CetZ1-dependent assembly and positioning of the motility machinery inhaloarchaea

Archaeal CetZ cytoskeletal proteins are a major group of the tubulin superfamily involved in the development of motile rod-shaped cells. In the model archaeon Haloferax volcanii , it has been unknown whether CetZs contribute to motility solely through the effects of rod cell shape on swimming speed, or in other ways too. Here, we used cryo-electron and fluorescence microscopy to observe cell surface filaments and the assembly of the motility machinery in cells carrying cetZ1 or cetZ2 deletion, overexpression, or dominant GTPase-deficient mutations. CetZ1 was found to play important roles in the polar assembly of motility marker proteins, including ArlD1, a constituent of the archaellum base, the chemotaxis sensory array adapter CheW1, and signal transducer CheY. Notably, overexpression of cetZ1 reduced motility and the assembly of the motility machinery markers but did not affect the rod shape of motile cells. Dual protein localization studies further suggested that CetZ1 is not a direct stoichiometric component of the motility machineries but promotes their assembly via a significant role in organisation or structure of the cell poles. Significance This study demonstrates that cytoskeletal proteins (CetZs) can contribute to assembly of the motility machinery in archaea. In the model organism, H. volcanii , this function is expected to contribute significantly to the overall impact of CetZ1 on swimming motility, in addition to its established role in rod cell formation generating a streamlined shape. Our study also provides the first evidence that archaeal CetZ tubulin superfamily proteins can have structural or organisational roles for the assembly of protein complexes on the cell surface and cell poles. The multiple functions and subcellular locations of CetZ1 during motile cell development are reminiscent of eukaryotic cytoskeletal proteins and the roles of tubulin in basal bodies during the development of the eukaryotic flagellum.