Recombinant Arabidopsis thaliana Putative RING-H2 finger protein ATL35 (ATL35)

What is ATL35 and what experimental evidence confirms its E3 ubiquitin ligase activity?

ATL35 (encoded by At4g09110) belongs to the extensive ATL family of RING-H2 finger proteins that function as E3 ubiquitin ligases in Arabidopsis thaliana. Like other ATL family members, ATL35 contains a characteristic RING-H2 domain that is essential for its E3 ligase activity .

Experimentally confirming E3 ligase activity typically requires:

• In vitro ubiquitination assays using recombinant ATL35 protein, E1 (ubiquitin-activating), E2 (ubiquitin-conjugating) enzymes, and ubiquitin

• Detection of polyubiquitin chains by western blotting

• Mutational analysis of key cysteine and histidine residues in the RING-H2 domain to abolish E3 activity

Similar to demonstrated activity in ATL9, researchers can confirm ATL35's E3 ligase activity through assays showing that disruption of the RING domain abolishes ubiquitination activity .

What structural domains characterize ATL35 and how do they compare to other ATL family members?

ATL35 contains multiple conserved domains characteristic of the ATL family:

The RING-H2 domain in ATL35 follows the consensus sequence C-X2-CL-X-E-X7-R-X2-P-X-C-X-H-X-FH-X2-C-X-D-X-W-X6-CP-X-C, where X represents any amino acid . This domain contains eight zinc-coordinating residues arranged in a C3HC4 pattern that is crucial for E3 ligase activity .

When comparing ATL35 to other family members, the RING-H2 domain shows the highest conservation, while other regions may vary, reflecting functional specialization .

What experimental systems are available for studying ATL35 protein expression and purification?

To study recombinant ATL35 protein, researchers can employ several expression systems:

• E. coli-based expression:

  • BL21(DE3) strain expressing His-tagged ATL35 (32-302 amino acids)

  • Induction with IPTG at optimal temperature (typically 18-25°C to enhance solubility)

  • Purification using Ni-NTA affinity chromatography

• Plant-based expression:

  • Transient expression in Nicotiana benthamiana

  • Stable transformation in Arabidopsis with epitope tags (FLAG, HA, GFP)

  • Native promoter vs. constitutive promoter (35S) constructs

• Cell-free expression systems:

  • Wheat germ extracts for rapid protein production

  • Useful for proteins that aggregate in bacterial systems

Key methodological considerations include:

• Using only the mature protein (residues 32-302) without transmembrane domains improves solubility

• Adding 6% trehalose as a stabilizing agent during lyophilization

• Storing aliquots at -80°C to minimize freeze-thaw cycles

How can researchers design experiments to identify specific substrates of ATL35?

Identifying E3 ubiquitin ligase substrates remains challenging. A multi-faceted approach is recommended:

• Yeast two-hybrid screening:

  • Use ATL35 as bait (consider using versions lacking transmembrane domains)

  • Library screening of Arabidopsis cDNAs

  • Validation through co-immunoprecipitation in planta

• Affinity purification coupled with mass spectrometry:

  • Express epitope-tagged ATL35 in Arabidopsis

  • Crosslink protein complexes prior to isolation

  • Identify interacting proteins by LC-MS/MS

  • Include proteasome inhibitors to stabilize ubiquitinated substrates

• Ubiquitination assays with candidate substrates:

  • Based on ATL31's targeting of 14-3-3 proteins, test related substrates with ATL35

  • Monitor substrate degradation kinetics in wild-type vs. atl35 mutants

  • Use proteasome inhibitors (MG132) to confirm proteasome-dependent degradation

• Global proteomics comparing wild-type and atl35 mutants:

  • Quantitative proteomics to identify proteins that accumulate in mutants

  • Focus on proteins showing increased ubiquitination in complemented lines

This integrated approach has successfully identified substrates for other ATL proteins, such as ABT1 for ATL5 and 14-3-3 proteins for ATL31 .

What are the optimal experimental designs for studying ATL35's role in stress responses?

