Self-Assembly of the Bacterial Flagellum: No Intelligence Required
In my last post, I explained why the bacterial flagellum remains so powerful an icon for the Intelligent Design (ID) movement: it looks and functions just like the outboard motor, a machine designed by intelligent human engineers. So conspicuous is the resemblance that it seems perfectly logical to infer a Designer for the flagellum.
Yet as we saw, appearances can be deceiving. ID advocates William Dembski and Jonathan Witt agree that “a careful investigator will be on guard against deceiving appearances. The sun looks like it rises in the east and sets in the west, but really the Earth spins on its axis as it revolves around the sun. A healthy skepticism about appearances is vital…To distinguish appearance from reality, the successful investigator must remain open to various possibilities and follow the evidence.”
Despite the strong appearance of special design, most scientists, myself included, believe the evidence points to a gradual development for the bacterial flagellum. We’ll delve into some of that evidence in future posts. First, however, I want to explain how flagella are assembled in bacteria. This amazing process gives me such delight in our Father’s world; I hope it does for you as well.
How does the flagellum assemble?
The bacterial flagellum may look like an outboard motor, but there is at least one profound difference: the flagellum assembles spontaneously, without the help of any conscious agent. The self-assembly of such a complex machine almost defies the imagination. As I showed with an earlier blog on the self-assembly of viruses (much simpler contraptions by comparison), all such phenomena seem astonishing and counterintuitive.
Because the tail of the flagellum extends well beyond the bacterial cell wall, many of its 40 or so components have to be extruded through an export apparatus that assembles first and makes up the base of the final structure. In general, assembly occurs as a linear process, with components in the base coming together first, followed by the formation of the hook, followed by formation of the filament (see figure).
First, the MS-ring (orange) assembles in the inner cell membrane, most likely in conjunction with some of the export proteins (light green; labeled Type III secretion system). The MS-ring serves as housing for the export apparatus and as a mounting plate for the rotor, which will assemble later.
Next, the stator (gray) assembles around the MS-ring, followed by the rotor (light blue; labeled C-ring). The stator remains fixed in the cell’s frame of reference, while the rotor spins; together, these two parts make up the proton-powered motor.
Now that the base of the flagellum is built, most of the remaining parts are assembled from proteins exported through its center. First comes the rod (yellow), made of four different kinds of proteins, guided by a fifth, the “rod cap,” which is believed to help break down the tough bacterial cell wall.
This “rod cap” is then displaced by a “hook cap,” which guides the formation of the hook structure (dark blue). The hook acts as a universal joint to connect the rod and the filament. When the hook reaches its characteristic length, several “junction zones” form, followed by the export of the “filament cap” protein. This cap structure, different than the rod or hook caps, guides the bundling of more than 20,000 copies of a protein called flagellin into a helical tail (dark green; labeled filament).
The helical filament is long and fragile, but breakage is not too serious a concern for the bacterium. Like a lizard, the flagellum can grow a new tail if it breaks, because flagellin proteins continue to move down the central channel from the cell body toward the tip. Other parts of the flagellum are dynamic as well: individual proteins in the rotor and stator, for example, can exchange with freely-diffusing proteins in the membrane. Such activity may be important for the bacterium’s direction-sensing capability.
How do we know all this?
Scientists are pretty clever at teasing out the workings of microscopic machines like the flagellum. The general order of assembly was meticulously worked out by removing individual protein components one at a time and observing what occurred. If you remove the flagellin protein, for instance, you get the base and the hook, but not the tail. This tells us that the tail forms late in the assembly process. If you remove one of the proteins that make up the MS-ring, on the other hand, the motor elements do not assemble and neither does the rest of the flagellum. That’s how we know the MS-ring isn’t just tacked on at the end.
Other scientists have looked at how the timing of the assembly process is controlled at the genetic level. The genes that contain the instructions for making all the protein components of the flagellum are organized in a number of clusters called operons. Each operon is read when its “master sequence” is activated like a light switch. When the switch is flipped, the genes in that particular operon are interpreted by the cell so that the corresponding proteins are made. It turns out that the genes needed to produce proteins in the base of the flagellum are activated first. Once the base is complete, a clever feedback mechanism flips the next switch, activating the next set of genes, which allows later stages of assembly to occur, and so on. (It’s actually more complicated than that, but you get the idea.) So the parts of the flagellum are made “just in time,” shortly before each piece is needed.
Natural forces work “like magic”
Nothing we know from every day life quite prepares us for the beauty and power of self-assembly processes in nature. We’ve all put together toys, furniture, or appliances; even the simplest designs require conscious coordination of materials, tools, and assembly instructions (and even then there’s no guarantee that we get it right!). It is tempting to think the spontaneous formation of so complex a machine is “guided,” whether by a Mind or some “life force,” but we know that the bacterial flagellum, like countless other machines in the cell, assembles and functions automatically according to known natural laws. No intelligence required.1
Video animations like this one by Garland Science beautifully illustrate the elegance of the self-assembly process (see especially the segment from 2:30-5:15). Isn’t it extraordinary? When I consider this process, feelings of awe and wonder well up inside me, and I want to praise our great God.
Several ID advocates, most notably Michael Behe, have written engagingly about the details of flagellar assembly. For that I am grateful—it is wonderful when the lay public gets excited about science! But I worry that in their haste to take down the theory of evolution, they create a lot of confusion about how God’s world actually operates.
When reading their work, I’m left with the sense that the formation of complex structures like the bacterial flagellum is miraculous, rather than the completely normal behavior of biological molecules. For example, Behe writes, “Protein parts in cellular machines not only have to match their partners, they have to go much further and assemble themselves—a very tricky business indeed” (Edge of Evolution, 125-126). This isn’t tricky at all. If the gene that encodes the MS-ring component protein is artificially introduced into bacteria that don’t normally have any flagellum genes, MS-rings spontaneously pop up all over the cell membrane. It’s the very nature of proteins to interact in specific ways to form more complex structures, but Behe makes it sound like each interaction is the product of special design. Next time I’ll review some other examples from the ID literature where assembly is discussed in confusing or misleading ways.
1. Some would say this kind of statement violates the sovereignty of God. Not so! I fully believe God is sovereign, but I don’t take that to mean he himself carries out everything that happens inside each cell.
Macnab, Robert M. “How Bacteria Assemble Flagella.” Annu. Rev. Microbiol. 57:77-100. 2003.
Kathryn Applegate is Program Director at The BioLogos Foundation. She received her PhD in computational cell biology at The Scripps Research Institute in La Jolla, Calif. At Scripps, she developed computer vision software tools for analyzing the cell's infrastructure, the cytoskeleton.