"Just how ATPases catch the energy of ATP is a long-standing question," says Tainer. The individual monomers in the FlaI crown process the ATP and reduce it to ADP by releasing a phosphate; here the "glue" that binds a FlaI monomer to its neighbor in the crown lets go. "In this phosphate release mechanism our team including Abrahjyoti Ghosh and Gareth Williams in Life Sciences were able to see a state never seen before, an intermediate conformation created by the released phosophate."
Instantly the whole FlaI monomer in the crown moves upward, pushing up the FlaB filament and opening a gap at its base where waiting FlaB subunits are added to the filament, causing it to grow. The process is similar to the mechanism in a bacterial Type IV pilus, although the resulting structures operate very differently.
That FlaI is the protein uniquely responsible for both assembly and motility of the archaellum was established by further genetic studies. A mutant strain that lacked only the first 29 amino acids in the NTD point was quite capable of assembling archaella, but incapable of making them rotate.
"This mutant was interesting to us because it raised the question of how altered FlaI proteins keep the ability to assemble archaella, yet they lose their motility," says Albers. "That the N-terminal-deletion mutant can assemble archaella, but the cells are not motile, implies that after assembly a switch is thrown, and the filament starts to rotate."
The biggest challenge for the team's continuing research is to learn how, as ATP releases energy, the upward movement of FlaI monomers is transferred to the rotation of the archaellum filament. Does the hexamer the whole FlaI crown rotate, or only the archaellum itself? And if so, how?<
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory