Structural determinants of endopilus assembly, stability and functional specificity in bacterial type II secretion

By integrating NMR, cryo-EM, and mutational analyses of the highly similar major pilins from *Dickeya dadantii* and *Klebsiella oxytoca*, this study reveals how subtle sequence variations in conserved type II secretion system endopili dictate their assembly, stability, and functional specificity across different ecological niches.

The Gist

Imagine bacteria as tiny, microscopic factories. To survive and cause disease, they often need to ship out special "tools" (proteins) to break down their surroundings or attack a host. To do this, they use a sophisticated delivery system called the Type II Secretion System (T2SS).

Think of this system as a biological catapult. At the heart of this catapult is a flexible, spring-like rope called the endopilus. This rope is made of many identical links (proteins) strung together. When the bacteria needs to launch a tool, this rope extends, grabs the tool, and pushes it out through a door in the outer wall of the cell.

This paper is a deep dive into two specific versions of this catapult rope, found in two different bacteria:

1. PulG: Found in Klebsiella, a human pathogen. It's like a specialized delivery truck that only carries one specific package (a starch-digesting enzyme).
2. OutG: Found in Dickeya, a plant pathogen. It's like a heavy-duty crane that carries a massive variety of tools (about 20 different enzymes) to chew up plant cell walls.

Even though these two ropes look 77% identical (like two cars from the same manufacturer but different models), they behave very differently. The scientists wanted to know: Why does one need a specific "fuel" to work, while the other is tough enough to work without it? And how does the rope know which tools to grab?

Here is the story of their findings, broken down with simple analogies:

1. The "Calcium Battery" vs. The "Super-Sturdy Rope"

The most surprising discovery was about calcium.

• The PulG Rope (Human Bacteria): This rope is like a sensitive high-tech gadget. It absolutely needs calcium ions (tiny charged particles) to stay together. If you take away the calcium (like removing the battery), the rope falls apart, and the bacteria can't launch its tools. The human body has plenty of calcium, so this bacteria doesn't need to worry; it can rely on this fragile, calcium-dependent design.
• The OutG Rope (Plant Bacteria): This rope is like a tough, military-grade cable. It has a special design that keeps it stable even if the calcium is removed. The plant environment can be tricky with calcium levels, so this bacteria evolved a "self-stabilizing" rope that doesn't need constant calcium to hold its shape.

The Secret: The scientists found that the "calcium battery" is located in a specific loop on the rope. When they swapped this loop between the two bacteria, the human bacteria's rope became tough (like the plant one), and the plant bacteria's rope became fragile (like the human one). The calcium site is the key to stability.

2. The "Velcro Patch" for Specificity

If the ropes are so similar, how does the plant bacteria know to grab its 20 different plant-eating tools, while the human bacteria only grabs its one starch tool?

The answer lies in a small, exposed patch of "Velcro" on the surface of the rope (a specific loop of amino acids).

• The scientists found that if they swapped this small patch on the human rope to look like the plant rope, the human bacteria suddenly started grabbing the plant tools!
• Conversely, if they changed the plant rope's patch to look like the human one, it lost its ability to grab the plant tools efficiently.

This patch acts like a magnetic key. It doesn't hold the rope together (that's the job of the calcium site); instead, it acts as a recognition signal that tells the rope, "Hey, I'm ready to pick up this specific package."

3. The "Extra Tail" That Doesn't Matter

The plant rope (OutG) has a long, floppy tail at the end that the human rope doesn't have. The scientists thought this tail might be important for grabbing the many different tools.

• The Surprise: They cut off this tail, and the rope still worked perfectly! It turned out this tail isn't essential for stability or for grabbing the tools in the lab. It might have a different job we haven't discovered yet, or perhaps it's just extra decoration that didn't get trimmed off during evolution.

The Big Picture: Evolution's Fine-Tuning

This paper teaches us a beautiful lesson about evolution. Even when two machines look almost identical (77% the same), nature has tweaked tiny, specific parts to make them perfect for their specific jobs:

• The Human Bacteria lives in a calcium-rich world, so it built a fragile, calcium-dependent rope that is easy to control.
• The Plant Bacteria lives in a variable world, so it built a super-stable, calcium-independent rope with a specialized "Velcro patch" to handle a wide variety of heavy-duty tools.

In summary: The scientists took apart two nearly identical bacterial delivery ropes, found the "battery" that keeps them stable, and identified the "magnetic hook" that decides what they carry. It's a perfect example of how small changes in a blueprint can lead to big differences in how a machine functions in its real-world environment.

Technical Summary

1. Problem Statement

Bacterial Type II Secretion Systems (T2SS) are essential nanomachines in Gram-negative bacteria that transport folded protein effectors across the outer membrane. A critical component of the T2SS is the endopilus (or pseudopilus), a periplasmic helical polymer composed of a major pilin subunit (GspG) and four minor pilins. While T2SS endopili are structurally related to Type IV pili (T4P), they possess a unique feature: a conserved calcium-binding site that stabilizes the major pilin.

Despite high sequence identity (>77%) between the major pilins of different species, there is a significant gap in understanding how subtle sequence variations dictate:

1. Stability: Why some pilins (like Klebsiella oxytoca PulG) are strictly calcium-dependent and unstable without it, while others (like Dickeya dadantii OutG) are stable regardless of calcium availability.
2. Specificity: How the endopilus recognizes and facilitates the secretion of specific substrates, given that T2SSs in different pathogens secrete vastly different repertoires of effectors (e.g., a single pullulanase in K. oxytoca vs. ~20 plant cell wall-degrading enzymes in D. dadantii).

