Archaellum

Archaellum (plural archaella) is the motility organelle of archaea, a rotating, extracellular filament that enables swimming in liquid environments. It is analogous to the bacterial flagellum in function but is structurally distinct and believed to have evolved separately.

The filament is composed of archaellin subunits encoded in fla gene clusters. These subunits polymerize to form a hollow, helical archaellum that protrudes from the cell surface. Filaments often bear post-translational modifications, such as glycosylation, which influence stability and assembly. A basal body anchored in the cell envelope provides a platform for motor assembly and filament export.

Powering of the archaellum comes from ATP hydrolysis carried out by a cytoplasmic ATPase (commonly FlaI) with accessory regulatory factors (such as FlaH). This ATP-driven motor drives rotation of the filament, a mechanism distinct from the proton motive force used by many bacterial flagella and closely related to Type IV pilus systems in its assembly logic.

Genetically, archaellum components are encoded in fla gene clusters that include both structural subunits and the motor/assembly machinery. Comparative studies show homology between the archaellum apparatus and type IV pilus assembly systems, supporting an evolutionary link between archaeal motility and bacterial pilus-based processes.

Archaella support swimming motility in a wide range of archaeal lineages, including Euryarchaeota and Crenarchaeota, enabling chemotaxis and colonization in diverse habitats from hypersaline seas to hot springs. Regulation of archaellum expression integrates environmental cues and cellular signaling pathways.