Asymmetric Hydration and Protonation Switching of Dual Aspartates Drive Flagellar Rotation

This study elucidates the rotational mechanism of the *Campylobacter jejuni* flagellar motor by demonstrating that asymmetric hydration and alternating protonation switching of dual D22 residues, coupled with plug removal and sidechain conformational changes, are essential for converting ion gradients into mechanical force.

The Gist

Imagine a tiny, microscopic engine inside a bacterium. This engine, called the flagellar motor, is what allows the bacteria to swim, hunt for food, and escape danger. It's one of nature's most impressive nanomachines.

For a long time, scientists knew that this engine worked, but they didn't quite understand how it turned chemical energy (from protons, which are tiny charged particles) into physical spinning. It was like knowing a car has an engine but not knowing how the fuel makes the wheels turn.

This paper is like a high-definition detective story that finally cracks the case. The researchers used super-powerful computer simulations and advanced microscopes to watch the engine in action, atom by atom. Here is the story of how they figured it out, using some simple analogies.

1. The Engine and the "Plug"

Think of the motor as a water wheel in a river. The river is the flow of protons (charged particles) outside the bacterium.

• The Stator (The Engine): This is the stationary part of the motor that sits in the cell wall. It has two main parts, like a pair of hands (MotA and MotB).
• The Plug: In its resting state, the engine has a "plug" (a piece of protein) jamming the water channel. This is like putting a cork in a bottle. No water (protons) can get in, so the wheel doesn't spin. The engine is "asleep."
• Unplugging: To wake up, the engine must pull this plug out. But the researchers found something surprising: Just pulling the plug isn't enough. Even with the plug gone, the engine won't spin unless something else happens.

2. The Two "Hands" and the Switch

Inside the engine, there are two special amino acids (building blocks of proteins) named D22. Let's call them Hand F and Hand G. They are the "proton carriers."

The big discovery is that these two hands don't work the same way. They are asymmetric (different from each other), like a left hand and a right hand.

• Hand F is usually wet (hydrated) and ready to grab a proton.
• Hand G is usually dry (dehydrated) and holding onto a proton tightly.

3. The "Magic Switch" (Protonation)

The engine runs on a cycle of switching. Imagine a seesaw where one side goes up while the other goes down.

1. The Grab: Hand F (which is wet) grabs a proton from the outside. This is like catching a ball.
2. The Spin: When Hand F catches the proton, it changes its shape (like a finger curling). This shape change pulls on the rest of the engine, giving it a little "kick" or "power stroke" that makes it rotate.
3. The Release: As the engine spins, Hand F gets pushed into a dry, tight spot where it can't hold the proton anymore. It drops the proton off inside the cell.
4. The Swap: Now, Hand G (which was holding a proton) gets wet and lets go of its proton, while Hand F gets dry and ready to grab a new one.

The Analogy: Imagine a relay race where two runners (Hand F and Hand G) are passing a baton (the proton).

• Runner A grabs the baton, runs a step, and passes it to Runner B.
• But here's the trick: The track is designed so that Runner A can only run when they are holding the baton, and Runner B can only run when they are not holding it.
• They take turns grabbing and dropping the baton, and this back-and-forth motion pushes the wheel forward.

4. The Role of Water (The Lubricant and the Brake)

The most fascinating part of this study is the role of water.

• Water acts as a switch: When a "hand" is surrounded by water, it's easier for it to let go of a proton (like dropping a wet ball). When it's dry, it holds the proton tightly (like a dry ball sticking to your hand).
• The Cycle: The engine's rotation physically moves the hands between wet and dry zones.
  • Dry Zone: The hand grabs the proton and holds it tight.
  • Wet Zone: The hand gets wet, the proton slips away, and the hand resets.

This means water isn't just a background fluid; it's an active part of the engine's control system. It tells the engine when to grab and when to let go.

5. The Shape-Shifting Dancers

The researchers also noticed that the "hands" (the D22 residues) change their physical shape (twisting like a dancer) depending on whether they are holding a proton or not.

