If you're here, Then I imagine that you are already familiar with what cell division is however, in brief Cell division is the process in which the parent cell divides into two daughter cells. This process is highly complex and regulated; it requires both adequate cell growth and DNA replication followed by division itself. Septal formation and invagination (constriction of the cell envelope) is a vital part of this process and if all goes well should result in two daughter cells of roughly equal size. It requires the transient assembly of more than 30 proteins that make up the "divisome" so I have a feeling this will be a WIP for a while and likely highly confusing to explain well, so i do apologise.... # Proto-ring formation and condensation The formation of the proto-ring is an extremely important part of cell division, it occurs early in the process and essentially sets up the site/support where division and invagination is going to take place, Its regulation and assembly is key for proper division. ![[Pasted image 20250813211212.png|250]] PDB: 6UNX FtsZ from *Escherichia coli* in complex with GTP **FtsZ** is the key protein in the Z-ring. It arrives at the midcell first and provides a scaffold for subsequent proteins. It is a tubulin homologue (tubulin is a super family of proteins that also contains a conserved GTPase domain). FtsZ localisation or more rather restriction to the midcell is mediated by MinCDE proteins. >**Min mediated restriction** > I think these are really interesting how they work! MinD localises to the poles and contains an ATPase and ATP binding domain when MinD is bound to ATP it is able to bind to the membrane and form clusters which in turn activate MinC. MinC inhibits the polymerisation of FtsZ. > MinE regulates the polar localisation of MinDC by forming a ring near each pole that hydrolyses the ATP bound to MinD causing minD to Unbind from the membrane there is a diffuse nature to the action of the MinCDE system and it oscillates. Over Expression of MinC results in filamentation (Huang, Meir and Wingreen, 2003) FtsZ assembles on the inner surface of the inner membrane into a dynamic ring structure. The filaments constantly break down and rebuild, Polymerising when bound to GTP but de-polymerising upon GTP hydrolysis, swapping subunits with the free cytoplasmic pool, which allows precise regulation of cell division. This constant restructuring allows for rapid disassembly if it is not time to divide (e.g. nucleoid occlusion) but also change in diameter in response to septum formation. It interestingly can assemble in the absence of any other protein but is unstable. **FtsA** is an essential actin-like protein in gram negative bacteria and acts as an ATPase; it can polymerise into antiparallel double filaments forming the **A-ring**. It is also recruited to the mid-cell at the earliest stage of the cell division process and relies on the prior localisation of FtsZ in order to localise itself. It anchors the Z-ring to the inner membrane and associates with the membrane through an amphipathic helix. **ZipA** is also an essential protein and also acts as a FtsZ anchor, anchoring the the Z-ring to the membrane these three assembly earliest in the division stages and form the "proto-ring" both FtsA and ZipA bind to the conserved [[c-terminal]] peptide of FtsZ and are required for recruitment of the rest of the divisome but can form the z-ring in tandem with FtsZ alone. ![[proto-ring.JPG]] **ZapABCD** (_FtsZ-associated proteins_) are the next series of proteins I will mention, while not essential they also co-localise with the Z-ring early in divisome formation although, **ZapE** co-localises later. they help to stabilise and condense the Z-ring. ZapA is a widely conserved protein, it co-localises with FtsZ. It is not essential in wild-type cells with typical levels of FtsZ, but is required when FtsZ concentrations are diminished it contributes to Z-ring condensation and bundling. ZapB interacts directly with ZapA and the pair can form structures independent of FtsZ filaments. It contributes to stabilising the Z-ring. >Bundling is the cross-linking of multiple, individual filaments together to form "Bundles". This is required