Bacteria are generally distinguished from the cells of fungi, plants, and animals (eukaryotes) not only by their much smaller size but also by the absence of certain subcellular structures such as nuclei, internal organelles, and microtubules. Using state-of-the-art microscopy, this paper shows that microtubules do exist in some bacteria. These bacterial microtubules are built from proteins that are closely related to the microtubule proteins in eukaryotes. Bacterial microtubules are smaller in diameter than their counterparts in eukaryotic cells but have the same basic architecture. This paper proposes that bacterial microtubules represent primordial structures that preceded eukaryotic microtubules evolutionarily. Because bacterial microtubules can be produced and handled in the lab more easily than their eukaryotic counterparts, they may become useful tools for microtubule research and anti-cancer drug screening.
Microtubules in Bacteria: Ancient Tubulins Build a Five-Protofilament Homolog of the Eukaryotic Cytoskeleton. (2011) PLoS Biol 9(12): e1001213. doi:10.1371/journal.pbio.1001213
Microtubules play crucial roles in cytokinesis, transport, and motility, and are therefore superb targets for anti-cancer drugs. All tubulins evolved from a common ancestor they share with the distantly related bacterial cell division protein FtsZ, but while eukaryotic tubulins evolved into highly conserved microtubule-forming heterodimers, bacterial FtsZ presumably continued to function as single homopolymeric protofilaments as it does today. Microtubules have not previously been found in bacteria, and we lack insight into their evolution from the tubulin/FtsZ ancestor. Using electron cryomicroscopy, here we show that the tubulin homologs BtubA and BtubB form microtubules in bacteria and suggest these be referred to as “bacterial microtubules” (bMTs). bMTs share important features with their eukaryotic counterparts, such as straight protofilaments and similar protofilament interactions. bMTs are composed of only five protofilaments, however, instead of the 13 typical in eukaryotes. These and other results suggest that rather than being derived from modern eukaryotic tubulin, BtubA and BtubB arose from early tubulin intermediates that formed small microtubules. Since we show that bacterial microtubules can be produced in abundance in vitro without chaperones, they should be useful tools for tubulin research and drug screening.