Recombinant Cucumis sativus Ethylene receptor 1 (ETR1)

What is the structure and function of cucumber ETR1?

Cucumber ETR1 is a membrane-bound receptor protein consisting of 740 amino acids that functions in ethylene perception. Structurally, it contains multiple transmembrane domains at the N-terminus where ethylene binding occurs, followed by a GAF domain, a histidine kinase domain, and a receiver domain . The protein functions as a negative regulator of ethylene responses, meaning that in the absence of ethylene, ETR1 actively suppresses ethylene responses, while ethylene binding inactivates this suppression mechanism . The complete amino acid sequence of cucumber ETR1 is:

METCYCIEPQWPADELLMKYQYISDFFIALAYFSIPLELIYFVKKSAVFPYRWVLVQFGA FIVLCGATHLINLWTFTMHSRTVAVVMTTAKVLTAVVSCATALMLVHIIPDLLSVKTREL FLKNKAAELDREMGLIRTQEETGRHVRMLTHEIRSTLDRHTILKTTLVELGRTLALEECA LWMPTRTGLELQLSYTLHQQNPVGYTVPINLPVISQVFSSNRAVKISPNSPVASLRPRAG RYVAGEVVAVRVPLLHLSNFQINDWPELSTKRYALMVLMLPSDSARQWRVHELELVEVVA DQVAVALSHAAILEESMRARDPLMEQNVALDLARREAETANHARNDFLAVMNHEMRTPMH AIIALSSLLQETELTPEQRLMVETILKSSNLLATLINDVLDLSRLEDGSLQLDIGTFNLH AVFKEVLNLIKPVTLVKKLSLTLHLGLDLPVFAVGDEKRLMQAILNVVGNAVKFSKEGSI SISAIVAKAETFREIRVPDFHPVPSDSHFYLRVQVKDTGSGISPQDIPKLFTKFAQTTVG PRNSCGSGLGLAICKRFVNLMEGHIWLESEGLGKGCTATFIVKLGIAEQSNESKLPFTSK IHENSIHTSFPGLKVLVMDDNGVSRSVTKGLLVHLGCEVTTAGSIEEFLRVVSQEHKVVF MDICTPGVDGYELAIRIREKFAKCHERPFMVVLTGNSDKVTKESCLRAGMDGLILKPVSI DKMRSVLSELIERRVLFETS

What expression systems are most effective for producing recombinant cucumber ETR1?

The most common expression system for recombinant cucumber ETR1 is Escherichia coli. When expressing membrane proteins like ETR1, several methodological considerations are important:

• Vector selection: Vectors with strong but inducible promoters (like T7) are preferable for controlling expression levels.

• Fusion tags: N-terminal His-tags are commonly used for cucumber ETR1 to facilitate purification without interfering with transmembrane domains .

• Expression conditions: Lower temperatures (16-20°C) after induction can increase soluble protein yield.

• Host strains: E. coli strains like BL21(DE3) that are deficient in certain proteases are recommended.

For cucumber ETR1, expression in E. coli yields functional protein that can be purified using affinity chromatography methods . The resulting protein typically has a purity greater than 90% as determined by SDS-PAGE analysis .

What is the optimal protocol for storage and reconstitution of recombinant cucumber ETR1?

For optimal stability and activity of recombinant cucumber ETR1, the following storage and reconstitution protocols are recommended:

Storage conditions:

• Store lyophilized protein at -20°C to -80°C upon receipt

• Aliquot the protein to avoid repeated freeze-thaw cycles, which significantly reduce activity

• Working aliquots can be stored at 4°C for up to one week

Reconstitution protocol:

• Briefly centrifuge the vial prior to opening to bring contents to the bottom

• Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• Add glycerol to a final concentration of 5-50% (standard is 50%)

• Create small aliquots for long-term storage at -20°C/-80°C

Buffer composition:

• Optimal storage buffer: Tris/PBS-based buffer, pH 8.0, containing 6% trehalose for stability

How does cucumber ETR1 interact with RTE1, and what is the functional significance of this interaction?

ETR1 and RTE1 (REVERSION-TO-ETHYLENE SENSITIVITY1) display a unique functional relationship that has significant implications for ethylene signaling. Studies primarily in Arabidopsis have demonstrated that:

• RTE1 functions are primarily dependent on ETR1 and can work independently of other ethylene receptors .

• The N-terminus of ETR1 is sufficient for the activation of RTE1 function .

• Ethylene insensitivity caused by RTE1 overexpression is largely masked by the etr1-7 mutation (a loss-of-function mutation), suggesting that RTE1's function requires the presence of functional ETR1 .

Methodologically, these interactions can be studied through:

• Co-immunoprecipitation assays with tagged versions of both proteins

• Yeast two-hybrid analysis to detect direct interactions

• Genetic studies using mutants and overexpression lines (particularly valuable are the etr1-7 and rte1-2 mutants)

• Functional complementation experiments using various domains of each protein

Researchers investigating this interaction in cucumber should consider designing experiments that examine the subcellular localization of both proteins, as this may provide insights into their functional relationship and signaling mechanism.

What role does ETR1 play in gibberellic acid-regulated sex determination in cucumber?

ETR1 appears to be a critical component in the GA-mediated regulation of sex expression in cucumber. Transcriptomic analyses reveal a complex interaction between GA signaling and ethylene perception:

• GA3 treatment significantly decreases ethylene production in cucumber plants .

• CsETR1 expression increases by 2.12-fold in shoot apices after 12 hours of GA3 treatment, suggesting a transcriptional response to GA3 .

• This upregulation occurs in parallel with downregulation of key ethylene biosynthesis genes (particularly the M gene/CsACS2) and ethylene-responsive transcription factors (ERFs) .

Table: Changes in gene expression after GA3 treatment in cucumber shoot apices

These data suggest that GA may promote maleness in cucumber by:

• Increasing ETR1 expression, potentially enhancing ethylene insensitivity

• Suppressing ethylene biosynthesis through inhibition of M gene expression

• Downregulating ethylene-responsive transcription factors

Researchers should consider designing experiments that manipulate ETR1 expression levels to test whether this affects the ability of GA to influence sex determination in cucumber.

What are the best methods for assessing ETR1 binding affinity and kinetics with ethylene?

Evaluating ETR1 binding properties with ethylene requires specialized techniques due to the gaseous nature of the ligand and the membrane-embedded binding domain. Recommended methodologies include:

• Radioligand binding assays: Using radioactive ethylene (14C-ethylene) to measure binding parameters:

  • Equilibrium dissociation constant (Kd)

  • Maximum binding capacity (Bmax)

  • Association and dissociation rates

• Surface plasmon resonance (SPR): For real-time kinetic measurements, requires:

  • Immobilization of purified ETR1 in nanodiscs or detergent micelles

  • Controlled ethylene delivery system

  • Careful temperature regulation

• Membrane fraction binding studies: Using microsomal preparations expressing recombinant ETR1:

  • Incubate membrane fractions with varying ethylene concentrations

  • Wash to remove unbound ethylene

  • Measure bound ethylene through gas chromatography

• Functional response measurements: Indirect assessment through downstream signaling events:

  • Histidine kinase activity assays

  • Receptor autophosphorylation state analysis

  • Measurement of interaction with downstream components

When performing these assays, it's critical to maintain the native conformation of the transmembrane domains where ethylene binding occurs, often requiring stabilization with appropriate detergents or lipid environments.

How can researchers effectively compare cucumber ETR1 function with ethylene receptors from other species?

Comparative analysis of ethylene receptors across species requires a multi-faceted approach:

• Sequence and structural analysis:

  • Multiple sequence alignment of ETR1 homologs from different species

  • Phylogenetic tree construction to understand evolutionary relationships

  • Homology modeling to predict structural differences

• Heterologous expression studies:

  • Express cucumber ETR1 in Arabidopsis etr1 mutants to test functional complementation

  • Compare expression with other ethylene receptors (ETR2, ERS1, ERS2, EIN4) to assess redundancy and specificity

• Domain swap experiments:

  • Create chimeric receptors with domains from different species to identify species-specific functional elements

  • Analyze which domains are responsible for specific signaling outputs

• Comparative transcriptomics:

  • Analyze differential gene expression patterns in response to ethylene with various receptor mutants

  • Compare cucumber ETR1-regulated genes with those regulated by ETR1 in other species

Research by Resnick et al. and others indicates that while the basic mechanism of ethylene perception is conserved, there are species-specific differences in receptor function, particularly in how they interact with other components like RTE1 . The Arabidopsis ethylene receptor family (with five members) has functional redundancy, yet specific receptors like ETR1 have unique roles that cannot be fully compensated by other family members .