for concentrating the Z-ring to the specific division site and stabilising its formation as a scaffold for the rest of the divisome. ZapC is another non-essential member of the divisome, also involved in Z-ring stability. It functions both to promote interactions between filaments but also to supress the GTPase activity of FtsZ. This role is important to promote prolonged polymerisation of FtsZ which can rapidly disassemble. Interestingly over-production of ZapC leads to division defects and elongation, likely as a result of a hyper-stable Z-ring. ZapD functions to crosslink FtsZ filaments and its over production can also result in similar defects as seen in ZapC over-production. While most Zap proteins are "non-essential" and single knockouts typically produce mild or negligible defects (e.g.,  _ΔzapD_ deletion alone shows no distinct morphotype), combined deletions exacerbate division defects. Notably,  _ΔzapC_ alongside  _ΔzapA_ or  _ΔzapD_ produces elongated cells, suggesting impaired or delayed divisome assembly.(Gong et al., 2024) # Recruitment of the other essential division proteins I will note here that there is some variation in the order that these proteins assemble to the divisome (such as in *Caulobacter crescentus*) but I will discuss them in an order seen in many gram-negative bacteria. **FtsEX** are the next in the divisome line up. They are a part of the ABC transporter family with FtsE acting as the cytoplasmic Nucleotide binding domain and FtsZ forming the transmembrane component with 4 transmembrane domains. It has a large periplasmic loop and is uncharacteristically un-charged comparative to other ABC transporters. It is believed that FtsE interacts directly with FtsZ at the c-terminal tail- this means that it can, in high concentrations, displace FtsA and ZipA (resulting in the breakdown of the Z-ring). The interaction of FtsZ with FtsA is important for recruitment of later divisome proteins. The ATPase function of FtsEX is not required for divisome assembly but is required for division and cell wall hydrolysis by regulating amidases. **FtsK** is an essential DNA translocase its job is to resolve chromosome dimers and to clear the division site of genetic material. It also functions to link later proteins to the divisome. Its structure is very cool: it has a cytoplasmic membrane anchoring domain containing 4 α-helices, then a linker domain that varies in length across species and then it has a cytoplasmic hexameric motor domain made of repeating C-terminal α, β ATPase subunits with γ regulating the hexamer and DNA dimer resolution. ![[Pasted image 20250901130842.png|250]] PDB: 2IUU from *Pseudomonas aeruginosa* C-terminal motor domain hexamer **FtsQLB** forms a complex late in the divisome that is involved in regulation and activation of FtsWI (FtsW is part of the shape, elongation, division, and sporulation (SEDS) family and FtsI is also known as PBP3). All three proteins in this complex contain [[bitopic]], [[N-terminal]] transmembrane domains and periplasmic domains (Nguyen et al., 2023). **FtsQ** contains two periplasmic domains a [[POTRA]] domain and a β domain, with the POTRA domain crucial for interactions with other divisome proteins and the β domain needed to help localise to the β domain of FtsB specifically. **FtsW** is a polytopic integral membrane protein; it contains 10 transmembrane helices with 4 of these helices forming an internal cavity containing conserved polar residues. As mentioned above is a part of the SEDS family, it is involved in the transport of Lipid II (PtG precursor) across the membrane acting as a flippase. Since it is a part of the SEDS family it is also likely that it has glycosyltransferase activity working in conjunction with its (loving) partner **FtsI aka PBP3 which specifically synthesises septal PtG** **FtsN** is often the final protein to localise to the Z-ring, Its arrival "activates" the divisome so septal PtG synthesis and division can begin (this kind of gives me beautiful imagery LOL!). FtsN is membrane anchored, the N-terminal interacts with FtsA and the C-terminus contains a SPOR (Sporulation-related repeat) domain which binds to peptidoglycan that do not have stem peptides ("denuded" glycans). It also functions to promote the activities of FtsWI # Tol-Pal you can have a look at [[Tol system]] for further insight in potential additional roles of Tol-Pal in outer-membrane maintenance and a more detailed look at it- granted I love the Tol-Pal system so I likely wont hold back here... # How does division actually happen? <u>**References**</u> - [Bhattacharya, A., Ray, S., Singh, D., Dhaked, H.P.S. and Panda, D. (2015). ZapC promotes assembly and stability of FtsZ filaments by binding at a different site on FtsZ than ZipA. _International journal of biological macromolecules_, [online] 81, pp.435–42. doi:https://doi.org/10.1016/j.ijbiomac.2015.08.030.](https://www.sciencedirect.com/science/article/pii/S0141813015005656?via%3Dihub) - [Condon, S.G.F., Mahbuba, D.-A., Armstrong, C.R., Diaz-Vazquez, G., Craven, S.J., LaPointe, L.M., Khadria, A.S., Chadda, R., Crooks, J.A., Rangarajan, N., Weibel, D.B., Hoskins, A.A., Robertson, J.L., Cui, Q. and Senes, A. (2018). The FtsLB subcomplex of the bacterial divisome is a tetramer with an uninterrupted FtsL helix linking the transmembrane and periplasmic regions. _Journal of Biological Chemistry_, 293(5), pp.1623–1641. doi:https://doi.org/10.1074/jbc.ra117.000426.](https://www.jbc.org/article/S0021-9258(20)38942-0/fulltext) - [Du, S. and Lutkenhaus, J. (2017). Assembly and activation of the Escherichia coli divisome. _Molecular Microbiology_, 105(2), pp.177–187. doi:https://doi.org/10.1111/mmi.13696.](https://pmc.ncbi.nlm.nih.gov/articles/PMC5517055/) - [Gong, H., Yan, D., Cui, Y., Li, Y., Yang, J., Yang, W., Zhan, R., Wan, Q., Wang, X., He, H., Chen, X., Lutkenhaus, J., Yang, X. and Du, S. (2024). The divisome is a self-enhancing machine in Escherichia coli and Caulobacter crescentus. _Nature Communications_, 15(1). doi:https://doi.org/10.1038/s41467-024-52217-5.](https://www.nature.com/articles/s41467-024-52217-5) - [ Huang, K.C., Meir, Y. and Wingreen, N.S. (2003). Dynamic structures in _Escherichia coli_ : Spontaneous formation of MinE rings and MinD polar zones. _Proceedings of the National Academy of Sciences_, 100(22), pp.12724–12728. doi:https://doi.org/10.1073/pnas.2135445100.](https://pmc.ncbi.nlm.nih.gov/articles/PMC240685/) - [Massey, T.H., Mercogliano, C.P., Yates, J., Sherratt, D.J. and Löwe, J. (2006). Double-stranded DNA translocation: structure and mechanism of hexameric FtsK. _Molecular Cell_, [online] 23(4), pp.457–469. doi:https://doi.org/10.1016/j.molcel.2006.06.019.](https://www.cell.com/molecular-cell/fulltext/S1097-2765(06)00436-9?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1097276506004369%3Fshowall%3Dtrue) - [Nguyen, H.T.V., Chen, X., Parada, C., Luo, A.-C., Shih, O., Jeng, U.-S., Huang, C.-Y., Shih, Y.-L. and Ma, C. (2023). Structure of the heterotrimeric membrane protein complex FtsB-FtsL-FtsQ of the bacterial divisome. _Nature Communications_, [online] 14(1), p.1903. doi:https://doi.org/10.1038/s41467-023-37543-4.](https://www.nature.com/articles/s41467-023-37543-4) - [Rueda, S., Vicente, M. and Mingorance, J. (2003). Concentration and Assembly of the Division Ring Proteins FtsZ, FtsA, and ZipA during the Escherichia coli Cell Cycle. _Journal of Bacteriology_, 185(11), pp.3344–3351. doi:https://doi.org/10.1128/jb.185.11.3344-3351.2003.](https://pmc.ncbi.nlm.nih.gov/articles/PMC155373/#:~:text=One%20of%20these%20events%20is,vitro%20(13%2C%2033).) - [Schumacher, M.A., Zeng, W., Huang, K.-H., Tchorzewski, L. and Janakiraman, A. (2016). Structural and Functional Analyses Reveal Insights into the Molecular Properties of the Escherichia coli Z Ring Stabilizing Protein, ZapC. _Journal of Biological Chemistry_, 291(5), pp.2485–2498. doi:https://doi.org/10.1074/jbc.m115.697037.](https://www.sciencedirect.com/science/article/pii/S0021925820360841#:~:text=The%20studies%20that%20have%20been,each%20of%20its%20two%20domains.) - [Yang, L., Chen, Y., Chang, S., Shen, C., Wang, X., Zhang, C., Zhang, Z., Ding, B.-S., Su, Z., Dong, H. and Tang, X. (2024). Structural insights into the activation of the divisome complex FtsWIQLB. _Cell Discovery_, [online] 10(1). doi:https://doi.org/10.1038/s41421-023-00629-w.](https://www.nature.com/articles/s41421-023-00629-w)