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What It Takes to Build a Hook for the Flagellum

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You’ve just been hired as a software engineer. Your first project is to write code that will operate robotic machines. The robots need to build a high-speed universal joint and fasten it to a shaft that will rotate at high speed. The code needs to select materials that can tolerate the high stresses they will face, and arrange them into flexible, mutually-reinforcing configurations that will provide high performance and fault tolerance over many cycles of switching between prograde and anterograde rotations. Oh, and it needs to be hollow so that other materials can pass through during operation. Good luck.

Would everyone agree a new hire facing this challenge had better have high intelligence, combined with a great deal of experience with materials and Einsteinian knowledge of physics? What if you were told the company is considering farming out the task to a brainless entity with no sensors that can only rely on sheer dumb luck?

New light has been shed on just one part of the molecular machine that started Michael Behe on his revolutionary role as a champion of intelligent design: the bacterial flagellum. We often hear the parts of this iconic molecular machine rattled off in a list: rotor, stator, hook, propeller etc. The elegant animations in our new documentary Revolutionary certainly make the point, but not shown are some additional levels of complexity beneath the exterior that put extra oomph into the case for ID. A new paper opens Darwin’s black box further.

In Nature Communications, five researchers from the Okinawa Institute of Science and Technology in Japans describe the “complete structure of the bacterial flagellar hook” in Campylobacter jejuni, a bacterium with a single flagellum at one end of its spiral-shaped rod-like cell (some varieties have two flagella, one at each pole). C. jejuni colonizes the digestive tracts of cattle and many birds without apparent harm, but we humans don’t want it in our guts, because it is a major cause of food poisoning. Proper cooking of poultry and meat usually prevents disease. But we digress; such matters are beyond the domain of intelligent design theory. The focus is on whether the flagellar hook is designed or not.

Before looking at C. jejuni‘s flagellum, let’s compare the outboard motor styles of different bacteria. The authors write:

Flagella are found in both gram-positive and gram-negative bacteria. Although flagellar hooks appear identical at first sight, the diversity of flagellar hook proteins suggests that the hooks have diverged to specifically fit the motility requirements of each bacterium. The cells of the food-borne pathogen, Campylobacter jejuni (C. jejuni) are spiral-shaped and are able to move using unipolar or bipolar flagella, in comparison with rod-shaped S[almonella] enterica cells, which move using many peritrichous flagella over the cell surface. Uniquely, the C. jejuni flagellar hook is also used to export virulence factors during colonization of the avian or human host. Intriguingly, the C. jejuni hook protein, FlgEcj, has one of the longest amino acid sequences compared with other bacterial FlgE proteins. Compared with FlgE from S. enterica, FlgEcj is about twice as large. We solved the structure of the hook of a fliK null mutant strain derived from C. jejuni strain 81116 (NCTC 11828) by electron cryo-microscopy, using single-particle averaging methods with image segment classification followed by systematic symmetry exploration. [Emphasis added.]

As stated, there appear to be “motility requirements” behind the different motor styles, even though the authors do not question whether they evolved:

Flagella, although macroscopically similar, have evolved features that will make them specially adapted to particular tasks. The intestinal jejunum is a viscous environment where C. jejuni is adapted for swimming. The results shown here tend to support the idea that additional strengthening of the hook in C. jejuni is necessary to enable motility in this viscous environment.

So they are really talking about adaptation, not evolution. Nowhere do they describe how this extra-strong flagellum originated or how it evolved from another species. One story they tell is beside the point:

Amino acid sequence variability in the central parts of FlgE proteins of C. jejuni strains was proposed to occur because of selection pressure during host invasion to generate variations in surface-exposed antigenic determinants. The variable regions do indeed correspond to the surface-exposed region of C. jejuni hook domains D3 and D4. The variability is tolerated because these regions are not essential for intra-molecular contacts that organize the hook.

That appears to be a case of degeneration, not innovation. It’s a type of variation not related to function.

What they really know is that this flagellar hook meets the design requirements for its particular environment. The point of their paper is that the hook region of this species’ outboard motor, “reveals extensive set of stabilizing interactions.”