What methodologies are recommended for studying the subcellular localization of cucumber ETR1?

Understanding the subcellular localization of ETR1 is crucial for elucidating its function in ethylene signaling. Several complementary approaches are recommended:

• Fluorescent protein fusion techniques:

  • Generate N- or C-terminal GFP/YFP fusions with ETR1

  • Validate that fusion proteins retain functionality through complementation studies

  • Use confocal microscopy for high-resolution imaging

  • Co-localize with established organelle markers

• Subcellular fractionation and western blotting:

  • Perform differential and density gradient centrifugation

  • Use sucrose gradient fractionation (as shown effective for Arabidopsis ETR1)

  • Detect ETR1 in fractions using specific antibodies

  • Compare with known organelle marker proteins

• Immunogold electron microscopy:

  • Provides highest resolution of subcellular localization

  • Requires specific antibodies against cucumber ETR1

  • Allow visualization of receptor distribution within membrane structures

• Biochemical extraction methods:

  • Test solubility in different detergents to infer membrane association properties

  • Use protease protection assays to determine topology

  • Analyze post-translational modifications that might affect localization

Evidence from Arabidopsis suggests that ETR1 is primarily associated with the endoplasmic reticulum (ER) . Researchers should investigate whether cucumber ETR1 shares this localization or has distinct distribution patterns that might explain species-specific responses to ethylene.

How can researchers effectively measure the impact of ETR1 on ethylene sensitivity in cucumber tissues?

To assess the functional impact of ETR1 on ethylene sensitivity, several experimental approaches are recommended:

• Triple response assay in etiolated seedlings:

  • Compare hypocotyl and root length, and apical hook formation in wildtype vs. ETR1-modified lines

  • Test responses to exogenous ethylene at concentrations ranging from 0.01-100 ppm

  • Include appropriate controls with ethylene biosynthesis inhibitors like AVG (aminoethoxyvinylglycine)

• Ethylene production measurement:

  • Use gas chromatography to quantify ethylene production in different tissues

  • Compare production rates in wildtype vs. ETR1-modified plants

  • Measure after various treatments (e.g., GA3 treatment showed decreased ethylene production)

• Expression analysis of ethylene-responsive genes:

  • qRT-PCR analysis of known ethylene-responsive genes

  • RNA-Seq for genome-wide transcriptional changes

  • Focus on ethylene-responsive transcription factors (ERFs) which show altered expression in response to hormonal treatments that affect ETR1

• Physiological response assays:

  • Flower development and sex determination (particularly relevant in cucumber)

  • Fruit ripening characteristics

  • Senescence timing and progression

When manipulating ETR1 levels or function, researchers should consider both gain-of-function approaches (overexpression) and loss-of-function approaches (RNAi, CRISPR/Cas9) to comprehensively understand its role in ethylene sensitivity.

What approaches are recommended for studying the potential interaction between ETR1 and hormone signaling pathways beyond ethylene?

ETR1's potential interactions with other hormone signaling pathways, particularly gibberellic acid (GA), represent an important area for investigation. Recommended methodological approaches include:

• Hormone cross-talk experiments:

  • Apply combinations of hormones (ethylene, GA, auxin, cytokinin, etc.) to ETR1 wildtype and modified lines

  • Measure physiological and molecular responses

  • Analyze how ETR1 modification alters responses to non-ethylene hormones

• Transcriptomic analysis:

  • Perform RNA-Seq on tissues treated with different hormones

  • Compare transcriptome changes between wildtype and ETR1-modified plants

  • Identify genes regulated by multiple hormone pathways

• Protein-protein interaction studies:

  • Use yeast two-hybrid, co-immunoprecipitation, or BiFC (Bimolecular Fluorescence Complementation) to identify interactions between ETR1 and components of other hormone signaling pathways

  • Test interactions with known GA signaling components given evidence of cross-talk

• Genetic interaction analysis:

  • Create double mutants between etr1 and mutants in other hormone pathways

  • Analyze phenotypes for additive, synergistic, or epistatic relationships

Research has shown that GA3 treatment increases ETR1 expression approximately 2.12-fold after 12 hours, suggesting a direct regulatory link between GA signaling and ethylene perception through ETR1 . This finding highlights the importance of studying hormone cross-talk in understanding ETR1 function in cucumber development.

What strategies can be employed to investigate post-translational modifications of cucumber ETR1?

Post-translational modifications (PTMs) can significantly impact ETR1 function, stability, and interactions. To investigate these modifications in cucumber ETR1, researchers should consider:

• Mass spectrometry-based approaches:

  • Immunoprecipitate ETR1 from plant tissues or heterologous expression systems

  • Perform tryptic digestion followed by LC-MS/MS analysis

  • Use both bottom-up (peptide) and top-down (intact protein) proteomics

  • Search for common PTMs including phosphorylation, ubiquitination, SUMOylation, and glycosylation

• Phosphorylation-specific analysis:

  • Use phospho-specific antibodies in western blotting

  • Perform in vitro kinase assays to identify potential kinases

  • Create phosphomimetic and phosphonull mutations at predicted sites

  • Analyze how phosphorylation status affects receptor function

• Protein stability and turnover studies:

  • Pulse-chase experiments with protein synthesis inhibitors

  • Proteasome inhibitor treatments to assess ubiquitin-mediated degradation

  • Analysis of ETR1 levels under various hormonal and environmental conditions

• Site-directed mutagenesis:

  • Modify predicted PTM sites and express in heterologous systems or transform into cucumber

  • Assess functional consequences on ethylene binding, signaling, and protein-protein interactions

  • Compare PTM patterns with those known in Arabidopsis ETR1 to identify conserved and divergent regulatory mechanisms

Understanding PTMs of cucumber ETR1 will provide valuable insights into how this receptor's function is dynamically regulated in response to developmental and environmental cues.

What are the most effective strategies for generating and characterizing ETR1 mutants in cucumber?

Creating and characterizing ETR1 mutants in cucumber requires careful consideration of several technical aspects:

• CRISPR/Cas9 genome editing:

  • Design sgRNAs targeting conserved functional domains

  • Target the N-terminal transmembrane region for ethylene binding disruption

  • Target the histidine kinase domain to affect signaling capability

  • Use cucumber-optimized promoters for Cas9 expression

  • Screen for mutations using PCR-based methods followed by sequencing

• RNAi-based silencing:

  • Design constructs targeting unique regions of CsETR1

  • Use inducible or tissue-specific promoters to control silencing

  • Validate knockdown using qRT-PCR and western blotting

  • Consider potential off-target effects on other ethylene receptor genes

• Overexpression and dominant-negative approaches:

  • Express wildtype CsETR1 under constitutive or inducible promoters

  • Create dominant-negative versions (e.g., by mutating conserved histidine residues)

  • Use tissue-specific promoters for targeted expression analysis

• Phenotypic characterization pipeline:

  • Ethylene triple-response assays in etiolated seedlings

  • Sex determination analysis (male/female flower ratio)

  • GA response assays (given the ETR1-GA relationship)

  • Ethylene production measurements by gas chromatography

  • RNA-Seq for global transcriptional changes

When generating ETR1 mutants, researchers should be aware that complete loss-of-function may have pleiotropic effects due to ETR1's fundamental role in plant development.

What challenges might researchers encounter when expressing the transmembrane domains of cucumber ETR1, and how can these be addressed?

The transmembrane domains of ETR1 present specific challenges for recombinant expression and functional studies:

• Expression and solubility issues:

  • Challenge: Membrane proteins often form inclusion bodies or misfold when overexpressed

  • Solution: Use specialized E. coli strains (C41, C43) designed for membrane protein expression

  • Solution: Optimize induction conditions (lower temperature, reduced IPTG concentration)

  • Solution: Consider expression as fusion with solubility-enhancing tags (MBP, SUMO)

• Purification challenges:

  • Challenge: Maintaining native conformation during extraction from membranes

  • Solution: Screen multiple detergents (DDM, LMNG, digitonin) for optimal extraction

  • Solution: Consider nanodiscs or liposome reconstitution for functional studies

  • Solution: Use styrene maleic acid lipid particles (SMALPs) to extract with surrounding lipids

• Functional assessment:

  • Challenge: Verifying proper folding and ethylene binding capability

  • Solution: Radioligand binding assays with 14C-ethylene

  • Solution: Circular dichroism to assess secondary structure

  • Solution: Limited proteolysis to probe conformation

• Structural studies:

  • Challenge: Obtaining structural information on transmembrane regions

  • Solution: Consider cryo-EM for full-length protein in nanodiscs

  • Solution: X-ray crystallography of individual domains

  • Solution: NMR studies of isotopically labeled fragments

When working with the transmembrane domains, researchers should pay particular attention to the amino acid sequence METCYCIEPQWPADELLMKYQYISDFFIALAYFSIPLELIYFVKKSAVFPYRWVLVQFGAFIVLCGATHLINLWTFTMHSR, which contains the ethylene-binding region of cucumber ETR1 .

