Recombinant Arabidopsis thaliana Cytoplasmic tRNA 2-thiolation protein 2 (CTU2)

What is the function of CTU2 in Arabidopsis thaliana?

CTU2 in Arabidopsis thaliana functions as a cytosolic thiouridylase that forms a complex with ROL5 (a homolog of yeast NCS6) to catalyze the 2-thiolation of cytosolic tRNAs. This complex is specifically required for the mcm5s2U modification of tRNAs, which ensures efficient decoding during translation. The tRNA thiolation is essential for plant immunity, as demonstrated by the hyper-susceptibility of ctu2-1 mutants to pathogen Pseudomonas syringae . The complex plays a crucial role in both transcriptome and proteome reprogramming during immune responses, highlighting its importance in translational control mechanisms that regulate plant defense responses .

What phenotypes are observed in Arabidopsis ctu2 mutants?

Arabidopsis ctu2 mutants exhibit several distinct phenotypes:

• Compromised immunity: The ctu2-1 mutant shows hyper-susceptibility to the pathogen Pseudomonas syringae (Psm ES4326) .

• Loss of tRNA thiolation: HPLC-MS analysis reveals that ctu2-1 mutants have undetectable levels of mcm5s2U modification while mcm5U levels are very high, indicating complete loss of tRNA thiolation activity .

• Altered immune signaling: Both transcriptome and proteome reprogramming during immune responses are compromised in ctu2 mutants .

• Reduced translation of specific proteins: Most notably, translation of the salicylic acid receptor NPR1 is reduced, resulting in compromised salicylic acid signaling .

How does the CTU2-ROL5 complex function biochemically?

The CTU2-ROL5 complex functions as an enzyme that catalyzes the 2-thiolation of the wobble uridine (U34) in the anticodon of specific cytosolic tRNAs. Biochemically:

• ROL5 (homolog of yeast NCS6) and CTU2 (homolog of yeast NCS2) physically interact to form a functional complex .

• This complex specifically catalyzes the addition of a thiol group (2-thiolation) to the mcm5U modification, converting it to mcm5s2U in the wobble position of certain tRNAs .

• In wild-type Arabidopsis, mcm5U is almost undetectable because it is efficiently transformed into mcm5s2U, whereas in rol5-c and ctu2-1 mutants, mcm5s2U is completely absent .

• This modification is proposed to be crucial for both restriction of wobble in the corresponding split codon box and efficient codon-anticodon interaction during translation .

How can researchers confirm the loss of tRNA thiolation in ctu2 mutants?

Researchers can confirm the loss of tRNA thiolation in ctu2 mutants using several complementary approaches:

• N-acryloylamino phenyl mercuric chloride binding assay: This compound specifically binds thiolated tRNAs, allowing for their detection. Previous studies have successfully used this method to show defective tRNA thiolation in rol5 and ctu2 mutants .

• HPLC-MS analysis: High-performance liquid chromatography coupled with mass spectrometry can quantitatively measure the levels of mcm5U and mcm5s2U in tRNA samples. In wild-type plants, mcm5U is almost undetectable while mcm5s2U is abundant, whereas in ctu2-1 mutants, mcm5s2U is undetectable and mcm5U levels are elevated . The specific protocol involves:

  • Isolation of total tRNA from plant tissues

  • Enzymatic digestion of tRNA to nucleosides

  • HPLC separation of nucleosides

  • Mass spectrometry detection and quantification of modified nucleosides

• APM gel electrophoresis: This technique causes a mobility shift in thiolated tRNAs, which can be followed by northern blotting with specific tRNA probes to identify affected tRNA species.

What expression systems are optimal for producing recombinant Arabidopsis CTU2 protein?