2. Methodology

The authors employed a multi-disciplinary approach combining structural biology, biophysics, mutagenesis, and in vivo functional assays to compare the T2SS systems of K. oxytoca (Pul system) and D. dadantii (Out system).

• Structural Determination:
  • NMR Spectroscopy: Solved the solution structure of the calcium-bound OutG monomer and analyzed chemical shift perturbations (CSP) to map calcium binding and conformational changes.
  • Cryo-Electron Microscopy (Cryo-EM): Determined high-resolution (3.6 Å) structures of both wild-type OutG and PulG pili. This allowed for the visualization of subunit interfaces and calcium-binding sites in the assembled state.
• Biophysical Analysis:
  • NanoDSF (Nano-Differential Scanning Fluorimetry): Measured thermal stability ($T_m$) and refolding capabilities of purified pilin monomers under varying conditions (buffer, +CaCl$_2$, +EGTA).
  • In Vitro Stability Assays: Incubated isolated pili at 60°C with calcium or chelators (EGTA) to quantify disassembly.
• Mutagenesis and Chimeras:
  • Generated chimeric proteins by swapping specific variable regions between OutG and PulG:
    • CTer: C-terminal extension.
    • L: $\alpha3-\beta1$ loop (residues 69–83).
    • Ca: Calcium-binding site (residues 114–123).
  • Created variants with truncated or swapped loops (e.g., $Li$ variants swapping only residues 69–81).
• In Vivo Functional Assays:
  • Pilus Assembly: Tested pilus formation on bacterial surfaces in the presence/absence of EGTA using immunodetection.
  • Secretion Assays: Measured the secretion efficiency of the substrate PelD (pectate lyase) in D. dadantii $\Delta outG$ mutants expressing various pilin variants.

3. Key Results

A. Calcium Dependence and Stability

• Phenotypic Difference: K. oxytoca PulG pili disassemble rapidly in the absence of calcium (EGTA treatment), whereas D. dadantii OutG pili remain stable.
• Monomer Stability: NanoDSF revealed that OutG has a higher intrinsic melting temperature ($T_m \approx 63.4^\circ$C) than PulG ($T_m \approx 53.8^\circ$C). Crucially, when calcium is removed, PulG cannot refold, while OutG can partially refold, indicating OutG is less strictly dependent on calcium for structural integrity.
• Structural Basis: Both proteins share a similar "lollipop" fold. The calcium-binding site involves residues D117, T122, D125, and backbone carbonyls. While the coordination geometry is similar, the local environment differs.

B. Structural Determinants of Stability

• The Calcium-Binding Site is Critical: Swapping the calcium-binding site region (residues 114–123) between the two proteins completely swapped their stability phenotypes.
  • OutG$^{Ca}$ (OutG with PulG's Ca-site): Became unstable and failed to assemble pili without calcium.
  • PulG$^{Ca}$ (PulG with OutG's Ca-site): Gained stability and assembled pili even in the presence of EGTA.
• Other Regions:
  • The C-terminal extension (CTer) and the $\alpha3-\beta1$ loop (L) did not determine the intrinsic thermal stability or calcium dependence of the monomer.

C. Structural Determinants of Secretion Specificity

• The $\alpha3-\beta1$ Loop Controls Specificity: While the loop did not affect stability, it dictated secretion specificity in D. dadantii.
  • PulG$^{L}$ (PulG with OutG's loop): Restored the ability to secrete PelD in D. dadantii.
  • OutG$^{L}$ (OutG with PulG's loop): Reduced secretion efficiency.
  • Fine Mapping: The effect was localized to residues 69–81; swapping the entire loop (including residues 82–83) had different effects due to buried interactions (salt bridges) at the pilus interface.
• Surface Electrostatics: The Pilus surface potentials differ significantly. PulG pili are highly negatively charged, while OutG pili have a more balanced distribution with positive patches. This may influence interactions with substrates.

D. Structural Insights from Cryo-EM

• Both pili form 1-start helical filaments with similar parameters.
• Inter-subunit Contacts: A key difference was identified at the interface between subunit P and P+1. In OutG, a salt bridge forms between D83 (subunit P) and K108 (subunit P+1). In PulG, the equivalent residues (G83 and Q108) do not form this bridge. This interaction reinforces the OutG pilus stability.
• Calcium Visibility: Calcium ions were clearly resolved at the surface of both pili in the cryo-EM maps.

4. Significance and Conclusions

• Evolutionary Adaptation: The study demonstrates that subtle sequence variations in conserved nanomachines are fine-tuned to specific ecological niches.
  • OutG (Plant Pathogen): Evolved a calcium-binding site and surface properties that confer stability even in fluctuating periplasmic calcium levels (common in plant environments) and reduced calcium competition with its diverse set of secreted enzymes (which also require calcium).
  • PulG (Human Pathogen): Remains strictly calcium-dependent, likely because calcium is abundant in the human host (mucus, blood), allowing for a regulatory layer where assembly is coupled to environmental calcium availability.
• Functional Decoupling: The study successfully decoupled the determinants of stability (controlled by the calcium-binding site) from secretion specificity (controlled by the $\alpha3-\beta1$ loop).
• Mechanistic Insight: The findings challenge the view that T2SSs are purely promiscuous; instead, the major pilin encodes specific determinants for substrate recognition and system integration. The $\alpha3-\beta1$ loop likely mediates specific interactions with substrates or other T2SS components (like the secretin or minor pilins) to ensure correct effector selection.

In summary, this work provides a high-resolution structural and functional map of how T2SS major pilins evolve to balance structural stability with functional specificity, adapting to the distinct physiological constraints of their bacterial hosts.