Recombinant Arabidopsis thaliana RING-H2 finger protein ATL63 (ATL63)

What is ATL63 and how is it classified within the ATL family?

ATL63 is a member of the ATL family of RING-H2 ubiquitin ligases in Arabidopsis thaliana. The ATL family consists of single-subunit RING finger E3 ubiquitin ligases characterized by a RING-H2 variation of the canonical RING finger domain and a transmembrane domain located at the N-terminal end. ATL63 shares specific sequence LOGOs with other ATL family members, particularly from conserved regions II and III, which distinguishes it within the broader family of 91 ATL members in A. thaliana .

The classification of ATL63 is based on phylogenetic analysis and motif organization. The ATL family has been classified into 9 distinct groups based on these criteria, with ATL63 sharing particularly close sequence homology with ATL2, especially in the glycine-leucine-aspartic acid (GLD) motif region .

What is the structural composition of the ATL63 protein?

The ATL63 protein contains several key structural elements that are characteristic of the ATL family:

• A RING-H2 domain with the specific arrangement of six cysteines and two histidines that coordinate zinc ligation

• A transmembrane domain at the N-terminal end

• A highly conserved GLD motif between the transmembrane domain and the RING-H2 domain

• An arginine-rich region (region III) that may be involved in E2 enzyme binding

The RING-H2 domain is a variation of the canonical RING finger where the fifth cysteine residue is replaced by a histidine. This domain contains a tryptophan residue spaced three residues downstream from the sixth zinc ligand, which is a highly conserved feature in most RING finger domains .

The following table compares the key structural features of ATL63 with other well-studied ATL family members:

Where is ATL63 localized in plant cells?

While specific localization studies for ATL63 have limited documentation in the provided search results, insights can be drawn from closely related ATL family members. Based on studies of ATL2, which shares significant sequence homology with ATL63, these proteins are predominantly localized to the plasma membrane.

Research methodologies to determine cellular localization include:

• Cell fractionation analysis: Western blot analysis of fractionated cell extracts from transgenic plants expressing tagged ATL proteins shows that these proteins are primarily found in the membrane fraction and undetectable in the soluble fraction .

• Live-cell confocal imaging: Studies using ATL proteins fused to GFP (Green Fluorescent Protein) and co-expressed with plasma membrane markers like AtPIP2A-mCherry demonstrate co-localization at the plasma membrane .

For researchers investigating ATL63 localization, it is recommended to use similar approaches, creating ATL63-GFP fusion constructs under an appropriate promoter and observing their expression in plant cells using confocal microscopy, alongside established membrane markers.

How is ATL63 gene expression regulated?

The regulation of ATL63 expression likely follows patterns similar to other well-characterized ATL family members. Based on studies of ATL2 and other ATLs:

• Pathogen-Associated Molecular Patterns (PAMPs) induction: Like ATL2, ATL63 expression may be rapidly and transiently induced by PAMPs such as chitin or cellulase preparations. The induction likely occurs within 15-30 minutes of exposure and may be independent of de novo protein synthesis .

• Defense hormone regulation: Expression may be modulated by defense hormones such as salicylic acid and jasmonic acid, as observed with ATL12 .

• MAPK cascade involvement: The expression may be linked to MAPK (Mitogen-Activated Protein Kinase) cascade activation, similar to ATL12 .

• Transcript stability control: ATL mRNAs typically have short half-lives, possibly regulated by DST elements within the 3′UTR, as predicted for ATL2 .

For studying ATL63 regulation, researchers should consider using promoter-GUS fusion constructs to monitor expression patterns in response to various elicitors and stress conditions, and RT-qPCR to quantify transcript levels at different time points following treatments.

What are the optimal conditions for expressing recombinant ATL63 in bacterial systems?

