After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein, FtsZ, directs the partition between the nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site.
A septum is formed between the nucleoids, extending gradually from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate. Binary Fission : These images show the steps of binary fission in prokaryotes. The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo karyokinesis and, therefore, have no need for a mitotic spindle.
However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules that make up the mitotic spindle fibers that are necessary for eukaryotes. FtsZ proteins can form filaments, rings, and other three-dimensional structures that resemble the way tubulin forms microtubules, centrioles, and various cytoskeletal components.
In addition, both FtsZ and tubulin employ the same energy source, GTP guanosine triphosphate , to rapidly assemble and disassemble complex structures.
FtsZ and tubulin are homologous structures derived from common evolutionary origins. In this example, FtsZ is the ancestor protein to tubulin a modern protein. While both proteins are found in extant organisms, tubulin function has evolved and diversified tremendously since evolving from its FtsZ prokaryotic origin. This was the case though the effect was partial; Supplementary Fig. They also provides a potentially useful tool the soluble bacterial enzyme for in vitro reconstitution of BARS-dependent fission.
The formation of VSVG-containing carriers from the TGN was visualized by immunofluorescence microscopy and quantified by inhibiting the fusion of these carriers with the plasma membrane with tannic acid 14 , 15 , 43 , which results in accumulating carriers close to the cell surface. Quantification is on the right see Methods. Insets: enlarged view of merged signals for the Golgi area. Glyceraldehyde 3-phosphate dehydrogenase GAPDH is shown for the internal protein levels and molecular weight standards kDa are indicated on the left of each panel in a — f.
These tubules represent carrier precursors that elongate out of the Golgi but do not detach to form mature transport intermediates 14 , 15 Supplementary Movies 1 and 2. When they rarely did detach, however, they could be seen to move towards, and fuse with, the plasma membrane Supplementary Movie 2. Finally, we probed a soluble basolateral cargo, the stably expressed constitutively secreted GFP-tagged variant of the human growth hormone hGH Bottom: representative western blotting with an anti-Flag antibody, for the transfection efficiencies of these proteins used for the LPAAT assays.
Molecular weight standards kDa in a — c , are indicated on the left of each panel. Dotted lines show cell borders. Quantification of VSVG-positive carriers right. Importantly, they are potentially sufficient, depending on substrate availability, to change the PA concentrations in the TGN rapidly and substantially.
See also Supplementary Fig. We then sought to manipulate the BARS levels acutely in in vitro lysates to exclude transcriptional or compensatory effects that might arise in siRNA-depletion experiments 16 , As a control, we tested the effects of overexpressing wild-type BARS of note, wild-type BARS and the dominant-negative mutants showed comparable expression levels in these experiments; see Supplementary Fig.
Glyceraldehyde 3-phosphate dehydrogenase GAPDH is shown for the internal protein levels and molecular weight standards kDa are indicated on the left of each panel. Finally, we repeated a few of the above experiments using a Golgi-membrane-enriched fraction Supplementary Fig. Also, as seen with total extracts Fig. A comparison of the donor decay lifetime in control cells cells not expressing the acceptor and in cells co-expressing donor and acceptor showed in the latter case a marked reduction of donor lifetime at the Golgi Fig.
Insets, right: magnification of the tubular carrier precursors in the Golgi area. Notably, the metabolism of PA has been implicated in various aspects of membrane dynamics by other groups, albeit generally based on indirect evidence 18 , 21 , 34 , 56 , 57 , Based on current and previous results, we propose the following working model for basolateral carrier formation. This produces a local increase in phosphatidylinositol 4-phosphate PtdIns4 P 59 , which supports the budding of tubular carrier precursors, most likely through recruitment of PtdIns4 P -binding proteins Carrier precursor budding is assisted by phospholipase A 2 PLA 2 via production of positively curved lysolipids, including LPA, which facilitate the bending of membranes into tubules 2 , 33 , 34 , 60 , LPA and PA have biophysical properties that may be relevant for fission.
PA has a highly charged headgroup close to the glycerol backbone, a tendency to form intramolecular and intermolecular hydrogen bonds and segregate into microdomains. In addition, under physiological conditions LPA and PA have strongly positive and negative spontaneous curvatures, respectively 32 ; thus the conversion from LPA to PA may generate negatively curved PA microdomains within overall positively curved membrane areas, which might lead to membrane destabilization.
And, finally, the formation of PA might lead to other potential fission-related lipid-based mechanisms, such as the enzymatic conversion of PA into diacylglycerol 57 , 64 , The precise role of the lipid and protein players considered in this study must now be defined. Most of the key components involved in BARS-dependent fission are available in pure form, and the possibility to reconstitute this fission pathway in artificial membranes using known components appears to be within reach.
All of the individually sourced antibodies for western blotting WB , immunofluorescence and the LPAAT assay were obtained and used as detailed in ref. Source images from relevant WB are available in the Supplementary Figs 11— After three washes with lysis buffer with 0. The beads were then washed three times with lysis buffer at pH 8. Full-scan images of all WB data are reported in Supplementary Figs 11— The fluorescent spots 0. The cells were immunostained with anti-VSVG antibody and the percentage of the cells with the ER staining was quantifed Wide-field microscopy was performed as described previously 15 , with some modifications.
