by Daniel P. Haeusser
Figure 1. Global phylogeny of the tubulin superfamily. Source.
General biology textbooks, like the one I use for teaching, often depict the prokaryotic cell as an oval of homogenous-appearing cytoplasm surrounded by two membranes. This simplified image of a Gram-negative bacterial cell contrasts with the depiction of a well-organized, feature-laden eukaryotic cell with its membrane-bound organelles. While many would agree that the absence of a membrane-bound nucleus defines the prokaryotic–eukaryotic boundary, the recent decade of research has given us a glimpse of the actual organizational complexity of bacteria, which includes encoding cytoskeletal elements. Just as eukaryotes have their tubulin, actin, and intermediate filaments, the prokaryotes have FtsZ, MreB, and crescentin (although the occurrence of each varies greatly).
The first of these prokaryotic cytoskeletal proteins to be discovered, FtsZ, is a distantly-related member of the tubulin family (Figure 1). FtsZ is a highly conserved protein involved in cytokinesis in most bacteria, certain archaea, chloroplasts, and in the mitochondria of some protists. Although FtsZ shows only ~17% amino acid identity with tubulin, their crystal structures are remarkably similar (a fine example of structure trumping sequence). Notably, FtsZ, unlike tubulins, is unable to form microtubule-like structures; instead it just self-associates into loosely bundled protofilaments. Although a recent paper shows that FtsZ forms tubule-like structures in vitro in the presence of a binding partner/assembly regulator, these tubules are quite distinct from eukaryotic microtubules in both size and structure, and their in vivo relevance is uncertain.