How can the histidine kinase activity of recombinant cucumber ETR1 be reliably measured?

Measuring the histidine kinase activity of ETR1 requires specialized biochemical approaches:

• Autophosphorylation assays:

  • Incubate purified ETR1 with [γ-32P]ATP

  • Analyze phosphorylation by SDS-PAGE and autoradiography

  • Compare activity with and without ethylene to assess regulatory effects

  • Include appropriate controls (kinase-dead mutants, phosphatase treatments)

• Phosphotransfer profiling:

  • Test phosphotransfer to potential response regulators

  • Use purified response regulator domains as substrates

  • Monitor transfer kinetics under varying conditions

  • Compare specificity with other histidine kinases

• Non-radioactive detection methods:

  • Phos-tag SDS-PAGE to detect phosphorylated species

  • Phospho-specific antibodies if available

  • Mass spectrometry to identify phosphorylation sites

  • FRET-based sensors for real-time kinase activity monitoring

• In vivo activity measurements:

  • Transform bacteria lacking endogenous histidine kinases with cucumber ETR1

  • Assess complementation of bacterial signaling pathways

  • Create chimeric proteins with bacterial components to enable in vivo assays

When designing kinase activity assays, researchers should consider that ethylene receptors like ETR1 function as negative regulators, with kinase activity potentially being inhibited rather than activated upon ethylene binding .

What emerging technologies might revolutionize our understanding of cucumber ETR1 function?

Several cutting-edge technologies hold promise for advancing ETR1 research:

• Single-molecule techniques:

  • Single-molecule FRET to observe conformational changes upon ethylene binding

  • Optical tweezers to measure force generation during signaling events

  • Super-resolution microscopy to visualize receptor clustering and dynamics

• Cryo-electron microscopy:

  • Determination of full-length ETR1 structure in different functional states

  • Visualization of ETR1 complexes with interacting partners like RTE1

  • Structural changes induced by ethylene binding

• Optogenetic approaches:

  • Engineering light-responsive ETR1 variants to control receptor activity

  • Spatiotemporal control of ethylene signaling in specific tissues

  • Real-time monitoring of downstream signaling events

• CRISPR-based technologies:

  • Base editing to create specific amino acid changes without double-strand breaks

  • CRISPRa/CRISPRi for precise transcriptional control of ETR1 expression

  • Prime editing for introducing specific mutations with minimal off-target effects

• Computational approaches:

  • Molecular dynamics simulations of ethylene binding and signal transduction

  • Machine learning to predict protein-protein interactions

  • Systems biology modeling of the entire ethylene signaling network

These technologies may help resolve outstanding questions about how ethylene binding to the transmembrane domains of ETR1 is translated into changes in histidine kinase activity and downstream signaling events.

How might comparative studies between cucumber ETR1 and homologs in other cucurbits advance our understanding of receptor evolution?

Comparative analyses across cucurbit species can provide valuable evolutionary insights:

• Functional diversification studies:

  • Compare ETR1 sequences and functions across cucumber, melon, watermelon, and squash

  • Identify conserved vs. divergent domains that may relate to species-specific traits

  • Analyze rates of selection across different protein domains

• Cross-species complementation:

  • Express cucumber ETR1 in other cucurbit species with ETR1 mutations

  • Test whether functional differences correlate with physiological differences

  • Identify species-specific interaction partners

• Developmental role comparison:

  • Compare ETR1 expression patterns during similar developmental events

  • Focus on sex determination given the known role in cucumber

  • Analyze correlation between ETR1 variants and sexual system diversity in cucurbits

• Hormone sensitivity analysis:

  • Compare ethylene and GA responses across species with different ETR1 variants

  • Analyze how ETR1 polymorphisms relate to domestication traits

  • Investigate whether ETR1 has been a target of selection during cucurbit domestication

This comparative approach may reveal how modifications to ETR1 structure and regulation have contributed to the diverse developmental patterns and environmental adaptations seen across the Cucurbitaceae family.