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Bistable forespore engulfment in Bacillus subtilis by a zipper mechanism in absence of the cell wall.

Ojkic N, López-Garrido J, Pogliano K, Endres RG - PLoS Comput. Biol. (2014)

Bottom Line: By systematic exploration of model parameters, we predict regions of osmotic pressure and membrane-surface tension that produce successful engulfment.Indeed, decreasing the medium osmolarity in experiments prevents engulfment in line with our predictions.Forespore engulfment may thus not only be an ideal model system to study decision-making in single cells, but its biophysical principles are likely applicable to engulfment in other cell types, e.g. during phagocytosis in eukaryotes.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Imperial College, London, United Kingdom; Centre for Integrative Systems Biology and Bioinformatics, Imperial College, London, United Kingdom.

ABSTRACT
To survive starvation, the bacterium Bacillus subtilis forms durable spores. The initial step of sporulation is asymmetric cell division, leading to a large mother-cell and a small forespore compartment. After division is completed and the dividing septum is thinned, the mother cell engulfs the forespore in a slow process based on cell-wall degradation and synthesis. However, recently a new cell-wall independent mechanism was shown to significantly contribute, which can even lead to fast engulfment in [Formula: see text] 60 [Formula: see text] of the cases when the cell wall is completely removed. In this backup mechanism, strong ligand-receptor binding between mother-cell protein SpoIIIAH and forespore-protein SpoIIQ leads to zipper-like engulfment, but quantitative understanding is missing. In our work, we combined fluorescence image analysis and stochastic Langevin simulations of the fluctuating membrane to investigate the origin of fast bistable engulfment in absence of the cell wall. Our cell morphologies compare favorably with experimental time-lapse microscopy, with engulfment sensitive to the number of SpoIIQ-SpoIIIAH bonds in a threshold-like manner. By systematic exploration of model parameters, we predict regions of osmotic pressure and membrane-surface tension that produce successful engulfment. Indeed, decreasing the medium osmolarity in experiments prevents engulfment in line with our predictions. Forespore engulfment may thus not only be an ideal model system to study decision-making in single cells, but its biophysical principles are likely applicable to engulfment in other cell types, e.g. during phagocytosis in eukaryotes.

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Bistable engulfment depends on zipper-molecule density.(A) Engulfment as a function of SpoIIQ protein surface density for   = 50 pN/µm and   = 500 Pa. The total binding energy was converted to SpoIIQ protein-surface density using the binding energy of a single SpoIIQ-SpoIIIAH bond (see Text S1) [12]. Consistent with experimental results shown in (D) (extracted from [8]), engulfment is threshold-dependent on number of SpoIIQ proteins expressed in forespores. Gray vertical arrows point to surface densities for which snapshots are shown in (C) at 5 s. (B) Simulations lead to bistable outcome at later times (  = 10 s) with two subpopulations of stalled and fully completed cups. (E) For each set of constraint parameters ( and ) we performed a surface-density scan as in (A). The lower bound on the critical number of SpoIIQ molecules necessary for engulfment ranges from  120 to  7200 molecules depending on constraint parameters.
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pcbi-1003912-g006: Bistable engulfment depends on zipper-molecule density.(A) Engulfment as a function of SpoIIQ protein surface density for  = 50 pN/µm and  = 500 Pa. The total binding energy was converted to SpoIIQ protein-surface density using the binding energy of a single SpoIIQ-SpoIIIAH bond (see Text S1) [12]. Consistent with experimental results shown in (D) (extracted from [8]), engulfment is threshold-dependent on number of SpoIIQ proteins expressed in forespores. Gray vertical arrows point to surface densities for which snapshots are shown in (C) at 5 s. (B) Simulations lead to bistable outcome at later times (  = 10 s) with two subpopulations of stalled and fully completed cups. (E) For each set of constraint parameters ( and ) we performed a surface-density scan as in (A). The lower bound on the critical number of SpoIIQ molecules necessary for engulfment ranges from 120 to 7200 molecules depending on constraint parameters.

Mentions: (A–D) Simulation snapshots for different parameter combinations for surface tension () and (pressure difference) at 5 s for fixed SpoIIQ-SpoIIIAH surface density . Simulations that reached full engulfment earlier than 5 s were terminated and last snapshots are displayed. (B) Percentage of forespore-surface area enclosed by mother membrane. White dashed line separates regions of full and partial engulfment. White cross shows the parameters used for Fig. 6A and B. (C and D) Volume and surface area of mother cell.


