Various mechanisms of flagella helicity formation in haloarchaea

The genome of a halophilic archaeon Haloarcula marismortui carries two flagellin genes, flaA2 and flaB. Previously, we demonstrated that the helical flagellar filaments of H. marismortui were composed primarily of flagellin FlaB molecules, while the other flagellin (FlaA2) was present in minor amounts. Mutant H. marismortui strains with either flagellin gene inactivated were obtained. It was shown that inactivation of the flaA2 gene did not lead to changes in cell motility and helicity of the filaments, while the cells with inactivated flaB lost their motility and flagella synthesis was stopped. Two FlaB flagellin forms having different sensitivities to proteolysis were found in the flagellar filament structure. It is speculated that these flagellin forms may ensure the helical filament formation. Moreover, the flagella of a psychrotrophic haloarchaeon Halorubrum lacusprofundi were isolated and characterized for the first time. H. lacusprofundi filaments were helical and exhibited morphological polymorphism, although the genome contained a single flagellin gene. These results suggest that the mechanisms of flagellar helicity may differ in different halophilic archaea, and sometimes the presence of two flagellin genes, in contrast to Halobacterium salinarum, is not necessary for the formation of a functional helical flagellum.