CetZ1-dependent polar assembly of the archaeal motility machinery

The archaeal tubulin-like cytoskeletal protein CetZ1 is required for rod-cell morphogenesis during the development of motility in Haloferax volcanii. This is expected to improve swimming speed and directionality. Here, we found that deletion of cetZ1 or expression of a GTPase-defective mutant caused a substantial defect in the assembly of the motility machinery, including the archaellum base marker protein ArlD1, the chemotaxis sensory array adapter CheW1, and signal transducer CheY. Furthermore, overexpression of cetZ1 reduced the assembly and polar placement of the motility machinery without detectably affecting the rod shape of motile cells. In contrast, deletion of the conserved paralog cetZ2 caused no defects in swimming or rod shape, although expression of the cetZ2 GTPase-defective mutant reduced motility whereas cetZ2 overexpression caused mild hyper-motility; these effects were dependent on the presence of cetZ1. A functional CetZ1-mTq2 fusion strongly localized at the poles of mature motile cells, where it partially co-localized with the motility machinery markers. These results suggest that CetZ1 has another role in the organisation or structure of the cell poles that promotes the assembly of the motility machinery. The multiple roles and locations of CetZ1 during motile cell development are reminiscent of the multiple functions of eukaryotic cytoskeletal proteins.