Microorganisms of the third domain of life, Archaea, are important to biotechnology and the environment, yet relatively little is known of their physiology and cell biology. Archaea have a prokaryotic cell structure, but are genetically more related to eukaryotes than bacteria. Our work has focused on the tubulin super-family of proteins that constitute the cytoskeleton in eukaryotic cells, and the cell division apparatus in bacteria. Interestingly, whereas bacterial cells contain a single tubulin-family member called FtsZ, many archaea encode multiple homologs. We show here that two of the eight homologs present in the model archaeon Haloferax volcanii are essential for normal cell division. Remarkably, however, deletion and double-deletion strains can be maintained indefinitely, despite the apparent complete absence of a cytokinetic mechanism and the development of very large misshapen cells. By developing GFP-tagging approaches for this species, combined with super-resolution and time-lapse video-microscopy, we revealed a dynamic pattern of cell division in which planes of cell division are established orthogonally in successive cell cycles.
The remaining six tubulin-family homologs in H. volcanii were not required for cell division. They are members of a distinct uncharacterized family that show greater similarity to eukaryotic tubulins. We discovered that one of these proteins, named CetZ, controls the development of H. volcanii cells from a pleomorphic cell type to a elongated rod-shaped form required for cell motility. Such a role in dynamic control of cell shape is reminiscent of tubulin’s role in regulating eukaryotic cell structure and has not been observed previously in prokaryotes to our knowledge. This new function and our crystal structures of CetZ proteins in single and proto-filament form suggest that the CetZ protein family represents a missing link in the evolutionary history of the eukaryotic tubulins and bacterial FtsZs.