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Archaeal flagella are similar functionally to bacterial flagella, but structurally they are completely different. Helical archaeal flagellar filaments are formed of protein subunits called flagellins (archaellins). Notwithstanding progress in studies of archaeal flagella achieved in recent years, many problems in this area are still unsolved. In this review, we analyze the formation of these supramolecular structures by the example of flagellar filaments of halophilic archaea. Recent data on the structure of the flagellar filaments demonstrate that their supramolecular organization differs considerably in different haloarchaeal species.

Modified flagella of halophilic archaea Halobacterium salinarum mineralized by cobalt oxide and applied to a conductive nickel grid support have been tested as a material for the negative electrode of a lithium-ion battery. It is found that the reversible capacity of such nanostructured samples exceeded 400 mAh/g and their stability in cycling increased significantly when flagella were fragmented by sonication prior to mineralization with cobalt oxide.

Archaeal flagella are among the most perspective biopolymer structures for use in nanotechnology and have certain advantages over both bacterial analogues and viral particles. Such advantages include their resistance to dissociating agents and their adhesion properties to certain types of surfaces. As a rule, archaeal flagella consist of several protein subunit types (flagellins). This allows multifunctional filaments to be designed with each flagellin differently modified. One of the reasons that archaeal flagella have not been used in nanotechnology is the lack of data about the spatial structure of subunits in flagella. We present an experimental approach for determining flagellin molecule sites which are exposed on the flagellum surface and suitable for directed insertion of groups capable of binding certain ligands. It was shown that modified flagella of halophilic archaea Halobacterium salinarum can be used as scaffolds for designing nowel nanomaterials.

Many Archaea use rotation of helical flagellar filaments for swimming motility. We isolated and characterized the flagellar filaments of Haloarcula marismortui , an archaeal species previously considered to be nonmotile. Two Haloarcula marismortui phenotypes were discriminated—their filaments are composed predominantly of either FlaB or FlaA2 flagellin, and the corresponding genes are located on different replicons. FlaB and FlaA2 filaments differ in antigenicity and thermostability. FlaA2 filaments are distinctly thicker (20–22 nm) than FlaB filaments (16–18 nm). The observed filaments are nearly twice as thick as those of other characterized euryarchaeal filaments. The results suggest that the helicity of Haloarcula marismortui filaments is provided by a mechanism different from that in the related haloarchaeon Halobacterium salinarum , where 2 different flagellin molecules present in comparable quantities are required to form a helical filament.

Filaments of the flagellum of the halophilic archaeon Halobacterium salinarum consist of five flagellins: A1, A2, B1, B2, and B3, which are encoded by five genes localized in tandem in two flgA and flgB operons. While the role of flagellins A1 and A2 has been determined, the role of the proteins, B operon products, is still unclear. A mutant strain of H. salinarum with deleted A and B flagellin genes (ΔflgAΔflgB) has been obtained for the first time. This strain has been used to create and analyze the strains carrying only individual B1 or B3 flagellin genes. Cells of the ΔflgAΔflgB strain were shown to have short filamentous formations, 7–8 nm thick, which we have named as X-filaments. It has been shown that X-filaments consist of a protein immunologically related to flagellins A and B. Expression of the B1 and B3 genes is suppressed in the absence of A1, A2, and B2. It has been shown that flagellins B1 and B3 cannot be substituted for flagellin B2 upon the formation of a curved hook-like structure, which serves as a connecting element between the flagellar filament and the motor axis. The multicomponent nature of flagella is discussed in the light of their possible involvement in other cell processes besides providing motility.

Haloarchaeal flagella are composed of a number of distinct flagellin proteins, specified by genes in two separate operons (A and B). The roles of these flagellins were assessed by studying mutants of H. salinarum with insertions in either the A or the B operon. Cells of the flgA- mutant produced abnormally short, curved flagella that were distributed all over the cell surface. The flgA2- strain produced straight flagella, mainly found at the poles. The flgB- mutant had flagella of the same size and spiral shape as wild-type cells, but these cells also showed unusual outgrowths, which appeared to be sacs filled with basal body-like structures. In broth cultures of this mutant, the medium accumulated flagella with basal body-like structures at their ends.

We examine distribution of flagellins along multicomponent flagellar filaments (FF) and protofilaments (PF) of the haloalkaliphilic archaeon Natrialba magadii using immunogold electron microscopy. A high specific polyclonal antibody raised to one of the flagellin types bound homogeneously to the undissociated FF along the full length. At the same time both uniformly labelled and completely unlabelled PF, outwardly indistinguishable one from another, were observed.