Bacterial actin and tubulin homologs in cell growth and division

Abstract

In contrast to the elaborate cytoskeletal machines harbored by eukaryotic cells, such as mitotic spindles, cytoskeletal structures detectable by typical negative stain electron microscopy are generally absent from bacterial cells. As a result, for decades it was thought that bacteria lacked cytoskeletal machines. Revolutions in genomics and fluorescence microscopy have confirmed the existence not only of smaller-scale cytoskeletal structures in bacteria, but also of widespread functional homologs of eukaryotic cytoskeletal proteins. The presence of actin, tubulin, and intermediate filament homologs in these relatively simple cells suggests that primitive cytoskeletons first arose in bacteria. In bacteria such as Escherichia coli, homologs of tubulin and actin directly interact with each other and are crucial for coordinating cell growth and division. The function and direct interactions between these proteins will be the focus of this review.

Figure 1

Different cellular morphologies and localization patterns in E. coli depending on the levels of FtsA, FtsZ or MreB. Shown are schematics of typical cell shapes and localization of FtsZ (green) or FtsA (red) under various conditions, along with a micrograph of a normal punctate E. coli Z ring (FtsZ in green) imaged by 3D-SIM. FtsAΔCt is missing the carboxy-terminal membrane-targeting sequence and forms large bundled polymers in the cell when overproduced that often causes the cells to curve. FtsZ overproduction results in Z rings near cell poles, which can divide off to yield minicells. The lack of MreB abolishes rod shape, forcing cells to grow and divide as spheroids, often with an asymmetric invagination as shown in both the schematic and the micrograph.

Figure 2

Roles of bacterial tubulin (FtsZ) and actin (FtsA) in assembly of the cytokinetic ring in E. coli. FtsZ consists of a large globular core domain required for GTP binding and head to tail polymerization, along with a flexible linker and a short conserved carboxy-terminal core, which serves as a hub for binding ZipA (not shown) and FtsA. Upon binding to GTP, FtsZ monomers form head-to-tail polymers that can hydrolyze GTP. FtsZ monomers resulting from polymer disassembly recycle rapidly and rebind GTP to continue the cycle. FtsA consists of 4 subdomains (1A, 1C, 2A, and 2B, shown in different colors) and a carboxy-terminal amphipathic helix that serves as a membrane anchor (see also Figure 3). Although not as well characterized, ATP binding seems to be required for formation of head-to-tail polymers of FtsA, which then may be able to interact with FtsZ polymers at the membrane as shown. In addition, bundles of FtsZ polymers may interact with bundles of FtsA polymers.

Figure 3

Diversity of selected prokaryotic tubulins and actins. (A) Phylogenetic tree of bacterial FtsZs, archaeal FtsZs (purple box), chloroplast FtsZ paralogs (dark green box), mitochondrial FtsZ paralogs (blue boxes), TubZs (orange box), BtubA (red box), and yeast tubulin (light green box). (B) Phylogenetic tree of bacterial FtsAs, MreB homologs (red box), ParM (purple box), MamK (blue box), crenactin in a crenarchaeon (orange box) and yeast actin (green box). Sequences were aligned in Uniprot and converted to trees in Geneious.

Figure 4

Atomic structures of MreB and FtsA polymers with bound ATP. FtsA is depicted at a different angle than MreB. Membrane targeting sequences are disordered in the crystal structures but depicted as brown boxes at the end of a flexible linker. Alpha-helical and beta-sheet secondary structures are shown in red and purple, respectively. For MreB, a residue important for inter-filament interactions between the two antiparallel polymers [136] is highlighted in green in the subunits of the left dimer. For FtsA, the FtsZ carboxy-terminal peptide that interacts with FtsZ [96] is shown in green in the bottom subunit of the dimer. PDB structures (4A2A for the Thermotoga maritima FtsA, 4CZJ for Caulobacter crescentus MreB) were manipulated using UCSF Chimera.

Figure 5

Typical localization patterns of FtsZ, FtsA and MreB in several diverse bacterial species. Shown are the transition between predivisional and divisional cells of E. coli, which harbors all three proteins; divisional cells of Corynebacterium glutamicum, which lack MreB and FtsA, Staphyloccccus aureus, which lack MreB, and reticulate bodies of the intracellular pathogen Chlamydia trachomatis, which lacks FtsZ and FtsA but uses MreB for cytokinesis instead.