CetZ1-dependent assembly and positioning of the motility machinery in haloarchaea

The tubulin-like CetZ proteins are archaeal cytoskeletal proteins that contribute to cell shape and swimming motility in the halophilic archaeon Haloferax volcanii. Currently it is unknown whether CetZs contribute to motility solely through their control of cell shape or in other ways too. Here, we used cryo-electron and fluorescence microscopy to observe H. volcanii cell surface filaments and the localisation of the motility machinery, respectively, in strains with cetZ1 or cetZ2 deletion, overexpression, and polymerisation-defective mutant backgrounds. Our results show that CetZ1 has an important role at the poles of mature motile rod cells for the assembly of key motility proteins, including ArlD1, a constituent of the archaellum base, and the chemotaxis sensory array adapter CheW1 and signal transducer CheY. Importantly, overproduction of CetZ1 and CetZ1-mTurquoise2 inhibited motility and reduced the frequency of localisation of the motility machinery markers but did not affect rod-shape in swimming cells. Our data suggest that CetZ1 acts as a polar cytoskeletal structure that orchestrates the assembly and positioning of both major components of the motility machinery. The multifunctionality and dynamic redeployment of CetZ1 during motile cell development is reminiscent of eukaryotic cytoskeletal proteins and the roles of tubulin at the base of the eukaryotic flagellum.