To investigate ATL35's potential role in stress responses, a comprehensive experimental design should include:

• Expression analysis under stress conditions:

  • qRT-PCR to measure ATL35 transcript levels under various stresses

  • Compare with known stress-responsive ATLs (ATL2, ATL6, ATL78L, ATL12)

  • Test multiple timepoints to capture early and late responses

• Phenotypic characterization of genetic resources:

  • T-DNA insertion mutants

  • CRISPR/Cas9-generated knockout lines

  • Overexpression lines using 35S or native promoters

  • Complementation lines to confirm phenotypes

• Stress treatment experimental design:

  • Between-subjects factorial design with genotype and stress conditions as factors

  • Include appropriate controls for each genotype and treatment combination

  • Measure multiple physiological parameters (growth, ROS production, photosynthetic efficiency)

• Molecular phenotyping:

  • Transcriptome analysis (RNA-seq) comparing wild-type and mutant responses

  • Measurement of stress-induced hormone levels (SA, JA, ABA)

  • Quantification of ROS production using luminol-based assays or H2DCFDA fluorescence

• Statistical analysis:

  • ANOVA for factorial experiments with post-hoc tests

  • Time series analysis for temporal expression patterns

  • Multiple testing correction for transcriptome data

How does ATL35 compare to other ATL family members in terms of E2 enzyme specificity?

E2 enzyme specificity determines downstream ubiquitination patterns and substrate targeting. To characterize ATL35's E2 specificity:

• E2 binding assays:

  • Yeast two-hybrid with the RING-H2 domain as bait against E2 library

  • In vitro pull-down assays with purified components

  • Biolayer interferometry to measure binding kinetics

• E2 functional assays:

  • In vitro ubiquitination assays testing multiple E2 enzymes

  • Focus on members of the Ubc4/Ubc5 subfamily, which interact with other ATL proteins

  • Quantify ubiquitination activity with different E2 pairs

• Structural analysis:

  • Homology modeling based on solved structures like EL5 (rice ATL)

  • Identify key residues for E2 interaction

  • Site-directed mutagenesis to confirm functional importance

Most ATL proteins interact primarily with E2 enzymes of the Ubc4/Ubc5 subfamily. For ATL2, this interaction was confirmed through yeast complementation experiments where only members of the Arabidopsis Ubc4/Ubc5 subfamily could restore ATL2 function in yeast ubc4 mutants . Similar approaches could determine whether ATL35 shares this E2 specificity or has unique preferences.

What methodological approaches can resolve contradictions in ATL35 cellular localization data?

E3 ubiquitin ligases function in specific subcellular compartments. To resolve potential contradictions in localization data:

• Multiple complementary imaging techniques:

  • Confocal microscopy with fluorescent protein fusions

  • Immunogold electron microscopy for higher resolution

  • FRET/FLIM to detect protein-protein interactions in situ

• Domain-specific localization analysis:

  • Test transmembrane domains independently

  • Create chimeric proteins with domains from well-localized proteins

  • Use deletion constructs to identify localization signals

• Biochemical fractionation:

  • Differential centrifugation to isolate cellular compartments

  • Western blotting with compartment-specific markers

  • Protease protection assays to determine membrane topology

• Controls to prevent artifacts:

  • Multiple tag positions (N-terminal vs. C-terminal)

  • Confirmation of functionality of tagged proteins

  • Expression at near-native levels to avoid mislocalization

• Experimental design considerations:

  • Between-subjects design with multiple biological replicates

  • Within-subjects design for time-course studies of dynamic localization

  • Control for environmental factors affecting localization

Other ATL family members typically localize to the plasma membrane or endoplasmic reticulum through their hydrophobic domains . If contradictory results emerge for ATL35, consider examining localization under different stress conditions, as some E3 ligases relocalize upon activation.

How can researchers analyze the ubiquitination targets and patterns produced by ATL35?

To characterize the ubiquitination patterns catalyzed by ATL35:

• Ubiquitin chain linkage analysis:

  • In vitro ubiquitination with wild-type and lysine-mutant ubiquitins

  • Mass spectrometry to identify ubiquitin branch points

  • Linkage-specific antibodies for western blotting

• Substrate ubiquitination site mapping:

  • Site-directed mutagenesis of candidate lysine residues

  • Mass spectrometry of ubiquitinated substrates

  • Bioinformatic prediction of likely ubiquitination sites

• Functional consequences of different ubiquitination patterns:

  • Protein stability assays (cycloheximide chase)

  • Proteasome inhibitor experiments

  • Analysis of non-degradative ubiquitination functions

• Experimental design recommendations:

  • Include proper controls (inactive RING domain mutants)

  • Use ubiquitin-remnant profiling for proteome-wide analysis

  • Compare with known ATL family members (ATL31, ATL9, ATL5)

Studies of ATL5 demonstrated that it mediates polyubiquitination of its substrate ABT1, targeting it for degradation via the 26S proteasome . Similar approaches could determine whether ATL35 promotes degradative K48-linked polyubiquitination or other forms of ubiquitin modification.

