Tubulin superfamily (TSF) proteins are widespread and known for multifaceted roles as cytoskeletal proteins underpinning many basic cellular functions including morphogenesis, division, and motility. In eukaryotes, tubulin assembles into microtubules, a major component of the dynamic cytoskeletal network of fibres, whereas the bacterial homolog FtsZ assembles the division ring at midcell. Functions of the lesser-known archaeal TSF proteins are beginning to be identified, and show surprising diversity, including homologs of tubulin and FtsZ, and a third archaea-specific family, CetZ, implicated in the regulation of cell shape and possibly other unknown functions. In this study, we defined sequence and structural characteristics of the CetZ family and CetZ1 and CetZ2 subfamilies, identified CetZ groups and diversity amongst archaea, and identified potential functional relationships through analysis of the genomic neighbourhoods of cetZ genes. At least three subfamilies of orthologous CetZ proteins were identified in the archaeal class Halobacteria, including CetZ1 and CetZ2 and a novel uncharacterized subfamily. CetZ1 and CetZ2 were correlated to one another and to cell shape and motility phenotypes across diverse Halobacteria. Amongst other known CetZ clusters in orders Archaeoglobales, Methanomicrobiales, Methanosarcinales, and Thermococcales, an additional uncharacterized group from Archaeoglobales and Methanomicrobiales affiliated strongly with Halobacteria CetZs, suggesting they originated via horizontal transfer. Subgroups of Halobacteria CetZ2 and Thermococcales CetZ genes were found adjacent to different type IV pili regulons, suggesting a potential utilization of CetZs by type IV systems. More broadly conserved cetZ gene neighbourhoods included nucleotide and cofactor biosynthesis (e.g., F420) and predicted cell surface sugar epimerase genes. The findings imply that CetZ subfamilies are involved in multiple functions linked to the cell surface, biosynthesis and motility.

Recent evidence obtained in murine atrial cardiomyocytes suggests that the behavior of tubulin, a constituent of microtubules, is influenced by audible sounds. In the present study, the sequence and the structure of murine and human tubulins were compared with those of human TMC1 (Transmembrane channel-like protein 1), a sound-sensitive protein that is responsible for forming the pore of mechanosensory transduction channels in the hair cells of the inner ear of vertebrates, and with those of bacterial FtsZ (Filamenting temperature-sensitive mutant Z), a bacterial protein homologous to tubulin. The results show that mouse and human alpha-tubulin are 100% identical, whereas sequence homology of human TMC1 and human tubulin alpha 1A chain is scarce, with only 17.31% identity. A higher degree of similarity was observed in the presence of aromatic amino acids as well as in the propensity to form alpha-helices. Based on these results, it is hypothesized that the response of tubulin to audible sounds is mediated by the external alpha-helices as it occurs in TMC1. The sequence homology of human TMC1 and FtsZ from Lactobacillus johnsonii, is moderate with 23.40% identity, however higher than that between TMC1 and tubulin. The overall presence of aromatic amino acids in FtsZ is scarce, but helix propensity is significant. These results are discussed in the context of Orch OR, a theory that postulates that consciousness emerges from quantum computations occurring in microtubules of neurons via information processing mediated by aromatic amino acids of tubulin.