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Assembly and function of the crenarchaeal flagellum

Final Report Summary - ARCHAELLUM (Assembly and function of the crenarchaeal flagellum)

Organisms of the three domains of life, Eukaryotes, Bacteria and Archaea, have developed different extracellular structures which are used for motility. While Eukaryotes employ cilia and bacteria use flagella to swim, Archaea developed the archaellum, a structure which is evolutionary and structurally related to archaeal and bacterial type IV pili. However, in contrast to type IV pili which in many bacteria can assemble and retract, the archaellum is a rotating structure.
Archaella are present in all motile archaea and are build up from 7- 13 proteins depending on the species. Whereas the archaellin, the filament protein, and the proteins FlaF, FlaG, FlaH, FlaI and FlaJ are conserved in all archaellum operons, some accessory proteins are present in the different archaeal phyla which most probably are adaptations to the presence or absence of the chemotaxis system. Before we started with the ERC project, no details were known about the function and interaction of the subunits of the archaellum nor about their assembly.
We have studied the archaellum of the thermophilic archaeon Sulfolobus acidocaldarius in vivo and using purified components by employing molecular biology, genetics, biochemistry and biophysical methods. We established a thermo-microscope to study the swimming behavior of S. acidocaldarius at 75oC and could use this microscope to show that archaella are rotating filaments. During the course of the project we have obtained the crystal structures of FlaF, FlaG, FlaH and FlaI, and the single particle structure of FlaX, an archaellum protein specific to S. acidocaldarius. In interaction studies we learned that FlaX acts as a scaffold protein for the assembly of the archaellum motor complex proteins FlaH, I and most probably FlaJ. FlaI, an ATPase, has two functions: first it assembles the archaellum filament and then switches to rotate the filament, generating thrust so that the cells can swim. The switch of the activity of FlaI between assembly and rotation is induced by FlaH, a high affinity nucleotide binding protein. The structure of FlaH showed high similarity with the cyanobacterial KaiC protein, therefore we are still investigating whether FlaH might act a “clock” protein to time the switch of FlaI between assembly and rotation of the archaellum.
FlaG and FlaF both contain an archaellin domain, but are not part of the archaellum filament itself, therefore we propose that they form the stator of the archaellum in the cell envelope and interact with the S-layer, the sole surface protein in many archaea.
With the information we obtained about the different subunits of the archaellum we gained a basic understanding of their interactions and can now propose a model how this unique motility structure uses ATP for first assembly of its filament and subsequently for its rotation to achieve motility in Archaea.