Min System

Displacement of the Z-ring and the Ter macrodomain in a long ΔslmA Δmin double mutant E. coli cell. Z-ring fluorescence is followed using a ZipA-GFP construct (green), while the chromosomal terminus is labeled with MatP-mCherry (red). A phase contrast image (gray) is overlaid to visualize the cell contour. The scale bar is 2 µm.

The Min System is a mechanism composed of three proteins MinC, MinD, and MinE used by E. coli as a means of properly localizing the septum prior to cell division. Each component participates in generating a dynamic oscillation of FtsZ protein inhibition between the two bacterial poles to precisely specify the mid-zone of the cell, allowing the cell to accurately divide in two. This system is known to function in conjunction with a second negative regulatory system, the nucleoid occlusion system (NO), to ensure proper spatial and temporal regulation of chromosomal segregation and division.

History

The initial discovery of this family of proteins is attributed to Adler et al. (1967). First identified as E. coli mutants that could not produce a properly localized septum, resulting in the generation of minicells [1][2] due to mislocalized cell division occurring near the bacterial poles. This caused miniature vesicles to pinch off, void of essential molecular constituents permitting it to exist as a viable bacterial cell. Minicells are achromosomal cells that are products of aberrant cell division, and contain RNA and protein, but little or no chromosomal DNA. This finding led to the identification of three interacting proteins involved in a dynamic system of localizing the mid-zone of the cell for properly controlled cell division.

Function

The Min proteins prevent the FtsZ ring from being placed anywhere but near the mid cell and are hypothesized to be involved in a spatial regulatory mechanism that links size increases prior to cell division to FtsZ polymerization in the middle of the cell.

The MinCDE system. MinD-ATP binds to a cell pole, also binds MinC, which prevents the formation of FtsZ polymers. The MinE ring causes hydrolysis of MinD’s bound ATP, turning it into ADP and releasing the complex from the membrane. The system oscillates as each pole builds up a concentration of inhibitor that is periodically dismantled.

Centering the Z-Ring

One model of Z-ring formation permits its formation only after a certain spatial signal that tells the cell that it is big enough to divide. [3] The MinCDE system prevents FtsZ polymerization near certain parts of the plasma membrane. MinD localizes to the membrane only at cell poles and contains an ATPase and an ATP-binding domain. MinD is only able to bind to the membrane when in its ATP-bound conformation. Once anchored, the protein polymerizes, resulting in clusters of MinD. These clusters bind and then activate another protein called MinC, which has activity only when bound by MinD. [4] MinC serves as a FtsZ inhibitor that prevents FtsZ polymerization. The high concentration of a FtsZ polymerization inhibitor at the poles prevents FtsZ from initiating division at anywhere but the mid-cell. [5]

MinE is involved in preventing the formation of MinCD complexes in the middle of the cell. MinE forms a ring near each cell pole. This ring is not like the Z-ring. Instead, it catalyzes the release of MinD from the membrane by activating MinD’s ATPase. This hydrolyzes the MinD’s bound ATP, preventing it from anchoring itself to the membrane.

MinE prevents the MinD/C complex from forming in the center but allows it to stay at the poles. Once the MinD/C complex is released, MinC becomes inactivated. This prevents MinC from deactivating FtsZ. As a consequence, this activity imparts regional specificity to Min localization. [6] Thus, FtsZ can form only in the center, where the concentration of the inhibitor MinC is minimal. Mutations that prevent the formation of MinE rings result in the MinCD zone extending well beyond the polar zones, preventing FtsZ to polymerize and to perform cell division. [7] MinD requires a nucleotide exchange step to re-bind to ATP so that it can re-associate with the membrane after MinE release. The time lapse results in a periodicity of Min association that may yield clues to a temporal signal linked to a spatial signal. In vivo observations show that the oscillation of Min proteins between cell poles occurs approximately every 50 seconds. [8] Oscillation of Min proteins, however, is not necessary for all bacterial cell division systems. Bacillus subtilis has been shown to have static concentrations of MinC and MinD at the cell poles. [9] This system still links cell size to the ability to form a septum via FtsZ and divide.

