Archaellum

Abstract

Each of the three domains of life contains its own unique swimming apparatus. The archaellum (formerly called archaeal flagellum) is a unique, ‘tail‐like’ structure used for motility by single‐celled organisms belonging to the domain Archaea. Although archaella are functionally similar to the flagella found on bacteria, they differ significantly in structure and mode of assembly. Archaella are evolutionarily related to type IV pili, and each of the two systems shares several key homologues required for assembly of the respective structures. Recent studies have contributed much to our knowledge concerning the regulation of the operon encoding the archaellum proteins. In addition, the elucidation of the structures of most of the Fla proteins involved in archaella structure and function, coupled with detailed atomic models of the archaellum, including the motor, has provided major insights into how this unique motility organelle is assembled and functions, often in harsh environments inhabited by many archaea. Key Concepts Each of the three domains of life has a unique motility apparatus, which have recently been assigned distinct names (Archaea: archaellum; Bacteria: flagellum and Eukaryotes: cilium). The archaellum is functionally equivalent to flagellum but is evolutionarily related to a type IV pilus, with the archaella and type IV pili systems sharing important homologues. Archaea have the ability to regulate the synthesis of archaella depending on growth conditions, such as available nutrients and temperature. There can be crosstalk in the regulation of archaella with adhesive pili, allowing the cells under certain conditions to make only one type of appendage, either for swimming or adhesion. N‐Glycosylation of the major filament proteins, the archaellins, is widespread and essential under normal conditions for assembly of filaments. The ATPase responsible for assembly of the archaellins into the filament, that is, FlaI, is also responsible for the hydrolysis of ATP that powers the rotation of archaella. In many archaea, the archaellum interacts with a bacterial‐like chemotaxis system that requires novel chemotaxis proteins to act as adaptors to connect the systems. Not all archaellated species have an associated chemotaxis system. Knowledge of the structure and assembly of archaella has greatly increased in the past 5 years with atomic models of several archaella structures as well as structures of most of the individual archaella proteins and their interaction partners.