• When they hold a proton, they twist one way.
• When they lose it, they twist the other way.
  This twisting is what physically pulls the engine's gears, turning the chemical energy of the proton into the mechanical energy of rotation.

The Big Picture: What Did We Learn?

Before this paper, scientists thought maybe just removing the "plug" was enough to start the motor. This study proves that removing the plug is just the first step. The engine also needs the proton switch to work.

The Unified Model:

1. Wake Up: Pull the plug.
2. Grab: A proton lands on one hand (Hand F) because it's in a dry spot.
3. Twist & Spin: Hand F changes shape, pulling the engine and making it rotate.
4. Drop: The rotation moves Hand F into a wet spot, forcing it to drop the proton.
5. Reset: The other hand (Hand G) gets ready to grab the next proton.

Why Does This Matter?

This isn't just about bacteria. It's about understanding how nature builds machines.

• Efficiency: This motor is incredibly efficient, converting almost all its energy into motion.
• Medical Applications: Understanding how these motors work could help us design new antibiotics that jam the engine, stopping bacteria from moving and infecting us.
• Bio-engineering: It gives us a blueprint for building our own tiny, artificial motors that run on chemical energy, which could be used for drug delivery inside the human body.

In short, the researchers solved the mystery of how a microscopic engine uses water, shape-shifting, and a relay race of protons to spin its wheels. It's a beautiful example of how nature uses simple physics to create complex life.

Technical Summary

1. Problem Statement

The bacterial flagellar motor (BFM) is a sophisticated nanomachine that converts chemical energy from ion gradients into mechanical rotation. While the general architecture of the stator unit (MotAB) is conserved, the precise molecular mechanism linking ion translocation (specifically proton flow) to rotational force generation remains elusive.

• Key Knowledge Gap: Previous studies identified the conserved aspartate residue D22 in MotB as critical for function, but the dynamic interplay between protonation states, conformational changes, and the role of water molecules (hydration) in driving unidirectional rotation was not fully understood.
• Limitations of Previous Models: Existing molecular dynamics (MD) studies often relied on coarse-grained (CG) models that oversimplified side-chain details and hydration effects. Furthermore, standard MD simulations with fixed charges cannot capture the dynamic protonation state transitions essential to the motor's mechanism.

2. Methodology

The authors employed a multi-scale computational approach integrating high-resolution structural biology with advanced molecular simulations:

• Cryo-EM Structure Refinement:
  • Reprocessed single-particle cryo-EM datasets of Campylobacter jejuni MotAB (CjMotAB) in both "plugged" (inactive) and "unplugged" (active) states.
  • Achieved resolutions of 2.6 Å (plugged) and 2.3 Å (unplugged), resolving previously ambiguous regions, specifically the N-terminal tail of MotB (residues C10–T40) and bound water molecules.
• Constant-pH Molecular Dynamics (CpHMD):
  • Performed explicit-solvent atomistic $\lambda$-dynamics CpHMD simulations using GROMACS.
  • Allowed titratable residues (Glu, Asp, His) to dynamically switch protonation states ($\lambda$) in response to conformational changes and pH (1–12).
  • Simulated four distinct models: Plugged, Unplugged, and Plug-removed variants with specific combinations of protonated/deprotonated D22 residues.
• Validation and Cross-Validation:
  • pKa Prediction: Compared CpHMD results with single-structure predictors (PropKa 3.0, PypKa, DeepKa).
  • Free Energy Perturbation (FEP): Calculated deprotonation free energy differences ($\Delta\Delta G$) using a thermodynamic cycle to rigorously validate pKa shifts.
• Mechanistic Analysis:
  • Analyzed side-chain dihedral angles ($\chi_1$), hydrogen bond networks, solvent accessibility (SASA), and water coordination numbers.
  • Utilized path collective variables (CV) to quantify MotA rotational dynamics relative to MotB.

