Within individual bacteria, we combine force-dependent polymerization dynamics of individual MreB protofilaments with an elastic model of protofilament bundles buckled into helical configurations. We use variational techniques and stochastic simulations to relate the pitch of the MreB helix, the total abundance of MreB, and the number of protofilaments. By comparing our simulations with mean-field calculations, we find that stochastic fluctuations are significant. We examine the quasi-static evolution of the helical pitch with cell growth, as well as timescales of helix turnover and denovo establishment. We find that while the body of a polarized MreB helix treadmills towards its slow-growing end, the fast-growing tips of laterally associated protofilaments move towards the opposite fast-growing end of the MreB helix. This offers a possible mechanism for targeted polar localization without cytoplasmic motor proteins.
Deep Dive into Steady-state MreB helices inside bacteria: dynamics without motors.
Within individual bacteria, we combine force-dependent polymerization dynamics of individual MreB protofilaments with an elastic model of protofilament bundles buckled into helical configurations. We use variational techniques and stochastic simulations to relate the pitch of the MreB helix, the total abundance of MreB, and the number of protofilaments. By comparing our simulations with mean-field calculations, we find that stochastic fluctuations are significant. We examine the quasi-static evolution of the helical pitch with cell growth, as well as timescales of helix turnover and denovo establishment. We find that while the body of a polarized MreB helix treadmills towards its slow-growing end, the fast-growing tips of laterally associated protofilaments move towards the opposite fast-growing end of the MreB helix. This offers a possible mechanism for targeted polar localization without cytoplasmic motor proteins.
The eukaryotic cytoskeleton organizes cell shape, cell polarity, cell division, and non-diffusive subcellular transport. F-actin, microtubules, and intermediate filaments comprise the cytoskeleton, and act with the associated proteins that provides spatial and dynamic control of cytoskeletal function [1]. Prokaryotic cells have cytoskeletal analogues, such as the FtsZ-ring associated with cell division [2] together with its "divisome" of associated proteins. Bacteria also have a number of polymerizing cytoplasmic proteins, such as ParM [3] and MinD [2] that exhibit distinctive helical structures within the cell.
Recently, the actin homologue MreB has been shown to play a cytoskeletal role in many bacteria [2,4,5]. MreB forms a continuous cytoplasmic helix that runs the length of nearly all rod-shaped prokaryotes, including Escherichia coli, Bacillus subtilis and Caulobacter crescentus [6], and it has been implicated in shape determination and polar protein localization.
In most Gram-positive bacteria MreB is present together with several paralogues, such as Mbl and MreBH in B. subtilis. The helical pitches for MreB or Mbl, separately observed by immunoflourescence microscopy, are reported to be 0.73 ± 0.12 µm and 1.7 ± 0.28 µm, respectively [7]. More recent measurements of fluorescent fusions of MreB and of Mbl report pitches of 0.6 ± 0.14 µm, with colocalization [8]. In Gram-negative species, such as E. coli and C. crescentus, only MreB is present. In E. coli, pitches of 0.46 ± 0.08 µm have been reported [9]. In all cases, the helices are dynamic, with elements moving along the main helix at reported speeds ranging from 6 nm/ s [10] to 70 nm/ s [11]. The helical structure has also been observed to condense into a ring at midcell near the time of division in E. coli [12], C. crescentus [13] and B. subtilis (where only MreBH coils) [14].
The MreB helix appears to be composed of a bundle of individual “protofilaments” [15][16][17]. Quantitative immunoblotting has been used to measure the molecular abundance of various MreBs. In B. subtilis, there are roughly 8000 MreB monomers and 12000 -14000 monomers of Mbl [7] while E. coli has roughly 17000 -40000 monomers of MreB [9]. Neglecting the cytoplasmic fraction of monomeric MreB, these abundances suggest a bundle thickness of about 10 protofilaments [18].
In B. subtilis Mbl is necessary for proper insertion of new peptidoglycan, which occurs in a helical fashion [6], while MreBH is necessary for the localization and function of the cell wall hydrolase LytE that is believed to recycle the outer layers of the cell wall, also in a helical fashion [14]. Cells with mutant mreB are wide, rounded and usually not viable [19]. Helical bundles of MreB may contribute to the spatial localization of associated MreC, MreD, and PBP2 that, in turn, help to coordinate cell wall synthesis. It has also been suggested that helical filaments of MreB paralogues under tension can lead to spiral morphologies [20].
Disruption of MreB leads to loss of proper polar localization of a number of proteins such as the chemotaxis protein Tar and the Shigella flexneri virulence factor IcsA in E. coli [21], and three integral membrane proteins (PleC, DivJ, CckA) in C. crescentus [22]. Polar localization in C. crescentus was disrupted by either underexpression or overexpression of MreB. When normal MreB expression was returned, polar localization was reestablished [22]. This suggests that MreB has a continual role in either direct polar trafficking of these proteins or in the maintenance of landmarks necessary for their proper positioning [4].
The polar proteins in C. crescentus are normally directed towards distinct (stalked and swarmer) poles in different stages (swarmer, stalked, and predivisional) of its life cycle. For example, PleC is localized to swarmer poles in swarmer and predivisional cells, DivJ is localized to stalked poles, while CckA is localized to both poles of predivisional cells [23]. After MreB expression is disrupted and restored, PleC and DivJ are restored randomly to either pole [23]. This suggests that MreB may be polarized within C. crescentus and that the polarity of the MreB helix is randomly restored after its disruption. However, tracking of individual YFP-MreB molecules shows unpolarized motion [10], raising questions about the mechanism of specific polar targeting. MreB-directed targeting to specific poles has not been reported in E. coli or B. subtilis.
MreB interacts with both RNA polymerase (RNAP) [23] and SetB, a chromosome defect suppressor [24], and has been implicated in the fast polar translocation of the origin-proximal regions (oriC) [27] of newly-replicated DNA in C. crescentus [25] and in E. coli [23] (see however [26]). Time-lapse microscopy has shown that the polar transport of oriC in B. subtilis had an average speed of 2.8 nm/s and a peak speed of 4.5 nm/s [27].
MreB is a homologue of the eukaryotic cytoskeletal protein actin [28,29]. Actin fi
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
