Archaeal cell biology: diverse functions of tubulin-like cytoskeletal proteins at the cell envelope

Abstract

The tubulin superfamily of cytoskeletal proteins is widespread in all three domains of life — Archaea, Bacteria and Eukarya. Tubulins build the microtubules of the eukaryotic cytoskeleton, whereas members of the homologous FtsZ family construct the division ring in prokaryotes and some eukaryotic organelles. Their functions are relatively poorly understood in archaea, yet these microbes contain a remarkable diversity of tubulin superfamily proteins, including FtsZ for division, a newly described major family called CetZ that is involved in archaeal cell shape control, and several other divergent families of unclear function that are implicated in a variety of cell envelope-remodelling contexts. Archaeal model organisms, particularly halophilic archaea such as Haloferax volcanii, have sufficiently developed genetic tools and we show why their large, flattened cells that are capable of controlled differentiation are also well suited to cell biological investigations by live-cell high-resolution light and electron microscopy. As most archaea only have a glycoprotein lattice S-layer, rather than a peptidoglycan cell wall like bacteria, the activity of the tubulin-like cytoskeletal proteins at the cell envelope is expected to vary significantly, and may involve direct membrane remodelling or directed synthesis or insertion of the S-layer protein subunits. Further studies of archaeal cell biology will provide fresh insight into the evolution of cells and the principles in common to their fundamental activities across the full spectrum of cellular life.

Improved growth and morphological plasticity of Haloferax volcanii

“…E218A) from a plasmid in a wild-type genetic background and then observed cell shapes. The E218A mutation in the GTPase active site strongly inhibits CetZ1 function via hyperstabilization of localized polymers, and was used previously for verification of CetZ1's involvement in rod development in motility soft agar [4]. We found that expression of cetZ1.…”

“…To verify that CetZ1 function is necessary for rod and tubule formation, we expressed a dominant-inhibitory mutant (CetZ1.E218A) from a plasmid in a wild-type genetic background and then observed cell shapes. The E218A mutation in the GTPase active site strongly inhibits CetZ1 function via hyper-stabilization of localized polymers, and was used previously for verification of CetZ1’s involvement in rod development in motility soft agar [4]. We found that expression of cetZ1 .E218A blocked rod cell and tubule formation in the wild-type strain background (), confirming its dominant influence on cell shape and the requirement for functional CetZ1 in trace-element-induced rod and tubule generation.…”

“…The model haloarchaeon, Haloferax volcanii, when first isolated, was described as mainly disc-shaped cells, with cell shape and size varying significantly [1]. In routine liquid cultures of H. volcanii, both pleomorphic discoid (plate) and rod cell morphologies may be observed, occasionally with more unusual shapes [2][3][4].…”

“…Furthermore, H. volcanii forms motile rods in soft agar, where rods are observed at the forefront of expanding colonies of swimming cells [6]. The tubulin-like cytoskeletal protein CetZ1 is required for rod formation, suggesting a connection between morphology and motility that may be expected based on improved hydrodynamics or directional movement of rods [4,6]. Conversely, the peptidase archaeosortase A (ArtA) and phosphatidylethanolamine biosynthesis enzymes PssA and PssD, which are required for the C-terminal processing and covalent lipid attachment of several H. volcanii surface proteins including the surface (S-layer) glycoprotein, are required for effective and stable plate-shaped cell formation [7,8].…”

“…In routine liquid cultures of H. volcanii , both pleomorphic discoid (plate) and rod cell morphologies may be observed, occasionally with more unusual shapes [2–4].…”