The cloned fragments were then completely sequenced and verified. COS7 cells were transiently transfected with 0. The lifetime values were extracted from the fitting of decay curves obtained from the donor emission E 0 GFP—BARS on the Golgi area, and global and pixel kinetic data analyses are computed by a commercially available software package SymPho Time, Version 5.
The measurements were carried out in fixed cells, for ease and speed of acquisition of a statistically reliable number of samples. The donor lifetime values were extracted from the fitting of decay curves obtained from the donor emission E 0 GFP—BARS in the Golgi area, and both global and pixel kinetic data analyses were computed by the commercially available software package SymPho Time, Version 5.
Changes in donor lifetime due to FRET were assessed by comparing the donor lifetime in cells expressing only the donor with the lifetime in cells co-transfected with donor and acceptor and fixed either under steady-state condition or during a traffic pulse. The cubes were immersed in 2. Electron microscopy images were acquired using FEI Tecnai electron microscope. The efficiency of interference was assessed by WB.
Golgi membranes from HeLa cells treated as described above were obtained as described previously 70 , with some modifications. The cell pellets were resuspended in vol. The cells were then passed six times each direction 12 passes total through the Balch homogenizer with a 7. After centrifugation 90, g , 2. The radiolabelled spots were quantified by gas ionization counting Beta-Imager Systems, Biospace Laboratories.
Dioleoyl [ 14 C]-PA was used as a standard. In Figs 5 , 6 , 7 and Supplementary Fig. The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files or are available from the corresponding authors upon request. How to cite this article: Pagliuso, A. Kozlov, M. Protein-driven membrane stresses in fusion and fission. Trends Biochem. Campelo, F. Membrane fission: the biogenesis of transport carriers.
Johannes, L. Induced domain formation in endocytic invagination, lipid sorting, and scission. Cell , — Frolov, V. Geometry of membrane fission. Lipids , — Weigert, R. Nature , — Roux, A.
Reaching a consensus on the mechanism of dynamin? FPrime Rep. Article Google Scholar. Ferguson, S. Dynamin, a membrane-remodelling GTPase. Cell Biol. Schmid, S. Dynamin: functional design of a membrane fission catalyst. Cell Dev. McMahon, H. Molecular mechanism and physiological functions of clathrin-mediated endocytosis.
Daumke, O. BAR domain scaffolds in dynamin-mediated membrane fission. Boucrot, E. Membrane fission is promoted by insertion of amphipathic helices and is restricted by crescent BAR domains. Lee, M. Lenz, M. Mechanical requirements for membrane fission: common facts from various examples. FEBS Lett. Bonazzi, M. Valente, C. Liberali, P. EMBO J. Haga, Y. Yang, J. Key components of the fission machinery are interchangeable. Hidalgo Carcedo, C. Science , 93—96 This force binds protons and neutrons together inside the nucleus, and it is most powerful when the nucleus is small and the nucleons are close together.
The electromagnetic force causes the repulsion between like-charged protons. The strong nuclear force acts to hold all the protons and neutrons close together, while the electromagnetic force acts to push protons further apart. In atoms with small nuclei, the strong nuclear force overpowers the electromagnetic force. As the nucleus gets bigger, the electromagnetic force becomes greater than the strong nuclear force.
These nuclei are called unstable, and this instability can result in radiation and fission. In order to initiate fission, a high-energy neutron is directed towards a nucleus, such as U. The combination of these two produces U, which is an unstable element that undergoes fission. The resulting fission process often releases additional neutrons, which can go on to initiate other U atoms, forming a chain reaction.
While nuclear fission can occur without this neutron bombardment, in what would be termed spontaneous fission, this is a rare occurrence; most fission reactions, especially those utilized for energy and weaponry, occur via neutron bombardment. If an element can be induced to undergo fission via neutron bombardment, it is said to be fissile.
Atomic bombs are nuclear weapons that use the energetic output of nuclear fission to produce massive explosions. These bombs are in contrast to hydrogen bombs, which use both fission and fusion to power their greater explosive potential.
Only two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. These two bombings resulted in the deaths of approximately , Japanese people—mostly civilians.
Atomic bombs are made up of a fissile element, such as uranium, that is enriched in the isotope that can sustain a fission nuclear chain reaction.
When a free neutron hits the nucleus of a fissile atom like uranium U , the uranium splits into two smaller atoms called fission fragments, plus more neutrons.
Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions. This creates the chain reaction. The very first uranium bomb, Little Boy, dropped on Hiroshima in , used 64 kilograms of 80 percent enriched uranium. In fission weapons, a mass of fissile material, either enriched uranium or plutonium, is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction.
The implosion method is considered more sophisticated than the gun method and only can be used if the fissile material is plutonium. The inherent radioactivity of uranium will then release a neutron, which will bombard another atom of U to produce the unstable uranium, which undergoes fission, releases further neutrons, and continues the process.
The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to uranium plus the extra neutron. The following equation shows one possible split, namely into strontium 95 Sr , xenon Xe , and two neutrons n , plus energy:.
Fission bomb assembly methods : Two methods have been applied to induce the nuclear chain reaction that produces the explosion of an atomic bomb. The gun-type assembly uses a conventional explosive to compress from one side, while the implosion assembly compresses from all sides simultaneously.
The immediate energy release per atom is about million electron volts Me. Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons.
This initial kinetic energy imparts an initial speed of about 12, kilometers per second. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second. By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees.
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