A Third Rotary Motor Has Now Been Found in Bacteria
Howard Berg's latest paper in Current Biology announces an exciting find: another rotary motor has been discovered in a bacterial cell. The Harvard expert on the bacterial flagellum (see him speaking about the "Marvels of Bacterial Behavior" above), along with two colleagues, describes a new kind of rotating motor in a bacterium, separate and distinct from ATP synthase and the kind of flagella found in E. coli. The short title of the paper is dramatic: "A Rotary Motor Drives Flavobacterium Gliding."
Cells of Flavobacterium johnsoniae, a rod-shaped bacterium devoid of pili or flagella, glide over glass at speeds of 2-4 ?m/s. Gliding is powered by a protonmotive force, but the machinery required for this motion is not known. Usually, cells move along straight paths, but sometimes they exhibit a reciprocal motion, attach near one pole and flip end over end, or rotate. This behavior is similar to that of a Cytophaga species described earlier.... To learn more about the gliding motor, we sheared cells to reduce the number and size of SprB filaments and tethered cells to glass by adding anti-SprB antibody. Cells spun about fixed points, mostly counterclockwise, rotating at speeds of 1 Hz or more. The torques required to sustain such speeds were large, comparable to those generated by the flagellar rotary motor. However, we found that a gliding motor runs at constant speed rather than at constant torque. Now, there are three rotary motors powered by protonmotive force: the bacterial flagellar motor, the Fo ATP synthase, and the gliding motor. [Emphasis added.]
This newly identified rotary motor is made up of protein parts that have no relationship to the other two rotary engines. Yet like them, it runs on proton-motive force. And it uses an entirely "novel" mechanism for movement.
The bacterium is studded with these motors that, as they spin, work cooperatively to make the cell glide along a surface. The majority of the motors (92 percent) were found to spin counterclockwise. When the team tethered one end of the rod-shaped bacterium to the surface, the rod spun around in circles at one turn per second (1 Hz). These bacteria generate a lot of torque! The authors listed the highlights of their discovery:
- The gliding motor, a novel rotary motor, spins tethered F. johnsoniae cells
- The gliding motor generates high torque
- The gliding motor runs at constant speed rather than at constant torque
Earlier studies of F. johnsoniae found that the gliding mechanism, however it worked, appeared to be combined with a secretory function. Since this secretion mechanism differed from the previously known types (I-VIII), it was classed as the Type IX Secretion System (T9SS). In 2013, two researchers found that without the secretion genes, the gliding stopped, because the mechanism could not be assembled. Publishing in the Journal of Bacteriology, they also noted that gliding motility is widespread among Bacteroidetes, a phylum of bacteria that is "large and diverse, with rapid gliding motility and the ability to digest macromolecules associated with many genera and species."
In May 2013, one of the authors of the new paper (A. Shrivastava), publishing in the Journal of Bacteriology, identified about a dozen protein parts of the gliding mechanism and noticed that they were secreted to the outer membrane of the cell via the T9SS. Apparently the team didn't recognize the rotary nature of the mechanism at the time, but observed that deletions of specific proteins broke it:
Nonpolar deletions of gldK, gldL, or gldM resulted in the absence of gliding motility and in T9SS defects. The mutant cells produced SprB and RemA proteins but failed to secrete them to the cell surface. The mutants were resistant to phages that use SprB or RemA as a receptor, and they failed to attach to glass, presumably because of the absence of cell surface adhesins.
The new paper by Berg, Shrivastava, and Lele now uncovers the secret: the gliding mechanism is a rotary motor. When mutations break the genes, the motor doesn't work and cannot even get built -- another indication of irreducible complexity.
The gliding motors have rotors and stators just like the other rotary engines. This gliding motor generates as much torque as a flagellum:
Torque ranged from 200-6,000 pN nm, with most cells running at ?1,000 pN nm (Figure 2D). Torques measured with motors of E. coli spinning latex beads (?1 ?m diameter) averaged ?1,300 pN nm; therefore, the torques generated by the gliding motor are comparable to those generated by a flagellar motor. Stator elements formed by MotA and MotB proteins act as force-generating units that generate torque for rotation of flagellar motors. It is likely that similar stator elements, albeit made up of different protein subunits, harvest protonmotive force to power rotation of the gliding motor.
The motor is fastened to a baseplate that slides along the outer membrane, propelled by the spinning "filaments" made of protein SprB. The connection between secretion and motility is intriguing. The T9SS and the rotary motor are interdependent, because parts of the motor need to be excreted to the outer membrane for proper assembly. "In our model," they say, "gliding motors formed complexes with T9SS, which spanned the inner and outer membranes, harvesting protonmotive force to power SprB rotation."
This gets interesting, because the authors state that the bacterial flagellum has a similar dependence on the Type-III Secretory System (T3SS), which some Darwinian evolutionists have tried to invoke as an ancestor to the flagellum:
Genome sequencing has shown that F. johnsoniae lacks proteins similar to components of the bacterial flagellar motor. GldJ is a putative component of the gliding motor. Presumably GldJ interacts with the Type-IX protein secretion system (T9SS) and is important for the movement of cell-surface adhesins. GldK, GldL, GldM, and GldN are core T9SS proteins, and cells lacking these proteins do not exhibit motility. The macromolecular structure of the gliding motor and its exact interaction with T9SS is unclear. GldL localizes to the cytoplasmic membrane, and it might act as an anchor for the gliding motor. Besides the core T9SS proteins, other Gld and Spr proteins might associate with this motor. The gliding motor appears to associate with T9SS in a manner analogous to the association of the bacterial flagellar motor with the Type-III secretion system (T3SS). In flagellated bacteria, T3SS is required for secretion of axial components of the flagellum. In F. johnsoniae, T9SS is required for secretion of the SprB filament and a mobile adhesin, RemA.
What this implies is even more irreducible complexity for the bacterial flagellum. Without the flagellum+T3SS working cooperatively, the flagellum will not work, because T3SS is "required for secretion of axial components" of the flagellum. This suggests that both the T3SS and the flagellum had to exist simultaneously for the flagellum to assemble properly and function.
While it's true that the T3SS works independently in non-flagellated bacteria as an "injectisome" (the mechanism by which some pathogens infect other cells), a case could be made that those pathogens lost their flagella rather than the other way around. This new motor supports the view that in both systems (the flagellum and the gliding motor), secretion and rotation comprise a single, unified system that is irreducibly complex. It is far easier to lose organelles than to make them. And it's easier to take an existing machine (like the T3SS) and shove new material through it, than to build one from scratch. It's a bit like using an elevator designed to transport bricks to deliver a bomb.
That's a suggestion that will surely be debated between evolutionists and design advocates. What's clear at this point is that another irreducibly complex, functional system for motility has been revealed, based on a rotary engine driven by proton-motive force. Harvard's announcement will certainly stimulate further research into this gliding motor and how it works. Not surprisingly, they feigned no hypothesis about how this system might have evolved.
This motor is just now coming to light two decades after the discovery of the rotary mechanism of the bacterial flagellum and ATP synthase. It makes you wonder how many other "novel" systems remain to be discovered in the simplest of living organisms.