Based on successful expression strategies for other RING-H2 proteins and ATL family members, the following methodological approach is recommended for recombinant ATL63 expression:

Expression system selection:

• E. coli BL21(DE3) strain is recommended for T7 promoter-based expression systems

• For potentially toxic proteins like ATL63, consider using C41(DE3) or C43(DE3) strains, which are designed for expressing toxic proteins

• BL21-CodonPlus (RIL) strain may be beneficial for overcoming codon bias issues, especially for AT-rich plant genes

Expression vector design:

• Clone ATL63 into a vector with an N-terminal tag (His-tag recommended for purification)

• Consider using the pRSETA vector, which has been successful for ATL family proteins

• Include a BamHI site in the forward primer and an EcoRI site in the reverse primer to facilitate subcloning

Induction and growth conditions:

• Grow cultures to mid-exponential phase (OD600 = 0.4-0.6) before induction

• Induce with IPTG (typically 0.5-1.0 mM)

• Consider lower temperature induction (16-25°C) to improve protein folding

• Extend expression time to 4-16 hours post-induction

Purification strategy:

• Lyse cells using appropriate buffer containing protease inhibitors

• For membrane-associated proteins like ATL63, include detergents in the lysis buffer

• Purify using nickel-charged affinity chromatography for His-tagged proteins

• Consider a two-step purification process for higher purity

Activity assessment:

• Verify E3 ligase activity using in vitro ubiquitination assays

• Include appropriate E1, E2 enzymes, ubiquitin, and ATP in the reaction

• Use SDS-PAGE and western blotting to detect ubiquitination products

How can the E3 ubiquitin ligase activity of ATL63 be experimentally verified?

Verifying the E3 ubiquitin ligase activity of ATL63 requires several complementary approaches:

In vitro ubiquitination assay:

• Express and purify recombinant ATL63 with an appropriate tag (MBP or His)

• Set up reaction mixtures containing E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), ubiquitin, ATP, and purified ATL63

• Include appropriate negative controls (reactions without ATP, E1, E2, or with a mutated ATL63)

• Generate a mutant ATL63 where the critical cysteine in the RING-H2 domain is substituted with alanine (C→A mutation) as a negative control

• Analyze reactions by SDS-PAGE and western blotting using anti-ubiquitin antibodies to detect poly-ubiquitinated products

Based on studies with ATL2, the E3 ligase activity of ATL63 likely depends on a critical cysteine residue in the RING-H2 domain. For instance, in ATL2, the Cys138 residue was found to be essential for E3 ligase activity, and substituting this residue with alanine completely abolished activity .

E2 enzyme specificity determination:
The E3 ligase activity of ATL proteins typically relies on members of the Ubc4/Ubc5 subfamily of E2 conjugating enzymes. To determine which E2 enzymes work with ATL63:

• Test multiple E2 enzymes in parallel ubiquitination reactions

• Focus on members of the Ubc4/Ubc5 subfamily, as ATL2 function depends on Ubc4

• Perform yeast two-hybrid or pull-down assays to confirm direct interaction between ATL63 and candidate E2 enzymes

How can ATL63 function be analyzed through mutant studies?

Functional analysis of ATL63 can be accomplished through the following mutant-based approaches:

Loss-of-function analysis:

• Obtain T-DNA insertion lines for ATL63 from repositories like the Arabidopsis Biological Resource Center (ABRC)

• Confirm homozygous T-DNA insertions using PCR with gene-specific and T-DNA-specific primers

• Verify loss of ATL63 expression using RT-PCR and RT-qPCR

• Phenotype the mutants under various stress conditions, particularly focusing on pathogen challenges

• Compare disease susceptibility, ROS production, and defense gene expression between wild-type and mutant plants

Gain-of-function analysis:

• Generate ATL63 overexpression lines under the control of the constitutive CaMV35S promoter

• Select multiple independent transgenic lines with varying levels of ATL63 expression

• Analyze phenotypes related to growth, development, and especially pathogen resistance

• Challenge plants with fungal pathogens like Alternaria brassicicola or Golovinomyces cichoracearum and measure lesion size and pathogen growth

Domain-specific mutations:

• Create point mutations in key residues of the RING-H2 domain, particularly the zinc-coordinating cysteines

• Generate GLD motif variants to assess its functional importance

• Express these mutated versions in the atl63 mutant background to test for complementation

• Assess whether the mutated proteins retain E3 ligase activity in vitro

Based on studies with ATL12, researchers should expect that loss of ATL63 function might lead to increased susceptibility to fungal pathogens, while overexpression might enhance resistance .