in vitro Reconstitution

The dynamic behavior of the Min proteins has been reconstituted in vitro using an artificial lipid bilayer as mimic for the cell membrane. MinE and MinD can self-organize into parallel and spiral protein waves by a reaction-diffusion-like mechanism. [10]

Additional study is required to elucidate the extent of temporal and spatial signalling permissible by this biological function. Efforts to reconstitute the Min System have been limited to an open, unrestricted geometry, on a supporting lipid bilayer, to demonstrate protein wave formation. These in vitro systems offered unprecedented access to features such as residence times and molecular motility, however, since the geometry of the cell directly influences the wave formation, parsing out additional characteristics of the system requires an improved in vitro model that would more closely mimic cell features.


References

  1. ↑ De Boer PA, Crossley RE, Rothfield LI (1989). "A division inhibitor and a topological specificity factor coded for by the minicell locus determine proper placement of the division septum in E. coli". Cell 56 (4): 641–649. doi:10.1016/0092-8674(89)90586-2. PMID 2645057.
  2. ↑ Adler HI, Fisher WD, Cohen A, Hardigree AA; Fisher; Cohen; Hardigree (1967). "Miniature Escherichia coli Cells Deficient in DNA". PNAS 57 (2): 321–326. Bibcode:1967PNAS...57..321A. doi:10.1073/pnas.57.2.321. PMID 335508.
  3. ↑ Weart RB, Levin PA (2003). "Growth Rate-Dependent Regulation of Medial FtsZ Ring Formation". J Bacteriol 185 (9): 2826–2834. doi:10.1128/JB.185.9.2826-2834.2003. PMC 154409. PMID 12700262.
  4. ↑ Hu Z, Gogol EP, Lutkenhaus J (2002). "Dynamic assembly of MinD on phospholipid vesicles regulated by ATP and MinE". Proc Natl Acad Sci USA 99 (10): 6761–6766. doi:10.1073/pnas.102059099. PMC 124476. PMID 11983867.
  5. ↑ Huang KC, Meir Y, Wingreen NS (2003). "Dynamic structures in Escherichia coli: Spontaneous formation of MinE rings and MinD polar zones". Proc Natl Acad Sci USA 100 (22): 12724–12728. doi:10.1073/pnas.2135445100. PMC 240685. PMID 14569005.
  6. ↑ Hu Z, Saez C, Lutkenhaus J (2003). "Recruitment of MinC, an Inhibitor of Z-Ring Formation, to the Membrane in Escherichia coli: Role of MinD and MinE". J Bacteriol 185 (1): 196–203. doi:10.1128/JB.185.1.196-203.2003. PMC 141945. PMID 12486056.
  7. ↑ Hu Z, Lutkenhaus J (2001). "Topological regulation of cell division in E. coli: spatiotemporal oscillation of MinD requires stimulation of its ATPase by MinE and phospholipid". Mol Cell 7 (6): 1337–1343. doi:10.1016/S1097-2765(01)00273-8. PMID 11430835.
  8. ↑ Dajkovic A, Lutkenhaus J (2006). "Z Ring as Executor of Bacterial Cell Division". J Mol Micro Bio 11 (3–5): 140–151. doi:10.1159/000094050. PMID 16983191.
  9. ↑ Marston AL, Thomaides HB, Edwards DH, Sharpe ME, Errington J (1998). "Polar localization of the MinD protein of Bacillus subtilis and its role in selection of the mid-cell division site". Genes Dev 12 (21): 3419–3430. doi:10.1101/gad.12.21.3419. PMC 317235. PMID 9808628.
  10. ↑ Loose M, Fischer-Friedrich E, Ries J, Kruse K, Schwille P (2008). "Spatial Regulators for Bacterial Cell Division Self-Organize into Surface Waves in Vitro". Science 320 (5877): 789–792. doi:10.1126/science.1154413. PMID 18467587.
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