3. Key Contributions

1. High-Resolution Structural Models: Provided the first high-resolution structural models of CjMotAB that include the previously unresolved N-terminal tail of MotB and explicit water molecules, crucial for understanding the proton pathway.
2. Asymmetric Protonation Mechanism: Demonstrated that the two homologous D22 residues in the MotB dimer (F-chain and G-chain) exhibit functional asymmetry in their protonation behavior and hydration, rather than acting identically.
3. Dual Prerequisites for Rotation: Established that MotAB rotation requires two simultaneous conditions: (1) removal of the plug domain and (2) the protonation of at least one D22 residue. Unplugging alone is insufficient.
4. Hydration-Driven Regulation: Identified that local hydration dynamics actively regulate protonation. Dehydration stabilizes the protonated state (raising pKa), while rehydration facilitates proton release.
5. Unified Mechanistic Model: Proposed a comprehensive, stepwise model where protonation switching, side-chain conformational changes ($\text{gauche} \leftrightarrow \text{trans}$), and asymmetric hydration drive the $\pi/5$ ($36^\circ$) rotational steps.

4. Key Results

• pKa Shifts and Asymmetry:
  • CpHMD and FEP calculations revealed massive pKa upshifts for MotB D22 residues compared to standard aqueous values.
  • F-D22: pKa $\approx$ 10.0.
  • G-D22: pKa $\approx$ 11.3 (and >12 in the plugged state).
  • FEP Validation: G-D22 showed a significantly higher deprotonation barrier ($\Delta\Delta G = 36.98$ kcal/mol) compared to F-D22 ($12.69$ kcal/mol), confirming G-D22 is much harder to deprotonate.
• Hydration Asymmetry:
  • F-D22: Highly hydrated (avg. 8–10 water molecules).
  • G-D22: Largely dehydrated (avg. 0–1 water molecules).
  • Dynamic Coupling: Rotation induces a swap in hydration states; as MotA rotates, F-D22 becomes dehydrated (favoring protonation) while G-D22 becomes hydrated (favoring deprotonation).
• Conformational Dynamics:
  • Side-chain $\chi_1$ Angle: Protonation triggers a dynamic transition between gauche and trans conformations.
  • Hydrogen Bonding:
    • G-D22 (Protonated): Forms a stable hydrogen bond with MotA T189 (in trans conformation).
    • F-D22 (Protonated): Forms a hydrogen bond with MotA T189 (in gauche conformation).
  • Rotation is driven by the breaking and forming of these hydrogen bonds, coupled with the gauche $\leftrightarrow$ trans switch.
• Rotational Requirements:
  • Simulations of plug-removed models showed that if both D22 residues are deprotonated, no rotation occurs.
  • Rotation only occurs when at least one D22 is protonated, confirming that protonation is the driving force, not just the removal of the plug.

5. Significance

• Mechanistic Clarification: This study resolves the long-standing question of how proton flow is converted into torque. It moves beyond the "proton gate" concept to a dynamic "proton switch" mechanism where hydration and conformational changes are inextricably linked.
• Methodological Advancement: It demonstrates the superiority of all-atom CpHMD over static predictors and coarse-grained models for studying membrane proteins where protonation states are dynamic and coupled to large conformational changes.
• Biological Insight: The findings explain how the motor achieves unidirectional rotation through an alternating cycle of two asymmetric sites. The "asymmetric hydration" acts as a regulatory valve, ensuring protons are captured when the channel is open and released when the channel is oriented toward the cytoplasm.
• Broader Impact: The proposed unified model provides a foundational framework for understanding diverse ion-driven biological motors (e.g., sodium-driven PomAB) and offers a template for investigating other chemo-mechanical transduction systems.

In summary, the paper elucidates that CjMotAB rotation is driven by an alternating cycle of asymmetric hydration and protonation of dual D22 residues, which modulates hydrogen bonding with MotA T189 and triggers side-chain conformational switches, effectively converting chemical energy into mechanical work.