The motility structure of this third domain of life has long been called a flagellum, a whip-like filament that, like the well-studied bacterial flagellum, rotates like a propeller. But although the archaeal structure has a similar function, it is so profoundly different in structure, genetics, and evolution that the researchers argue it deserves its own name: archaellum. 
This unique motor is highly conserved in all motile archaeal species. Its structure most resembles that of the bacterial Type IV pilus, the filamentary "grappling hook" by which bacteria attach to surfaces and pull themselves along – and which is responsible for pathogenicity in many bacteria, including deadly strains of E. coli. 
Since archaea may also be important players in the microbiota of the human gut, knowing the archaellum's structure will help scientists understand how archaea interact with human cells. The Berkeley Lab-MPI research team reports its findings in the journal Molecular Cell.

Finding the key protein 
Sulfolobus acidocaldarius was the model organism used in the analysis, says the research team's co-leader Sonja-Verena Albers, who heads the MPI's Molecular Biology of Archaea research group, "because this is one of the few well established model systems in which genetics works well. We have the genetic tools to mutate and precisely modify the Sulfolobus genome. We can combine in vivo experiments with the atomic structure of our proteins to see the effect of modifications." 
A protein called FlaI (pronounced "flah-eye") was a leading candidate for archaella assembly and rotation, but the team had to find proof. FlaI is an ATPase – an enzyme that releases energy from adenosine triphosphate, or ATP – and was known to be involved in the assembly and function of Type IV pili in bacteria and the secretion of proteins in many microorganisms. But FlaI's role in archaella was uncertain.