Hormone-dependent receptor docking controls calcium channel activity in plants

Technical Summary: Hormone-dependent receptor docking controls calcium channel activity in plants

Problem Statement
The phytohormone auxin is a central coordinator of plant growth and development. While its canonical pathway involves nuclear TIR1/AFB receptors acting as molecular glues to degrade Aux/IAA transcriptional repressors via the SCF ubiquitin ligase complex, a distinct "rapid auxin pathway" operates independently of transcription. This pathway, mediated by the AFB1 receptor, triggers ultra-rapid calcium (Ca²⁺) influx and root growth inhibition within seconds. Although the AFB1 receptor and the downstream Cyclic Nucleotide-Gated Channel 14 (CNGC14) have been identified as essential components, the molecular mechanism connecting the cytoplasmic receptor to the plasma membrane (PM) ion channel remained enigmatic. Specifically, it was unclear how AFB1, which functions independently of the SCF complex in the cytoplasm, physically interacts with and activates the PM-localized CNGC14 channel.

Methodology
The authors employed a multidisciplinary approach combining plant genetics, live-cell imaging, structural biology, and proteomics:

• Orthogonal Receptor System: The study utilized an engineered Arabidopsis thaliana line expressing the orthogonal receptor ccvAFB1, which binds exclusively to the synthetic ligand cvxIAA. This system allowed for the isolation of the rapid AFB1 pathway from the complex endogenous auxin signaling network.
• Proximity Labeling (TurboID): To identify transient interactors of the activated receptor, the authors generated a ccvAFB1-turboID-YFP line. They compared the "proxiome" of the ligand-bound (active) receptor against the unbound (inactive) receptor and a free TurboID control.
• Live-Cell Imaging: High-resolution spinning disk microscopy and FRET-FLIM (Förster Resonance Energy Transfer - Fluorescence Lifetime Imaging Microscopy) were used to visualize subcellular localization, colocalization, and direct protein-protein interactions in root epidermal cells.
• Structural Modeling: AlphaFold3 was utilized to model the interaction between AFB1, auxin, and the CNGC14 tetramer to predict binding interfaces.
• Biochemical Assays: Co-immunoprecipitation (co-IP) and pull-down assays using purified proteins and synthetic peptides were performed to validate interactions in vitro.
• Functional Assays: Root growth inhibition assays and real-time Ca²⁺ flux measurements (using GCaMP3 reporters in microfluidic devices) served as in vivo readouts for pathway activity.

Key Contributions and Results

1. Identification of the Direct Interaction: Proximity labeling revealed that the active ccvAFB1 receptor specifically associates with CNGC14 and Armadillo-Repeat Only (ARO) proteins. Unlike the canonical pathway, this interaction does not require the SCF complex or CUL1.
2. Ligand-Dependent Relocalization: The study demonstrates that the AFB1 receptor undergoes a binary localization switch. In the absence of auxin, AFB1 is purely cytoplasmic. Upon perception of the ligand (IAA or cvxIAA), AFB1 is rapidly recruited to the plasma membrane, forming foci that colocalize with CNGC14 and ARO2.
3. Mechanism of Activation: FRET-FLIM data confirmed a direct, ligand-dependent physical interaction between AFB1 and CNGC14. Crucially, this recruitment and subsequent Ca²⁺ influx were abolished in cngc14 mutants but persisted in aro2/3/4 mutants, indicating that CNGC14 is the primary docking partner required for receptor recruitment, while AROs are required for channel function but not for the initial docking event.
4. Structural Basis of the Interaction: AlphaFold3 modeling predicted that auxin acts as a "molecular glue" bridging the auxin-binding pocket of AFB1 and the C-terminal helix (residues 683–707) of CNGC14. This interaction is specific to AFB1 (which contains a unique F78Y mutation in the binding pocket) and the CNGC14 C-terminus.
5. Functional Validation:
   • Mutating the predicted interface residue in AFB1 (Y78F) prevented PM recruitment and abolished root growth inhibition, despite the mutant retaining the ability to bind auxin and form the canonical nuclear complex.
   • Pull-down assays using synthetic peptides confirmed that the CNGC14 C-terminal helix binds AFB1 in an auxin-dependent manner, and this binding is lost when key phenylalanine residues (F686, F695) in the CNGC14 helix are mutated.
   • The study ruled out the involvement of cyclic nucleotides (cAMP/cGMP) in this specific activation, as mutations in the putative adenylate/guanylate cyclase domains of AFB1 did not affect Ca²⁺ influx or receptor recruitment.

Significance
The paper establishes a novel paradigm in plant hormone signal transduction where a hormone receptor directly docks onto a downstream effector ion channel to trigger a rapid physiological response.

• CNGC14 as a Co-receptor: The findings position CNGC14 as an unprecedented component of the AFB1 auxin receptor complex, functioning as a direct co-receptor rather than a downstream target of a second messenger cascade.
• Alternative Signaling Paradigm: The study reveals that TIR1/AFB family receptors can utilize substrate recognition (in this case, the CNGC14 C-terminus) not for ubiquitin-mediated degradation, but for a localization shift that activates ion channels. This suggests the existence of unforeseen branches in auxin signaling where auxin acts as a molecular glue between F-box receptors and non-nuclear substrates.
• Mechanistic Insight: By defining the specific interaction interface (AFB1 auxin pocket and CNGC14 C-terminal helix), the work provides a structural basis for how auxin triggers ultra-rapid Ca²⁺ influx, a critical step in processes like root gravitropism.

The authors conclude that this hormone-dependent receptor docking represents an unprecedented mechanism of signal transduction in eukaryotic cells, opening new avenues for understanding how rapid, non-transcriptional responses are orchestrated in plants.