An N-degron proteolytic pathway modulates recipient susceptibility to T6SS DNase effectors

1. Problem Statement

The Type VI Secretion System (T6SS) is a contractile nanoweapon used by Gram-negative bacteria to inject toxic effector proteins into neighboring cells, providing a competitive advantage. While the biochemical mechanisms of T6SS effectors are well-characterized, the molecular determinants of recipient susceptibility (RS)—specifically, how recipient cell factors modulate the toxicity of delivered effectors—remain poorly understood.

Previous work by the authors identified the Escherichia coli ClpAP protease complex (specifically the unfoldase ClpA and protease ClpP) as an RS factor that enhances susceptibility to Agrobacterium tumefaciens C58 T6SS attacks. However, the mechanism by which a recipient's housekeeping protease facilitates the toxicity of an invading DNase effector was unknown. The central question was: How does the recipient ClpAPS protease system promote the lethality of the T6SS DNase effector Tde2?

2. Methodology

The study employed a multidisciplinary approach combining bacterial genetics, interbacterial competition assays, proteomics, and structural biology:

• Bacterial Strains & Mutants:
  • Attacker: Agrobacterium tumefaciens C58 wild-type (WT) and derivatives with specific deletions of T6SS effectors (Δtde1, Δtde2, Δtde1/2) or catalytic dead variants (Tde2-H439A/D442A).
  • Recipients: E. coli BW25113 Keio collection mutants (ΔclpP, ΔclpA, ΔclpS, ΔclpX), Dickeya dadantii mutants, and Agrobacterium C58 Δ3TIs (effector-null) mutants with clpP/clpS deletions.
• Interbacterial Competition Assays:
  • Co-cultured attacker and recipient strains on AK agar.
  • Quantified killing efficiency using a Susceptibility Index (SI), calculated as the log10 difference in recipient Colony Forming Units (CFUs) between the T6SS-defective control and the active attacker.
• Ectopic Expression & Toxicity Assays:
  • Expressed His-tagged Tde2 (WT and catalytic dead) in E. coli recipients under L-arabinose induction.
  • Assessed growth inhibition, plasmid DNA degradation (agarose gel), and DNA double-strand breaks via TUNEL assays (flow cytometry).
• Proteomics (LC-MS/MS):
  • Performed Ni-NTA pull-down assays using His-Tde2 expressed in a ΔclpP background (where Tde2 accumulates).
  • Identified Tde2-associated proteins via Liquid Chromatography-Tandem Mass Spectrometry to find potential inhibitory substrates.
• Biochemical Validation:
  • Co-immunoprecipitation (Co-IP): Verified physical interactions between Tde2 variants and candidate substrates (e.g., GuaC).
  • Complementation & Overexpression: Tested the effect of overexpressing candidate substrates on T6SS killing efficiency.
  • Mutagenesis: Generated catalytic site mutants of candidate substrates (e.g., GuaC-C186A) to test if enzymatic activity was required for inhibition.

3. Key Contributions & Results

A. Tde2 Toxicity is Strictly Dependent on Recipient ClpAPS

• Specificity: The ClpAPS requirement was specific to the Tde2 effector. Agrobacterium strains delivering only Tde2 (Δtde1) showed drastically reduced killing of ΔclpP, ΔclpA, and ΔclpS recipients compared to WT recipients. Tde1-mediated killing was unaffected.
• Protease Activity: Restoration of susceptibility required both the ClpA-ClpP complex formation (binding) and ClpP protease activity. Catalytic dead ClpP variants failed to restore susceptibility.
• Conservation: This dependency was observed across species, including E. coli, Dickeya dadantii, and Agrobacterium C58 (intra-species competition).

B. Paradoxical Accumulation of Tde2 in Protease Mutants

• In ΔclpP/A/S recipients, ectopically expressed Tde2 accumulated to much higher levels than in WT recipients, yet caused no growth inhibition or DNA damage.
• This indicated that Tde2 is not the direct substrate of ClpAPS for activation. Instead, the protease likely degrades an inhibitory factor that binds to Tde2. In the absence of ClpAPS, this inhibitor accumulates, sequestering Tde2 and preventing its DNase activity.

C. Identification of N-degron Substrates

• LC-MS/MS of Tde2 pull-downs from ΔclpP cells identified eight candidate N-degron substrates (proteins with N-terminal degrons targeted by ClpS/ClpAP): RapA, SlyB, GuaC, MreB, FtsY, ParC, AccC, and PoxB.
• Functional Screening:
  • Overexpression of GuaC (GMP reductase) and PoxB significantly reduced Tde2-mediated killing.
  • Overexpression of AccC reduced killing by both Tde1 and Tde2.
  • Deletion mutants of these genes showed variable resistance, likely due to functional redundancy.

D. Mechanism of Inhibition: GuaC Binding

• Specific Interaction: GuaC was identified as the strongest specific inhibitor of Tde2.
• Domain Mapping: The DNase domain of Tde2 (residues 261–526) was sufficient to bind GuaC.
• Catalytic Independence: The inhibition of Tde2 by GuaC did not require GuaC's enzymatic activity (GMP reductase function), as the catalytic dead mutant (GuaC-C186A) still inhibited Tde2.
• Binding Motif: The interaction depended on the intact HxxD catalytic motif of Tde2, suggesting GuaC binds specifically to the active conformation or the catalytic pocket of Tde2, thereby blocking its DNase function.

4. Proposed Model

The authors propose a model of "Indirect Activation":

1. Delivery: Agrobacterium injects Tde2 into the recipient cytoplasm.
2. Inhibition: In the recipient, Tde2 binds to an inhibitory N-degron substrate (e.g., GuaC), which blocks its DNase activity.
3. Proteolytic Clearance: The recipient's ClpAPS protease system recognizes and degrades the inhibitory substrate (GuaC) via the N-end rule pathway.
4. Activation: Once the inhibitor is cleared, Tde2 is free to degrade recipient DNA, leading to cell death.
5. Failure: If ClpAPS is defective, the inhibitor persists, sequestering Tde2 and rendering the recipient resistant.

5. Significance

• Novel RS Mechanism: This study reveals a previously uncharacterized layer of recipient susceptibility where a conserved housekeeping proteostasis pathway (ClpAPS/N-degron) is co-opted by the attacker to remove cellular "brakes" on the toxin.
• Paradigm Shift: It challenges the view that T6SS effectors act solely on their own; instead, their efficacy is contingent on the recipient's proteolytic capacity to clear specific inhibitors.
• Evolutionary Insight: The dependence on a conserved host pathway (ClpAPS) suggests that T6SS effectors may have evolved to exploit fundamental cellular quality control mechanisms, potentially reducing the selective pressure for effectors to evolve independent activation mechanisms.
• Therapeutic Potential: Understanding that bacterial competition relies on the recipient's proteolytic state could inform strategies to modulate microbiome dynamics or develop anti-virulence therapies that target these susceptibility pathways.

In summary, the paper demonstrates that the recipient ClpAPS protease system is a critical determinant of T6SS-mediated killing, acting indirectly by degrading inhibitory N-degron substrates (specifically GuaC) that otherwise neutralize the Tde2 DNase effector.