domain-containing genes could generate semi-dwarf phenotypes without the negative effects sometimes associated with mutations in other brassinosteroid pathway components.

• Abiotic Stress Tolerance Enhancement:
  The chloroplastic localization of CBSDUFCH2 and the presence of CBS domains suggests potential roles in energy sensing and stress responses. Modulating the expression or activity of DUF21 proteins could enhance:

  • Drought tolerance through optimized water use efficiency

  • Heat stress resilience by maintaining chloroplast function under elevated temperatures

  • Cold tolerance through regulation of membrane fluidity and photosynthetic efficiency

  • Nutrient use efficiency by optimizing growth based on available resources

• Photosynthetic Efficiency Improvement:
  As a chloroplast-localized protein potentially involved in photosynthesis regulation, CBSDUFCH2 homologs could be targets for improving:

  • Carbon fixation efficiency

  • Light harvesting optimization

  • Photoprotection mechanisms

  • Source-sink relationships within the plant

• Molecular Breeding Applications:

  • Develop molecular markers based on natural variation in DUF21 genes for marker-assisted selection

  • Identify superior alleles of DUF21 genes in crop germplasm collections

  • Create targeted gene edits that fine-tune growth responses without compromising yield

  • Stack optimized DUF21 alleles with other beneficial traits

• Future Research Priorities:
  To fully realize the potential applications, future research should focus on:

  • Characterizing DUF21 protein function across diverse crop species

  • Determining how variation in these genes correlates with agronomic traits

  • Investigating potential trade-offs between altered plant architecture and yield components

  • Developing precision breeding strategies that target specific regulatory mechanisms

The dual role of DUF21 proteins in growth regulation and potential stress response makes them particularly valuable targets for crop improvement, as they may allow simultaneous enhancement of both stress resilience and yield potential, addressing a common trade-off in plant breeding.

What are the emerging technologies that could advance research on CBSDUFCH2 and related proteins?

Several cutting-edge technologies are poised to significantly advance our understanding of CBSDUFCH2 and related DUF21 domain-containing proteins in the coming years:

• Genome Editing Innovations:

  • Prime Editing: Enables precise nucleotide substitutions without double-strand breaks, allowing subtle modifications to regulatory regions or coding sequences of CBSDUFCH2 with reduced off-target effects.

  • Base Editing: Facilitates C→T or A→G conversions without DNA cleavage, ideal for creating specific amino acid changes to probe structure-function relationships.

  • CRISPR-Cas Systems Beyond Cas9: Alternative systems like Cas12a (Cpf1) offer different PAM requirements and cutting patterns, potentially expanding targetable regions in the CBSDUFCH2 gene.

  • Multiplexed Genome Editing: Simultaneous modification of multiple genes within DUF21 family to address functional redundancy challenges.

• Advanced Imaging Technologies:

  • Super-Resolution Microscopy: Techniques like PALM, STORM, or STED could reveal subcellular dynamics of CBSDUFCH2 within chloroplasts at nanometer resolution.

  • Live-Cell Imaging with Split Fluorescent Proteins: Enables visualization of protein-protein interactions in real-time within living plant cells.

  • Correlative Light and Electron Microscopy (CLEM): Combines functional information from fluorescence microscopy with ultrastructural context from electron microscopy.

  • Label-Free Imaging Approaches: Raman microscopy and second harmonic generation imaging provide chemical and structural information without potentially disruptive tags.

• Protein Structure and Interaction Analysis:

  • AlphaFold and Related AI Systems: Deep learning approaches for accurate protein structure prediction, particularly valuable for challenging proteins like CBSDUFCH2.

  • Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS): Maps protein dynamics, conformational changes, and interaction interfaces with high sensitivity.

  • Crosslinking Mass Spectrometry (XL-MS): Identifies spatial relationships between proteins in complexes, helping map interaction networks.

  • Time-Resolved Structural Methods: Techniques like time-resolved crystallography or cryo-EM to capture dynamic structural changes during function.

• Single-Cell and Spatial Omics:

  • Single-Cell Transcriptomics: Reveals cell-type specific expression patterns and responses.

  • Spatial Transcriptomics: Maps gene expression patterns within tissues with spatial resolution.

  • Single-Cell Proteomics: Emerging techniques for protein profiling at cellular resolution.

  • Metabolite Imaging: Maps distribution of metabolites (e.g., brassinosteroids) with spatial information.

• Systems Biology Integration:

  • Multi-Omics Data Integration Platforms: Computational approaches to integrate transcriptomic, proteomic, metabolomic, and phenomic data.

  • Network Inference Algorithms: Advanced methods to reconstruct regulatory networks involving CBSDUFCH2.

  • Digital Twin Modeling: In silico plant models incorporating molecular-level information to predict phenotypic outcomes of genetic modifications.

  • Federated Learning Approaches: Collaborative analysis of distributed datasets across research institutions to accelerate discovery.

These emerging technologies will collectively enable more precise, comprehensive, and integrative studies of CBSDUFCH2 function, potentially revealing unexpected roles and regulatory mechanisms that could inform both fundamental understanding and applied crop improvement strategies.