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Shape-Shifting Protein in Bacterial Flagellum Controls Spin Direction

Casey Luskin

It’s been long known that the bacterial flagellum can spin in one direction and then quickly reverse directions and spin in the other. A recent issue of Nature has an article titled, “Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching” which elucidates some of the biomechanical properties of the FliG motor protein that allows this rotation switch to occur:

The flagellar motor drives the rotation of flagellar filaments at hundreds of revolutions per second, efficiently propelling bacteria through viscous media. The motor uses the potential energy from an electrochemical gradient of cations across the cytoplasmic membrane to generate torque. A rapid switch from anticlockwise to clockwise rotation determines whether a bacterium runs smoothly forward or tumbles to change its trajectory. A protein called FliG forms a ring in the rotor of the flagellar motor that is involved in the generation of torque through an interaction with the cation channel- forming stator subunit MotA. FliG has been suggested to adopt distinct conformations that induce switching but these structural changes and the molecular mechanism of switching are unknown. Here we report the molecular structure of the full-length FliG protein, identify conformational changes that are involved in rotational switching and uncover the structural basis for the formation of the FliG torque ring. This allows us to propose a model of the complete ring and switching mechanism in which conformational changes in FliG reverse the electrostatic charges involved in torque generation.

(Lawrence K. Lee, Michael A. Ginsburg, Claudia Crovace, Mhairi Donohoe & Daniela Stock, “Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching,” Nature, Vol. 466:996-1000 (August 19, 2010) (internal citations omitted).)

In essence, the FliG protein can undergo rapid changes to its shape while the flagellum is spinning. In one conformation, the “open” shape, the driveshaft of the flagellum turns in a clockwise diction. In its “closed” conformation, the shape of FliG causes the proton motor force to spin to driveshaft in the opposite, counter-clockwise direction. A supplemental movie available free online from Nature shows this process in motion. Much like water ballet dancers, FliG proteins change their conformations synchronously to mediate the spin direction of the flagellum.

No wonder a recent paper (looking at a different system) stated that “Molecules that extend and contract under external stimuli are used to build molecular machines with nanomechanical functions”!
The Nature article on the flagellum further explains that key stretches of amino acids in FliG, called the ARMC or ARMM+1 motifs, are involved in FliG’s mechanical shape-changing ability. They must have specific amino acid sequences to function or assemble properly:

Combined, these data indicate that the ARMC-ARMM+1 interaction is a real biological interaction and this has several profound implications. First, FliG forms part of the flagellar motor known as the switch complex, which contains two other proteins, FliM and FliN. All three proteins are required for flagellar assembly. Mutations at the base of ARMC and ARMM and on the face of ARMM, which contains a highly conserved EHPQR motif, can disrupt flagellar assembly and FliM binding. On the basis of the structure of a single FliG monomer, it seems evident that ARMC and ARMM are separate FliM binding sites. However the ARMC-ARMM+1 superhelix indicates that ARMC and ARMM from adjacent monomers interact to create a surface for a single FliM binding site, which is consistent with all other known ARM superhelices that stack to form a surface for protein-protein interactions. Second, in addition to its interactions with FliF and FliM, proper assembly of the bacterial flagellum also requires a FliG-FliG interaction that is independent of other components of the flagellar motor. This is well supported by mutagenesis studies that demonstrate the requirement of at least five hydrophobic residues at the ARMC-ARMM+1 interface for flagellar assembly. Last, because ARMC forms a right-handed superhelix with ARMM+1, it follows that ARMC+1 has the same interaction with ARMM+2. Thus, the ARMC-ARMM+1 superhelix is the structural basis for the formation of FliG multimers. (internal citations omitted)

No wonder in her 2005 article in Nature, “Assembly line inspection,” Sarah A. Woodson (who is not pro-ID as far as I know) marveled that “The cell’s macromolecular machines contain dozens or even hundreds of components. But unlike man-made machines, which are built on assembly lines, these cellular machines assemble spontaneously from their protein and nucleic-acid components. It is as though cars could be manufactured by merely tumbling their parts onto the factory floor.”

But there’s no intelligent design to see here folks. None whatsoever.

In fact, in their 2006 paper on the flagellum, Mark Pallen & Nick Matzke bluffed that “designing an evolutionary model to account for the origin of the ancestral flagellum requires no great conceptual leap.” But the most they were able to say about the origin of these proteins that are fundamental to the flagellum’s spinning ability is that FliG has “structural” (not sequence, but structural) similarities to a protein involved in magnesium transport. As for FliM and FliN, these proteins have homologies to one another–a useless observation when trying to understand how the flagellar proteins evolved from non-flagellar proteins. They do “share a C-terminal SpoA domain” with components of the Type III Secretory System, a system thought to have arisen after the flagellum. But of course, the c-terminal is found at the end of the protein and finding some homology there hardly accounts for the origin of the entire protein and its specific functional amino acid sequence. Their observations haven’t given the slightest hint about how these complex biomechanical structures evolved.

But there’s no intelligent design to see here. None whatsoever.


Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.



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