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Two Flagella Are Better than One

flagellar assembly2.jpg

As imaging improves, so does knowledge of the workings of the bacterial flagellum. Two new papers point out new findings about these outboard motors that contribute to the argument that they are irreducibly complex and intelligently designed. As could be predicted, neither attempts any explanation of how they could have evolved.

Secondary Flagella

One paper in PNAS by German scientists explores the advantage of having two flagella, one at the rear and another one or two on the sides. If you were a blind bacterium trying to find your way up a gradient, you would only have one trick in your steering kit: the "run-reverse-flick" move. Trouble is, when you operate that move, it often turns you 90 degrees. That's not helpful when you want to make progress up the gradient. The scientists found that having a secondary flagellum reduces that angle, even when it doesn't not provide extra power:

Flagella-mediated motility is an important or even crucial propagation factor for many bacteria. A number of polarly flagellated species possess a distinct secondary flagellar system, which, as current models suggest, allows more effective swimming under conditions of elevated viscosity or across surfaces. In this study, we demonstrate that such a secondary flagellar system may also exert beneficial effects in bacterial spreading by increasing the directional persistence through lowering the cellular turning angles. The strategy of increasing directional persistence to improve animal spreading efficiency has been proposed previously by theoretical modeling, and here we provide a specific example of how this strategy is used by bacteria. (Emphasis added.)

Corking the Torque

During assembly of a flagellum, the bacterium must avoid starting the engines before they are anchored in place. This is similar to fastening an outboard motor to a boat: turning the motor on could be dangerous. Another paper in PNAS describes how a particular protein in the stator plugs its ion channel until the stator is properly positioned in the membrane. In essence, it waits for a signal that assembly is complete, then undergoes a conformational change that allows the ions that drive the motor to flow.

Stator is the energy-converting membrane protein complex in the flagellar motor. Its ion-conducting activity is only activated when incorporated into the motor, but the mechanism for assembly-coupled activation remains a mystery. In this study, we solved the structure of a C-terminal fragment of the sodium-driven stator protein PomB (PomBC), the region responsible for anchoring the stator unit, at 2.0-� resolution. In vivo disulfide cross-linking studies of PomB double-Cys mutants and their motility assay suggested that the N-terminal region of PomBC changes its conformation, which is expected for MotB, the counterpart of PomB in the proton-driven Salmonella motor, in the final step of the stator assembly around the rotor.

It's remarkable that scientists can now look at parts of machines at two angstrom resolution -- two 10 billionths of a meter! The remarkable team at Nagoya University in Japan, who produced beautiful animations of flagella, has done it again, uncovering new aspects of these amazing molecular machines. The particular part of one protein essentially plugs up the ion channel, like a cork in a bottle:

Because the cross- linking did not affect stator assembly, we suspected that the cross-linking inhibits the ion conductivity of the stator channel. PomB/MotB has a periplasmic short segment called a "plug" just at the C terminal to their single transmembrane region...

The scientists mention in passing that the sodium-driven motors like in Vibrio routinely operate at over 100,000 RPM (1,700 Hz). Proton-driven flagella in E. coli and Salmonella are typically slower, about 300 Hz. The high-performance flagella have extra parts for their turbo-charged activity, just like one would expect to find in a Ferrari:

The basal body of the Vibrio motor has two unique ring structures, the T-ring and the H-ring. These extra rings are thought to reinforce the motor to resist the high-speed rotation. Recent structural study demonstrated that FlgT acts as an assembly base or scaffold for both the ring structures. The T-ring is made up of MotX and MotY, and is located beneath the P-ring, which is a part of a bushing structure for the rod, thereby believed not to rotate. The T-ring is an essential component to incorporate the stator into the motor. The periplasmic region of PomB is likely to bind to MotX, and MotX is connected to the basal body through the N-terminal domain of MotY. Thus, the stator of the sodium-driven motor is tightly fixed not only to the PG layer but also to the basal body through the interaction between PomB and the T-ring. Despite the rigid anchoring structure, the stator of the sodium-driven motor still shows a dynamic behavior dependent on the binding of sodium ion to PomB.

In this excerpt from their final discussion, notice how they describe the stepwise, coordinated assembly of parts before the ion-drive motor goes into action:

On the basis of this study and together with our previous results, we propose a model for activation mechanism of the Vibrio sodium-driven motor (Fig. S6). The stator diffusing in the cell membrane is in an inactive state. When the stator reaches around the rotor, PomA interacts with FliG. This interaction triggers opening of a"plug," allowing sodium ion to translocate into the channel of the stator. The sodium flow may induce the binding of PomB to the T-ring. This step probably includes a conformational change of the disordered N-terminal region of the PEM. After that, the N-terminal two-thirds of ?1 changes its conformation to an extended form to anchor to the PG layer [peptidoglycan layer, part of the external membrane].

This is just what Dr. Scott Minnich pointed out in Unlocking the Mystery of Life 12 years ago. The assembly instructions, he said, are even more irreducibly complex than the motor itself. Parts are arriving on time and moving into place in a programmed sequence, with feedback to the nucleus affecting how many parts are to be manufactured. Dr. Jonathan Wells added, "What we see is irreducible complexity all the way down." Twelve years of closer looks at these astonishing machines have only amplified those conclusions.