Joined: May 2002
Although the basics of how the eubacterial flagellum works are reasonably well-known
(see Howard Berg's article in Physics Today, Motile Behavior of Bacteria for a good introduction)
...one remaining mystery surrounds the exact mechanism by which the flow of H+ powers rotary motion.
Here is Berg's figure of the flagellum:
H+, aka protons, hydrogen ions, or "acid", flow from the inter-membrane space (where ATPases, photosynthesis, and other processes store them at high concentration) into the cell (down, in Berg's figure) where they are at lower concentration.
Somehow this energy is converted into motion of the cell. A number of diverse models have been proposed for how exactly this might occur. Mike Gene briefly reviews a couple of them in his argument for the nonevolution of the flagellum here.
He includes a figure derived from a paper on the question of motor mechanism:
The grey round thing represents the FliG protein (the part of the C-ring that interacts with the motor proteins, MotA and MotB) and the pinkish things with the H+ or Na+ flowing through them represent the MotAB complex.
[WARNING: beginning marginally informed speculation section. Treat as one would treat a mathematical "conjecture" -- or rather just a conjecture, I'm sure math is more formal about such things]
Perhaps we can imagine two main classes of models:
1) Those in which the H+ flow plays a *direct* role in rotating the flagellum (e.g., the "proton turbine model" in the left of the figure). This is, BTW, an appealingly simple model for motor operation: the protons just flow on through and the charges interact with the diagonally-positioned charges on the C-ring, sort of like wind blowing on a windmill. Presumably a conformation change in FliC could reverse the diagonal direction and presto, rotation in the opposite direction.
2) Those in which the H+ flow causes some conformation (shape) change in the MotAB complex, which then interacts to "push" (speaking very basically; could be a Brownian ratchet for instance) on the C-ring. How a conformation change in FliC would produce reverse rotation on this model is obscure to me, although I'm sure there's a way.
Based on the idea that MotAB had flagellum-independent origins in the form of proteins homolgous to ExbB (and ExbD IIRC), can we make any guess as to which of these might be more likely?
The difficulty with #1 would appear to be that both the proto-FliC and proto-ExbB would have to be at least crudely adapted to accepting proton flow.
It seems to me that #2 might be more likely if MotAB evolved from ExbB-like ion channels and if those channels functioned by conducting H+ through them and using the resulting conformation changes to perform work elsewhere at a distance. The proto-MotB (already adapted for performing H+-powered work on some other protein) might be the only thing that would have to mutate in order to get some crude purchase on the proto-FliC (the base of a transport system, but that's another part of the story).
Anyway, the basic idea is that it would be cool, for instance, if one could predict which functional model to prefer based on an evolutionary based on homology with ExbB...
All of this depends upon further understanding of how the Exb complex works however. Something to work on...
(PS: THis is one of the Mot-homolog articles, it appears that they go for the conformation-change model also):
Biochemistry 2001 Oct 30;40(43):13041-50
Conformational change in the stator of the bacterial flagellar motor.
Kojima S, Blair DF.
Department of Biology, University of Utah, Salt Lake City, Utah 84112, USA.
MotA and MotB are integral membrane proteins of Escherichia coli that form the stator of the proton-fueled flagellar rotary motor. The motor contains several MotA/MotB complexes, which function independently to conduct protons across the cytoplasmic membrane and couple proton flow to rotation. MotB contains a conserved aspartic acid residue, Asp32, that is critical for rotation. We have proposed that the protons energizing the motor interact with Asp32 of MotB to induce conformational changes in the stator that drive movement of the rotor. To test for conformational changes, we examined the protease susceptibility of MotA in membrane-bound complexes with either wild-type MotB or MotB mutated at residue 32. Small, uncharged replacements of Asp32 in MotB (D32N, D32A, D32G, D32S, or D32C) caused a significant change in the conformation of MotA, as evidenced by a change in the pattern of proteolytic fragments. The conformational change does not require any flagellar proteins besides MotA and MotB, as it was still seen in a strain that expresses no other flagellar genes. It affects a cytoplasmic domain of MotA that contains residues known to interact with the rotor, consistent with a role in the generation of torque. Influences of key residues of MotA on conformation were also examined. Pro173 of MotA, known to be important for rotation, is a significant determinant of conformation: Dominant Pro173 mutations, but not recessive ones, altered the proteolysis pattern of MotA and also prevented the conformational change induced by Asp32 replacements. Arg90 and Glu98, residues of MotA that engage in electrostatic interactions with the rotor, appear not to be strong determinants of conformation of the MotA/MotB complex in membranes. We note sequence similarity between MotA and ExbB, a cytoplasmic-membrane protein that energizes outer-membrane transport in Gram-negative bacteria. ExbB and associated proteins might also employ a mechanism involving proton-driven conformational change.
Edited by niiicholas on Feb. 17 2003,17:53