For producing recombinant Arabidopsis thaliana CTU2 protein, researchers should consider the following expression systems and optimization strategies:

• E. coli expression system:

  • BL21(DE3) or Rosetta strains are recommended for expressing plant proteins

  • pET or pGEX vectors containing N-terminal affinity tags (6xHis or GST) facilitate purification

  • Expression conditions should be optimized by testing different temperatures (16-30°C), IPTG concentrations (0.1-1 mM), and induction times (4-24 hours)

  • Co-expression with ROL5 may improve solubility and stability of CTU2

• Insect cell expression system:

  • Baculovirus expression in Sf9 or Hi5 cells may provide better folding for eukaryotic proteins

  • This system is particularly useful if E. coli expression yields insoluble protein

• Plant expression systems:

  • Transient expression in Nicotiana benthamiana using Agrobacterium-mediated transformation

  • This system provides the most native environment for plant protein expression

• Purification strategy:

  • Initial affinity chromatography (Ni-NTA for His-tagged proteins)

  • Ion exchange chromatography as an intermediate purification step

  • Size exclusion chromatography as a final polishing step

  • Protein quality assessment using SDS-PAGE, Western blotting, and activity assays

How can researchers measure the impact of CTU2 on translation efficiency in vivo?

To measure the impact of CTU2 on translation efficiency in vivo, researchers can employ several complementary approaches:

• Ribosome profiling (Ribo-seq): This technique provides genome-wide information on ribosome positioning on mRNAs with codon-level resolution.

  • Compare wild-type and ctu2 mutant plants with and without pathogen treatment

  • Identify mRNAs with altered translation efficiency

  • Analyze ribosome stalling at specific codons that might depend on thiolated tRNAs

• Polysome profiling: This method separates mRNAs based on the number of associated ribosomes.

  • Extract polysomes from wild-type and ctu2 mutant plants

  • Fractionate using sucrose gradient centrifugation

  • Analyze the distribution of specific mRNAs across polysome fractions by RT-qPCR

  • Perform RNA-seq on polysome fractions (polysome-seq) to identify mRNAs with altered translation efficiency globally

• Metabolic labeling: Use radioisotope or non-radioactive labeling to measure protein synthesis rates.

  • Pulse-labeling with 35S-methionine followed by immunoprecipitation of specific proteins

  • SUnSET method using puromycin incorporation followed by anti-puromycin antibody detection

  • Analyze global or protein-specific synthesis rates in wild-type versus ctu2 mutants

• Reporter gene assays: Construct reporters with varying codon usage to test translation efficiency.

  • Design GFP or luciferase reporters with codons that depend on thiolated tRNAs

  • Express in wild-type and ctu2 mutant backgrounds

  • Measure reporter protein levels by fluorescence, luminescence, or Western blotting

How does CTU2-mediated tRNA thiolation specifically affect plant immune responses?

CTU2-mediated tRNA thiolation plays a critical role in plant immunity through several mechanisms:

• Translational control of defense proteins: The ctu2 mutant shows compromised translation of the salicylic acid receptor NPR1, a key regulator of plant immunity, resulting in reduced salicylic acid signaling . This suggests that tRNA thiolation selectively enhances the translation of specific defense-related proteins.

• Global proteome reprogramming: High-throughput proteome analysis identified 2215 proteins differentially accumulated after Pseudomonas syringae infection in Arabidopsis, and this proteomic reprogramming is compromised in ctu2 mutants . This indicates that tRNA thiolation is required for the extensive proteome changes that occur during immune responses.

• Codon bias utilization: Many defense-related genes likely have a biased codon usage that depends on thiolated tRNAs for efficient translation. Without proper tRNA thiolation, these defense proteins cannot be synthesized at sufficient levels during pathogen attack.

• Temporal regulation of immune responses: tRNA thiolation may provide a mechanism for rapid translational upregulation of defense proteins upon pathogen perception, allowing the plant to mount a timely immune response.

Can CTU2 function be modulated to enhance plant immunity?

Based on the available research, several strategies could potentially be employed to modulate CTU2 function for enhanced plant immunity:

These approaches would require rigorous testing to validate their effectiveness and ensure they don't cause unintended consequences on plant growth and development.

How does the role of CTU2 in Arabidopsis compare with its role in other organisms?

CTU2 functions in a highly conserved pathway for tRNA thiolation across eukaryotes, but with species-specific roles and consequences:

• Yeast and nematodes: In fission yeast and nematodes, the Ctu1-Ctu2 complex is responsible for 2-thiolation of cytosolic tRNAs. Inactivation leads to thermosensitive decrease of viability, ploidy abnormalities, and aberrant development . The complex is critical for genome stability, with defects possibly resulting from both misreading and frame shifting during translation .