What is the role of ATL63 in plant defense responses?

While specific information about ATL63's role in defense is limited in the provided search results, insights can be drawn from related ATL family members, particularly ATL2 and ATL12, which share structural similarities with ATL63.

Early defense signaling:
ATL63 likely plays a role in early defense signaling, as ATL family members are typically rapidly induced after exposure to pathogen-associated molecular patterns (PAMPs) like chitin. The expression of ATL2, for example, is induced within 15-30 minutes of chitin exposure .

Hormone-mediated defense pathways:
ATL63 may function at the intersection of salicylic acid (SA) and jasmonic acid (JA) signaling pathways, similar to ATL12, which is upregulated after treatment with both hormones .

NADPH oxidase-mediated ROS production:
Based on ATL12's function, ATL63 might regulate the expression of respiratory burst oxidase homolog proteins (RBOHs), which are crucial for reactive oxygen species (ROS) production during pathogen attack .

Methodological approaches to study ATL63's defense role:

• Pathogen challenge assays:

  • Challenge atl63 knockout mutants and ATL63-overexpressing plants with various pathogens

  • Measure disease symptoms, pathogen growth, and defense marker gene expression

  • Use trypan blue staining to quantify fungal growth in infected tissues

• Defense hormone response:

  • Treat plants with defense hormones (SA, JA, ethylene) and monitor ATL63 expression

  • Test whether atl63 mutants show altered responses to these hormones

  • Analyze expression of hormone-responsive marker genes in atl63 mutants

• ROS production measurement:

  • Use luminol-based assays or DAB (3,3'-diaminobenzidine) staining to measure ROS production

  • Compare ROS production between wild-type and atl63 mutants after PAMP treatment

  • Analyze expression of RBOH genes in atl63 mutants

What bioinformatic approaches are useful for predicting ATL63 substrates?

Identifying the substrates of E3 ubiquitin ligases like ATL63 is challenging but can be approached using several bioinformatic methods:

Protein-protein interaction prediction:

• Use algorithms that predict protein-protein interactions based on sequence, structure, and evolutionary information

• Focus on region VII of ATL63, as this region in ATL proteins is likely involved in substrate recognition

• Search for proteins with motifs that complement the predicted binding sites in ATL63

Co-expression analysis:

• Analyze transcriptomic data to identify genes co-expressed with ATL63 under various conditions

• Focus on stress and pathogen exposure conditions, as ATL63 is likely involved in defense responses

• Prioritize proteins that show expression patterns inverse to ATL63, as these might be degradation targets

Comparative analysis with known ATL substrates:

• Compile known substrates of other ATL family members

• Search for homologous proteins in the predicted Arabidopsis proteome

• Analyze these proteins for conserved degron motifs that might be recognized by ATL63

Structural modeling:

• Generate homology models of ATL63 based on structures of related E3 ligases

• Perform molecular docking simulations with candidate substrate proteins

• Prioritize interactions with favorable binding energies and biologically relevant orientations

Machine learning approaches:

• Train models on known E3-substrate pairs using features like sequence composition, structural properties, and cellular context

• Apply these models to predict potential ATL63 substrates

• Validate top predictions experimentally using co-immunoprecipitation or yeast two-hybrid assays

What are the challenges in expressing full-length ATL63 in heterologous systems?

