title: Bistable forespore engulfment in Bacillus subtilis by a zipper mechanism in absence of the cell wall.
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.
authors.affiliation: Department of Life Sciences, Imperial College, London, United Kingdom; Centre for Integrative Systems Biology and Bioinformatics, Imperial College, London, United Kingdom.
authors.forename: Nikola
authors.initials: N
authors.lastname: Ojkic
authors.affiliation: Division of Biological Sciences, University of California, San Diego, La Jolla, California, United States of America.
authors.forename: Javier
authors.initials: J
authors.lastname: López-Garrido
authors.affiliation: Division of Biological Sciences, University of California, San Diego, La Jolla, California, United States of America.
authors.forename: Kit
authors.initials: K
authors.lastname: Pogliano
authors.affiliation: Department of Life Sciences, Imperial College, London, United Kingdom; Centre for Integrative Systems Biology and Bioinformatics, Imperial College, London, United Kingdom.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Endres
mesh_terms: D001412:Bacillus subtilis / Q000166:cytology* / Q000502:physiology*; D055592:Biophysical Phenomena / Q000502:physiology; D048430:Cell Shape / Q000502:physiology; D002473:Cell Wall; D019295:Computational Biology; D008954:Models, Biological; D013171:Spores, Bacterial / Q000166:cytology* / Q000502:physiology*