• Mammals/Cancer: In human cancer cells, CTU2 is implicated in promoting tumor development. In hepatocellular carcinoma, CTU2 enhances cell proliferation by promoting lipogenesis . CTU2 is elevated in breast tumors and promotes metastasis by supporting specific translation of oncogenic factors . In melanoma, CTU2-linked tRNA modification promotes growth by regulating HIF1α codon-dependent translation .

• Plants/Arabidopsis: In Arabidopsis, CTU2 forms a complex with ROL5 (homolog of yeast NCS6) to catalyze tRNA thiolation. This modification is essential for plant immunity, with ctu2 mutants showing hyper-susceptibility to pathogens due to compromised salicylic acid signaling .

This comparison reveals that while the biochemical function of CTU2 in tRNA thiolation is conserved, its physiological roles have diverged during evolution, regulating distinct cellular processes in different organisms.

What is the molecular mechanism by which CTU2 and ROL5 coordinate tRNA thiolation?

The molecular mechanism of CTU2 and ROL5-mediated tRNA thiolation likely follows these steps, based on current understanding:

• Complex formation: ROL5 (homolog of yeast NCS6) physically interacts with CTU2 (homolog of yeast NCS2) to form a functional enzymatic complex . This interaction is likely mediated by specific protein domains that could be identified through structural studies.

• Substrate recognition: The complex recognizes specific tRNAs, likely those with mcm5U modification at the wobble position (U34) in the anticodon loop. The molecular determinants for tRNA recognition remain to be fully elucidated but may involve both sequence and structural elements.

• Thiolation chemistry: The complex catalyzes the addition of a thiol group to the 2-position of the modified uridine (mcm5U), converting it to mcm5s2U. This likely involves activation of a sulfur donor molecule and its transfer to the target nucleoside.

• Regulation: The activity of the complex may be regulated in response to cellular conditions or stress, providing a mechanism to adjust translation efficiency according to the cell's needs.

To fully elucidate this mechanism, structural studies of the CTU2-ROL5 complex, along with biochemical assays using purified components and defined tRNA substrates, would be necessary.

How do researchers identify specific mRNAs whose translation depends on CTU2-mediated tRNA thiolation?

To identify specific mRNAs whose translation depends on CTU2-mediated tRNA thiolation, researchers can employ the following comprehensive approach:

• Ribosome profiling with RNA-seq: Compare ribosome-protected fragments (RPFs) and total mRNA levels between wild-type and ctu2 mutant plants to calculate translation efficiency for each mRNA.

  • Isolate and sequence both RPFs and total mRNA

  • Calculate translation efficiency (TE) as the ratio of RPF to mRNA abundance

  • Identify mRNAs with significantly reduced TE in ctu2 mutants

• Codon usage analysis: Examine the codon composition of mRNAs with altered translation in ctu2 mutants.

  • Focus on codons corresponding to tRNAs that require thiolation

  • Develop algorithms to identify potential "thiolation-dependent codons"

  • Calculate the frequency of these codons in affected versus unaffected mRNAs

• Polysome profiling: Analyze the distribution of specific mRNAs across polysome fractions.

  • Perform sucrose gradient fractionation of polysomes

  • Analyze specific mRNAs by RT-qPCR or perform RNA-seq on each fraction

  • Compare polysome association patterns between wild-type and ctu2 mutants

• Proteomics-transcriptomics integration: Identify proteins whose abundance decreases in ctu2 mutants without corresponding changes in mRNA levels.

  • Perform quantitative proteomics (TMT-MS) and RNA-seq

  • Calculate protein-to-mRNA ratios

  • Identify proteins with reduced ratios in ctu2 mutants

• Reporter assays: Construct reporters with varying codon usage to test translation dependency on thiolated tRNAs.

  • Design GFP or luciferase reporters with candidate codons

  • Express in wild-type and ctu2 mutant backgrounds

  • Measure protein and mRNA levels to calculate translation efficiency

What is the relationship between tRNA thiolation and cancer development?

The relationship between tRNA thiolation and cancer development represents an important area where plant research can inform human disease studies:

• CTU2 upregulation in cancer: CTU2 has been found to be elevated in several cancer types, including breast tumors, melanoma, and hepatocellular carcinoma (HCC) . In HCC specifically, CTU2 enhances proliferation of cancer cells and promotes tumor development .