Bistable forespore engulfment in Bacillus subtilis by a zipper mechanism in absence of the cell wall.

Ojkic N, López-Garrido J, Pogliano K, Endres RG - PLoS Comput. Biol. (2014)

Bistable engulfment depends on zipper-molecule density.(A) Engulfment as a function of SpoIIQ protein surface density for   = 50 pN/µm and   = 500 Pa. The total binding energy was converted to SpoIIQ protein-surface density using the binding energy of a single SpoIIQ-SpoIIIAH bond (see Text S1) [12]. Consistent with experimental results shown in (D) (extracted from [8]), engulfment is threshold-dependent on number of SpoIIQ proteins expressed in forespores. Gray vertical arrows point to surface densities for which snapshots are shown in (C) at 5 s. (B) Simulations lead to bistable outcome at later times (  = 10 s) with two subpopulations of stalled and fully completed cups. (E) For each set of constraint parameters ( and ) we performed a surface-density scan as in (A). The lower bound on the critical number of SpoIIQ molecules necessary for engulfment ranges from  120 to  7200 molecules depending on constraint parameters.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4214620&req=5

pcbi-1003912-g006: Bistable engulfment depends on zipper-molecule density.(A) Engulfment as a function of SpoIIQ protein surface density for  = 50 pN/µm and  = 500 Pa. The total binding energy was converted to SpoIIQ protein-surface density using the binding energy of a single SpoIIQ-SpoIIIAH bond (see Text S1) [12]. Consistent with experimental results shown in (D) (extracted from [8]), engulfment is threshold-dependent on number of SpoIIQ proteins expressed in forespores. Gray vertical arrows point to surface densities for which snapshots are shown in (C) at 5 s. (B) Simulations lead to bistable outcome at later times (  = 10 s) with two subpopulations of stalled and fully completed cups. (E) For each set of constraint parameters ( and ) we performed a surface-density scan as in (A). The lower bound on the critical number of SpoIIQ molecules necessary for engulfment ranges from 120 to 7200 molecules depending on constraint parameters.
Mentions: (A–D) Simulation snapshots for different parameter combinations for surface tension () and (pressure difference) at 5 s for fixed SpoIIQ-SpoIIIAH surface density . Simulations that reached full engulfment earlier than 5 s were terminated and last snapshots are displayed. (B) Percentage of forespore-surface area enclosed by mother membrane. White dashed line separates regions of full and partial engulfment. White cross shows the parameters used for Fig. 6A and B. (C and D) Volume and surface area of mother cell.

Bottom Line: By systematic exploration of model parameters, we predict regions of osmotic pressure and membrane-surface tension that produce successful engulfment.Indeed, decreasing the medium osmolarity in experiments prevents engulfment in line with our predictions.Forespore engulfment may thus not only be an ideal model system to study decision-making in single cells, but its biophysical principles are likely applicable to engulfment in other cell types, e.g. during phagocytosis in eukaryotes.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Imperial College, London, United Kingdom; Centre for Integrative Systems Biology and Bioinformatics, Imperial College, London, United Kingdom.

ABSTRACT
To survive starvation, the bacterium Bacillus subtilis forms durable spores. The initial step of sporulation is asymmetric cell division, leading to a large mother-cell and a small forespore compartment. After division is completed and the dividing septum is thinned, the mother cell engulfs the forespore in a slow process based on cell-wall degradation and synthesis. However, recently a new cell-wall independent mechanism was shown to significantly contribute, which can even lead to fast engulfment in [Formula: see text] 60 [Formula: see text] of the cases when the cell wall is completely removed. In this backup mechanism, strong ligand-receptor binding between mother-cell protein SpoIIIAH and forespore-protein SpoIIQ leads to zipper-like engulfment, but quantitative understanding is missing. In our work, we combined fluorescence image analysis and stochastic Langevin simulations of the fluctuating membrane to investigate the origin of fast bistable engulfment in absence of the cell wall. Our cell morphologies compare favorably with experimental time-lapse microscopy, with engulfment sensitive to the number of SpoIIQ-SpoIIIAH bonds in a threshold-like manner. By systematic exploration of model parameters, we predict regions of osmotic pressure and membrane-surface tension that produce successful engulfment. Indeed, decreasing the medium osmolarity in experiments prevents engulfment in line with our predictions. Forespore engulfment may thus not only be an ideal model system to study decision-making in single cells, but its biophysical principles are likely applicable to engulfment in other cell types, e.g. during phagocytosis in eukaryotes.

Show MeSH
Related in: MedlinePlus