The Tubulin Superfamily in Archaea

Abstract

In comparison with bacteria and eukaryotes, the large and diverse group of microorganisms known as archaea possess a great diversity of cytoskeletal proteins, including members of the tubulin superfamily. Many species contain FtsZ, CetZ and even possible tubulins; however, some major taxonomic groups do not contain any member of the tubulin superfamily. Studies using the model archaeon, Halferax volcanii have recently been instrumental in defining the fundamental roles of FtsZ and CetZ in archaeal cell division and cell shape regulation. Structural studies of archaeal tubulin superfamily proteins provide a definitive contribution to the cytoskeletal field, showing which protein-types must have developed prior to the divergence of archaea and eukaryotes. Several regions of the globular core domain - the "signature" motifs - combine in the 3D structure of the common molecular fold to form the GTP-binding site. They are the most conserved sequence elements and provide the primary basis for identification of new superfamily members through homology searches. The currently well-characterised proteins also all share a common mechanism of GTP-dependent polymerisation, in which GTP molecules are sandwiched between successive subunits that are arranged in a head-to-tail manner. However, some poorly-characterised archaeal protein families retain only some of the signature motifs and are unlikely to be capable of dynamic polymerisation, since the promotion of depolymerisation by hydrolysis to GDP depends on contributions from both subunits that sandwich the nucleotide in the polymer.

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

“…Biochemistry and Immuno-fluorescence microscopy studies indicated that archaeal FtsZ proteins are GTPases and localized to the mid-cell plane of division, much like their bacterial counterpart [30][31][32]35]. Whereas most bacterial species studied so far contain only one FtsZ, many archaea are now known to carry two FtsZ homologues, one from each of two distinct sub-families, with FtsZ1 being apparently more closely related to bacterial and plant FtsZ, with FtsZ2 being more phylogenetically distinct [5,6,36]. The early studies had identified only the FtsZ1 homolog but not the FtsZ2 homolog that was also present in the archaea chosen for study [32,35].…”

“…The early studies had identified only the FtsZ1 homolog but not the FtsZ2 homolog that was also present in the archaea chosen for study [32,35]. One exception was the FtsZ2 homologue studied in H. salinarum [6,30], which appeared to cause altered cell morphology after its overexpression in H. salinarum [30].…”

“…These proteins are widely distributed across the three domains of life: Archaea, Bacteria, and Eukarya [5]. Archaea house some of the most diverse members of the tubulin superfamily including bacteria-like and eukaryotelike tubulin homologs, as well as family members specific for archaea [6].…”

“…The archaea also show apparently greater diversity amongst the most wide-spread cytoskeletal proteins found in all three domains, including diverse homologs from the actin and tubulin superfamilies. In the tubulin superfamily, archaea house a particularly diverse collection, including FtsZ in many of the major taxa, tubulin in some groups, as well as some archaea-specific families, some of which are widespread-others highly divergent [6]. Indeed, many archaea encode multiple tubulin superfamily homologs that have differing functions [4,6].…”

“…In the tubulin superfamily, archaea house a particularly diverse collection, including FtsZ in many of the major taxa, tubulin in some groups, as well as some archaea-specific families, some of which are widespread-others highly divergent [6]. Indeed, many archaea encode multiple tubulin superfamily homologs that have differing functions [4,6]. Remarkably, almost all members of the Crenarchaeota phylum do not possess a clear tubulin superfamily homolog and use alternative structural/scaffolding components, including a cell division apparatus based on homologs of the eukaryotic Endosomal Sorting Complex Required for transport (ESCRT)-III complex [11,12].…”