Free SepF interferes with recruitment of late cell division proteins

Anchors: A way for FtsZ filaments to stay membrane bound

Most bacteria use the tubulin homolog FtsZ to organize their cell division. FtsZ polymers initially assemble into mobile complexes that circle around a ring-like structure at the cell midpoint, followed by the recruitment of other proteins that will constrict the cytoplasmic membrane and synthesize septal peptidoglycan to divide the cell. Despite the need for FtsZ polymers to associate with the membrane, FtsZ lacks intrinsic membrane binding ability. Consequently, FtsZ polymers have evolved to interact with the membrane through adaptor proteins that both bind FtsZ and the membrane. Here, we discuss recent progress in understanding the functions of these FtsZ membrane tethers. Some, such as FtsA and SepF, are widely conserved and assemble into varied oligomeric structures bound to the membrane through an amphipathic helix. Other less-conserved proteins, such as EzrA and ZipA, have transmembrane domains, make extended structures, and seem to bind to FtsZ through two separate interactions. This review emphasizes that most FtsZs use multiple membrane tethers with overlapping functions, which not only attach FtsZ polymers to the membrane but also organize them in specific higher-order structures that can optimize cell division activity. We discuss gaps in our knowledge of these concepts and how future studies can address them.

The archaeal protein SepF is essential for cell division in Haloferax volcanii

In most bacteria, cell division depends on the tubulin homolog FtsZ and other proteins, such as SepF, that form a complex termed the divisome. Cell division also depends on FtsZ in many archaea, but other components of the divisome are unknown. Here, we demonstrate that a SepF homolog plays important roles in cell division in Haloferax volcanii, a halophilic archaeon that is known to have two FtsZ homologs with slightly different functions (FtsZ1 and FtsZ2). SepF co-localizes with both FtsZ1 and FtsZ2 at midcell. Attempts to generate a sepF deletion mutant were unsuccessful, suggesting an essential role. Indeed, SepF depletion leads to severe cell division defects and formation of large cells. Overexpression of FtsZ1-GFP or FtsZ2-GFP in SepF-depleted cells results in formation of filamentous cells with a high number of FtsZ1 rings, while the number of FtsZ2 rings is not affected. Pull-down assays support that SepF interacts with FtsZ2 but not with FtsZ1, although SepF appears delocalized in the absence of FtsZ1. Archaeal SepF homologs lack a glycine residue known to be important for polymerization and function in bacteria, and purified H. volcanii SepF forms dimers, suggesting that polymerization might not be important for the function of archaeal SepF.

Control of septum thickness by the curvature of SepF polymers

Gram-positive bacteria divide by forming a thick cross wall. How the thickness of this septal wall is controlled is unknown. In this type of bacteria, the key cell division protein FtsZ is anchored to the cell membrane by two proteins, FtsA and/or SepF. We have isolated SepF homologs from different bacterial species and found that they all polymerize into large protein rings with diameters varying from 19 to 44 nm. Interestingly, these values correlated well with the thickness of their septa. To test whether ring diameter determines septal thickness, we tried to construct different SepF chimeras with the purpose to manipulate the diameter of the SepF protein ring. This was indeed possible and confirmed that the conserved core domain of SepF regulates ring diameter. Importantly, when SepF chimeras with different diameters were expressed in the bacterial host Bacillus subtilis, the thickness of its septa changed accordingly. These results strongly support a model in which septal thickness is controlled by curved molecular clamps formed by SepF polymers attached to the leading edge of nascent septa. This also implies that the intrinsic shape of a protein polymer can function as a mold to shape the cell wall.

ylm Has More than a (Z Anchor) Ring to It!

The division and cell wall (dcw) cluster is a highly conserved region of the bacterial genome consisting of genes that encode several cell division and cell wall synthesis factors, including the central division protein FtsZ. The region immediately downstream of ftsZ encodes the ylm genes and is conserved across diverse lineages of Gram-positive bacteria and Cyanobacteria In some organisms, this region remains part of the dcw cluster, but in others, it appears as an independent operon. A well-studied protein coded from this region is the positive FtsZ regulator SepF (YlmF), which anchors FtsZ to the membrane. Recent developments have shed light on the importance of SepF in a range of species. Additionally, new studies are highlighting the importance of the other conserved genes in this neighborhood. In this minireview, we aim to bring together the current research linking the ylm region to cell division and highlight further questions surrounding these conserved genes.

Systematic Identification of Target Genes for Cellular Morphology Engineering in Synechococcus elongatus PCC7942

Cyanobacteria are serving as promising microbial platforms for development of photosynthetic cell factories. For enhancing the economic competitiveness of the photosynthetic biomanufacturing technology, comprehensive improvements on industrial properties of the cyanobacteria chassis cells and engineered strains are required. Cellular morphology engineering is an up-and-coming strategy for development of microbial cell factories fitting the requirements of industrial application. In this work, we performed systematic evaluation of potential genes for cyanobacterial cellular morphology engineering. Twelve candidate genes participating in cell morphogenesis of an important model cyanobacteria strain, Synechococcus elongatus PCC7942, were knocked out/down and overexpressed, respectively, and the influences on cell sizes and cell shapes were imaged and calculated. Targeting the selected genes with potentials for cellular morphology engineering, the controllable cell lengthening machinery was also explored based on the application of sRNA approaches. The findings in this work not only provided many new targets for cellular morphology engineering in cyanobacteria, but also helped to further understand the cell division process and cell elongation process of Synechococcus elongatus PCC7942.

Mutation of G51 in SepF impairs FtsZ assembly promoting ability of SepF and retards the division of Mycobacterium smegmatis cells

The role of FtsZ-associated proteins in the regulation of the assembly dynamics of Mycobacterium smegmatis FtsZ is not clear. In this work, we examined the effect of M. smegmatis SepF on the assembly and stability of M. smegmatis FtsZ polymers. We discovered a single dominant point mutation in SepF (G51D or G51R) that renders the protein inactive. SepF promoted the polymerization of FtsZ, induced the bundling of FtsZ filaments, stabilized FtsZ filaments and reduced the GTPase activity of FtsZ. Surprisingly, both G51D-SepF and G51R-SepF neither stabilized FtsZ filaments nor showed a discernable effect on the GTPase activity of FtsZ. The binding affinity of SepF to FtsZ was found to be stronger than the binding affinity of G51R/D-SepF to FtsZ. Interestingly, the binding affinity of SepF to G51R-SepF was determined to be 45 times stronger than FtsZ. In addition, the interaction of SepF with G51R-SepF was found to be 2.6 times stronger than SepF-SepF interaction. Furthermore, G51R-SepF impaired the ability of SepF to promote the assembly of FtsZ. In addition, the overexpression of G51R-SepF in M. smegmatis mc² 155 cells retarded the proliferation of these cells and increased the average length of the cells. The results indicated that SepF positively regulates the assembly of M. smegmatis FtsZ and the G51 residue has an important role in the functioning of SepF.