The archaellum: an old motility structure with a new name.

Motility structures, called flagella, have been described in all three domains of life: Bacteria, Archaea and Eukarya. These structures are well studied in both Bacteria and Eukarya. However, already in eukaryotes there exists some confusion as to whether these structures should actually be called cilia. With increased studies conducted on organisms of the third domain of life, the Archaea, it has become clear that the archaeal flagellum only functionally appears similar to the bacterial flagellum, whereas it structurally resembles a bacterial type IV pilus. To resolve confusion due to unclear nomenclature, we propose renaming the archaeal flagellum as the 'archaellum'. This will make clear that the archaellum and the bacterial flagellum are two distinct structures that happen to both be used to enable microorganisms to swim.