What are the critical quality control steps for recombinant ATL35 protein production?

Ensuring high-quality recombinant ATL35 protein is essential for reliable results:

• Protein purity and integrity assessment:

  • SDS-PAGE with Coomassie staining (>90% purity recommended)

  • Western blotting with anti-His antibodies

  • Mass spectrometry to confirm intact protein

• Functional validation:

  • In vitro auto-ubiquitination assay

  • Circular dichroism to confirm proper folding

  • Dynamic light scattering to assess aggregation state

• Storage stability testing:

  • Freeze-thaw stability

  • Activity testing after storage at different temperatures

  • Use of stabilizers (trehalose, glycerol) for optimal preservation

• Batch consistency:

  • Establish standard operating procedures

  • Include positive controls in functional assays

  • Document lot-to-lot variation

For recombinant ATL35 produced in E. coli, researchers should reconstitute lyophilized protein in deionized sterile water to 0.1-1.0 mg/mL, add 5-50% glycerol as a cryoprotectant, create single-use aliquots, and store at -80°C to maintain activity .

How can researchers overcome challenges in experimental design when studying ATL35 in planta?

Plant experiments present unique challenges that require careful experimental design:

• Genetic material preparation:

  • Generate multiple independent transgenic lines

  • Select lines with comparable expression levels

  • Backcross T-DNA insertion lines to remove background mutations

• Experimental controls:

  • Include empty vector controls for overexpression studies

  • Use catalytically inactive versions (RING domain mutants)

  • Employ complementation with wild-type ATL35 to confirm phenotypes

• Statistical design considerations:

  • Use randomized complete block designs to control environmental variation

  • Calculate appropriate sample sizes through power analysis

  • Control for position effects in growth chambers/greenhouses

• Controlling confounding variables:

  • Standardize growth conditions (light, temperature, humidity)

  • Use plants of identical developmental stages

  • Consider circadian regulation of ubiquitination processes

• Measurement standardization:

  • Harvest tissues at consistent times of day

  • Establish clear phenotypic scoring criteria

  • Use internal controls for molecular analyses

What emerging technologies could advance our understanding of ATL35 function?

Novel methodologies that could deepen our understanding of ATL35 include:

• Proximity labeling methods:

  • BioID or TurboID fusions to identify transient interactors

  • Spatial mapping of ATL35 interaction networks

  • Identification of substrates that may be missed by traditional methods

• CRISPR-based technologies:

  • Base editing for introducing point mutations

  • CRISPRi/CRISPRa for tunable gene expression

  • Prime editing for precise genetic modifications

• Single-cell approaches:

  • Single-cell RNA-seq to capture cell-type specific responses

  • Spatial transcriptomics to map expression patterns

  • Single-molecule imaging of ATL35 dynamics

• Structural biology advances:

  • Cryo-EM structures of ATL35 with E2 enzymes and substrates

  • Hydrogen-deuterium exchange mass spectrometry

  • Integrative structural modeling

• Systems biology integration:

  • Multi-omics data integration

  • Network modeling of ATL35 within ubiquitination systems

  • Machine learning to predict new substrates and functions

These technologies could help resolve current knowledge gaps regarding ATL35's biological function, especially in the context of plant stress responses, where other ATL family members like ATL12 and ATL78L have demonstrated roles .

How can researchers design experiments to determine if ATL35 is involved in stress responses similar to other ATL family members?

To investigate potential stress response functions of ATL35:

• Transcriptional profiling:

  • Monitor ATL35 expression under diverse stress conditions

  • Compare with known stress-responsive ATLs (ATL2, ATL12, ATL78L)

  • Use qRT-PCR and promoter-reporter constructs

• Phenotypic characterization:

  • Test atl35 mutants under multiple stresses (cold, drought, pathogens)

  • Measure stress-specific physiological parameters

  • Compare with wild-type and complemented lines

• Molecular mechanisms:

  • Analyze hormone signaling pathway components (SA, JA, ABA)

  • Measure ROS production and scavenging

  • Test for NADPH oxidase regulation (as shown for ATL12)

• Experimental design recommendations:

  • Use factorial designs with multiple stress types/intensities

  • Include time-course measurements to capture dynamic responses

  • Control for developmental stage differences

Studies of other ATL family members provide models: ATL12 regulates chitin-induced ROS production through NADPH oxidases , ATL78L confers abiotic stress tolerance in tomato , and ATL31/ATL6 regulate C/N responses . Similar experimental approaches could reveal whether ATL35 shares these functions or has unique roles.