Expressing transmembrane proteins like ATL63 presents several challenges:

Membrane integration issues:

• ATL63 contains a transmembrane domain that may cause protein aggregation when expressed in bacterial systems

• Expression of membrane proteins often leads to formation of inclusion bodies

Solution strategies:

• Express truncated versions lacking the transmembrane domain

• Based on studies with Atl proteins, creating N-terminal truncations may improve expression levels

• Consider sequential deletions from the N-terminus to determine optimal constructs

• Use specialized E. coli strains like C41(DE3) or C43(DE3) designed for membrane protein expression

Toxicity concerns:

• ATL63, like ATL2, may be toxic when overexpressed in yeast and potentially in bacterial systems

• This toxicity might be related to targeting essential proteins for degradation

Expression optimization:

• Use lower temperatures (16-20°C) during induction to slow protein synthesis and improve folding

• Reduce inducer concentration to limit expression rate

• Use tightly controlled inducible promoters to minimize leaky expression

• Co-express molecular chaperones to assist in proper folding

The table below compares expression strategies that have worked for related proteins:

How can researchers optimize experimental design for studying ATL63 function in plants?

Designing robust experiments to study ATL63 function requires careful planning:

Control selection:

• Include both positive and negative controls in all experiments

• Use multiple independent transgenic lines or mutants to account for position effects

• Consider using related ATL family members (e.g., ATL2, ATL12) as comparative controls

Staggered rollout experimental design:
For time-course experiments studying ATL63 expression:

• Implement a non-adaptive experimental design where treatment assignment decisions are made prior to starting the experiment

• For studying early response genes like ATL63, use a staggered rollout design where the fraction entering treatment each period is initially low, then high, and finally low again

• This approach can reduce the opportunity cost of experiments by over 50% compared to static designs

Genetic redundancy considerations:

• Generate double or triple mutants with closely related ATL genes to address potential functional redundancy

• Use CRISPR/Cas9 to create multiplex gene knockouts

• Consider conditional knockout strategies for essential genes

Phenotypic analysis optimization:

• Define clear, measurable phenotypes related to expected ATL63 function

• Use automated image analysis to quantify phenotypes objectively

• Implement blinded assessment of phenotypes to reduce bias

• Apply appropriate statistical methods for data analysis, accounting for multiple testing

Documentation and reproducibility:

• Maintain detailed records of all experimental procedures

• Use electronic lab notebooks with standardized templates

• Share raw data and analysis scripts to enable reproducibility

• Follow the FAIR principles (Findable, Accessible, Interoperable, Reusable) for data management

What analytical techniques are most effective for studying ATL63 interactions with E2 enzymes?

Understanding ATL63's interactions with E2 ubiquitin-conjugating enzymes is crucial for characterizing its function:

Yeast two-hybrid screening:

• Use ATL63 RING-H2 domain as bait to screen for interacting E2 enzymes

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

• Validate interactions using point mutations in key residues of the RING-H2 domain

In vitro binding assays:

• Express and purify recombinant ATL63 RING-H2 domain and candidate E2 enzymes

• Perform pull-down assays using tagged proteins

• Use surface plasmon resonance (SPR) or isothermal titration calorimetry (ITC) to determine binding kinetics and affinity

• Compare wild-type and mutant versions of ATL63 to identify key interaction residues

Structural studies:

• Attempt co-crystallization of ATL63 RING-H2 domain with interacting E2 enzymes

• Use NMR spectroscopy to determine the solution structure of the complex

• Based on studies with rice ATL EL5, focus on key amino acid residues in the RING-H2 domain that mediate E2 binding

Computational modeling:

• Generate homology models of ATL63 based on related RING-H2 structures

• Perform molecular docking simulations with candidate E2 enzymes

• Use molecular dynamics simulations to analyze the stability of predicted complexes

Functional validation:

• Test whether identified E2 enzymes support ATL63-mediated ubiquitination in vitro

• Generate mutations in the arginine-rich region (region III) and the GLD motif to test their role in E2 binding

• Investigate whether the E2 specificity of ATL63 is similar to ATL2, which functions with the Ubc4 E2 enzyme