"Our goal was to solve as many structures as possible to see the protein in all its states and learn about its conformational changes," Reindl says. "If you can find different orientations between the bound states of ATP and ADP, you can assume the protein is performing a certain movement not that you can ever get thousand-percent proof, but you know it's likely."
Reindl was able to crystallize FlaI bound to ADP, but not to ATP. Attempts to soak ATP into the crystals dissolved them, making x-ray crystallography of the ATP-bound state impossible.
"We then used SAXS at the ALS's SIBYLS beamline to look at FlaI bound to ATP in solution," she says. SAXS stands for small-angle x-ray scattering, and SIBYLS stands for Structurally Integrated Biology for Life Sciences, an ALS beamline maintained by the Life Sciences Division for which Tainer is director.
Says Reindl, "That had disadvantages without a crystal we couldn't get atomic resolution but in some ways the advantages were greater, because we could see the overall conformation of the protein in solution, a more normal physiological state. By combining x-ray crystallography and SAXS data we could deduce how the structure changed."
When bound to ATP, individual FlaI monomers arrange themselves into flat, six-unit rings, hexamers, with the ATPs serving as glue to hold them together. The result resembles a crown, with the CTD units forming the circlet and the free-to-move NTDs as the points.
Seven different conformations were recorded, revealing a dynamic play among the protein's components in a changing, asymmetric assembly. From the detailed images, much of the action of the archaellum motor assembly could be deduced.
How FlaI builds an archaellum
The FlaI "crown" both assembles the archaellum and causes it to rotate, but it doesn't work alone. Other important components are the protein FlaJ, which serves as
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory