Archaella

Archaella are the motility organelles of many archaea, serving a function analogous to bacterial flagella but with distinct composition and mechanism. They are slender, helical filaments composed primarily of archaellin subunits that extend from a basal body embedded in the cell envelope and rotate to propel the cell through liquid.

Archaellum assembly resembles type IV pilus systems and depends on a cytoplasmic ATPase complex, typically involving FlaI and its regulator FlaH, along with membrane components such as FlaJ, FlaF, and FlaG. Archaellin subunits usually carry N-terminal signal peptides that are cleaved by a prepilin peptidase during assembly, enabling the filament to form.

Rotation is powered by ATP hydrolysis, not the proton motive force used by many bacterial flagella. The motor resides at the base of the structure and couples to cytoplasmic signaling proteins to generate torque. Movement is often governed by archaeal chemotaxis-like networks that respond to environmental cues.

Archaella are widespread across archaeal lineages, and some species harbor multiple archaellin genes, contributing to diversity in filament properties. In hyperthermophiles, archaella can exhibit notable stability under extreme conditions. The archaellum is evolutionarily distinct from bacterial flagella, reflecting a separate solution to microbial motility that arose from a type IV pilus–like assembly system.