That’s where things get interesting, because a hook is really a universal joint. A universal joint has to be able to transmit rotary power from a shaft through a range of angles. The laws of physics come into play here: torque, angular momentum, and material strain. An outboard motor with a universal joint will have to meet design requirements to handle extra strain, especially when the viscosity of its medium increases. Think of the stresses on a universal joint operating in oil instead of water, for instance. Now rev up the RPMs, and you can see what the software engineer is up against!

Bacterial flagella have long been studied, but many aspects of their structure and function are still eluding us. The structure of the hook can be described as a tubular helical structure. The hook functions like a universal joint. It transmits the torque, produced by the motor located in the cell membrane, to the filament that acts like a propeller. The assembly of about a 100 copies of a single protein, FlgE, makes the bacterial flagellar hook. An exported molecular ruler protein, FliK, controls the hook length. Cells bearing mutations in the fliK gene produce abnormally long ‘polyhook’ structures. The bacterial flagellar motor rotates at frequencies that vary between 100 Hz and 2,000 Hz depending on bacterial species. The hook undergoes multiple conformational changes while rotating around its axis. During these conformational changes, the interactions between the FlgE molecules must secure the stability while enabling the dynamic nature of the hook.

Get that calculator out. 100 Hz is 6,000 RPM; 2,000 Hz is, let’s see, yikes! 120,000 revolutions per minute! Our software engineer is getting really worried. His application only has one shaft, but the universal joint has to handle extreme torque in viscous fluid. The strain on the material is going to be enormous! How can he make it strong enough without compromising its flexibility?

Extra domains in FlgE, found only in Campylobacter and in related bacteria, bring more stability and robustness to the hook. Functional experiments suggest that Campylobacter requires an unusually strong hook to swim without its flagella being torn off.

If the engineer is smart, he will just mimic the elegant solution of the bacterium C. jejuni. Take a look at Figure 2 in the open-access paper. It’s beautiful. Multiple strands of the FlgE protein (remember, it’s twice as long as its counterparts in other species) intertwine in five layers. Each layer contains complex interactions with the other layers, providing the strength needed for the application, “while still allowing it to curve.” Even more amazing, “the interactions between the molecules that make the hook are transient interactions where interacting amino acid residues will constantly change partners during the rotation of the hook.”

A committed Darwinist might still be able to weave a just-so story about how this hook evolved. Once upon a time, Campylobacter having a simpler hook with fewer strands of FlgE invaded a new environment that was more viscous. By accident, a mutant with duplicate layers of FlgE succeeded in swimming without its flagella being torn off, so its descendents survived. By another accident, the ones with longer FlgE proteins also survived, perhaps by duplication. Somehow, blindly, complex transient interactions arose… etc. Sound convincing?

Evolutionists might argue that the hook only employs FlgE proteins, so it is not irreducibly complex. Nothing about Behe’s irreducible complexity principle, however, requires multiple types of materials. It’s their arrangement for function that matters. All the parts for a mousetrap, for instance, could be made of iron. In the case of the flagellar hook, the arrangement of the proteins in multiple layers with multiple transient interactions is the key to its operation. Remove these layers and interactions, and the flagellum would be torn off when the bacterium tried to swim.

We haven’t even addressed the issue of how this hook is assembled in the cell. To be heritable, mutations would have had to occur in the DNA. Those mistakes would have to evade the proofreading mechanisms of the cell during transcription and translation. But even if the mutant proteins arrived at the construction site (at the right time and in the right quantities, don’t forget), how would other molecular machines know what to do with them? Unless they got assembled correctly so that the transient interactions would take place properly during operation, they would provide no benefit; there would be no “selection pressure” to maintain them. The evolutionary story seems even more implausible at the level of coding and construction.

So there you have it: even the hook of the famous flagellum is irreducibly complex. It is amazingly well designed for its task within a task, that of being an essential part of a multi-part molecular machine. As Jonathan Wells says, “What we have here is irreducible complexity all the way down.”

Image: Campylobacter jejuni, by De Wood, Pooley, USDA, ARS, EMU. [Public domain], via Wikimedia Commons.

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