• Mechanisms in cancer progression:

  • Enhanced translation of oncogenic factors: In breast cancer, CTU2 promotes metastasis by supporting specific translation of the oncogenic factor LEF1 in an internal ribosome entry site (IRES) dependent manner .

  • Regulation of HIF1α translation: In melanoma, CTU2-linked tRNA modification promotes growth by regulating HIF1α codon-dependent translation .

  • Promotion of lipogenesis: In HCC, CTU2 participates in lipogenesis by directly enhancing the synthesis of lipogenic proteins, which is essential for rapidly proliferating cancer cells .

• Regulatory pathways: CTU2 has been identified as a target gene of Liver X receptor (LXR) in HCC, with a typical LXR element in the CTU2 promoter . CTU2 expression is activated by LXR agonist and depressed by LXR knockout .

• Therapeutic implications: Inhibition of CTU2 expression can synergize the anti-tumor effect of LXR ligands by inducing tumor cell apoptosis and inhibiting cell proliferation . This identifies CTU2 as a promising target for cancer treatment, particularly in combination with LXR agonists.

The conservation of the tRNA thiolation pathway between plants and humans suggests that fundamental insights gained from studying Arabidopsis CTU2 could be leveraged for developing novel therapeutic approaches targeting human CTU2 in cancer treatment.

What emerging technologies could advance CTU2 research in Arabidopsis?

Several emerging technologies hold promise for advancing CTU2 research in Arabidopsis:

• CRISPR base editing and prime editing: These technologies allow for precise modification of specific nucleotides without double-strand breaks, enabling the creation of specific CTU2 variants to study structure-function relationships.

• Nanopore direct RNA sequencing: This technology can directly detect modified nucleosides in native RNA molecules without prior conversion to cDNA, potentially allowing for comprehensive profiling of tRNA modifications in wild-type and ctu2 mutant plants.

• Cryo-electron microscopy: High-resolution structural determination of the CTU2-ROL5 complex bound to substrate tRNAs would provide crucial insights into the mechanism of tRNA recognition and thiolation.

• Single-cell proteomics and transcriptomics: These approaches could reveal cell-type specific roles of CTU2 in plant immunity and development, particularly in specialized cell types involved in pathogen responses.

• Metabolic tracing with stable isotopes: Using stable isotope-labeled sulfur donors could help trace the flux through the tRNA thiolation pathway and identify the source of sulfur atoms for the thiol modification.

• Proximity labeling proteomics (BioID or TurboID): Identifying proteins that interact transiently with CTU2-ROL5 could reveal additional components of the tRNA modification machinery and regulatory factors.

• In vitro translation systems derived from Arabidopsis: Developing plant-specific cell-free translation systems would allow for direct testing of how tRNA thiolation affects the translation of specific mRNAs in a controlled environment.

Can knowledge of CTU2 function be applied to improve crop resistance to pathogens?

The knowledge of CTU2 function could potentially be applied to improve crop resistance to pathogens through several strategies:

• Genetic engineering approaches:

  • Fine-tuning CTU2 expression levels in crops to optimize tRNA thiolation for enhanced defense protein translation

  • Creating targeted modifications in CTU2 to potentially enhance its activity or stability

  • Engineering key defense genes with optimized codon usage to reduce dependency on thiolated tRNAs

• Screening and breeding strategies:

  • Identifying natural variants of CTU2 with enhanced activity or regulation

  • Developing molecular markers based on CTU2 sequence or activity for marker-assisted selection

  • Creating TILLING populations to identify beneficial CTU2 variants

• Chemical approaches:

  • Developing compounds that enhance CTU2 activity specifically during pathogen attack

  • Identifying molecules that could stabilize thiolated tRNAs under stress conditions

• Translational research considerations:

  • Testing if the immunity-enhancing effects of optimal tRNA thiolation observed in Arabidopsis translate to crop species

  • Determining if enhanced CTU2 activity affects yield or other agronomic traits

  • Evaluating the durability of enhanced resistance across different pathogen types and environmental conditions

• Integration with other immunity approaches:

  • Combining CTU2 optimization with other established methods for enhancing disease resistance

  • Using systems biology to predict how altered tRNA thiolation would interact with other defense pathways

The application of this knowledge would require extensive testing to ensure that modulating tRNA thiolation doesn't negatively impact plant growth, development, or yield under normal growing conditions.