Bacillus subtilis is a Gram-positive microorganism which is able to differentiate during process called sporulation. A hallmark of sporulation in B. subtilis is the polar cell division. As occurs during vegetative cell division, the tubulin-like GTPase FtsZ forms a ring-like structure at mid-cell. At the onset of sporulation, however, the division apparatus is not assembled at this site, instead the Z-ring migrates from mid-cell on a spiral trajectory to the two cell poles in a process that depends on sporulation-specific overexpression of ftsAZ and the presence of SpoIIE. SpoIIE colocalizes with the polar Z-rings. One of the Z-rings matures into the sporulation septum while the other dissolves. Asymmetric cell division otherwise appears to involve the same set of proteins as constitute the divisome during vegetative cell division. However, the resulting sporulation septum is much thinner. Interestingly, SpoIIE is the only sporulation-specific protein whose deletion or mutation causes changes in the ultrastructure of the asymmetric septum. spoIIE null mutants are defective in sporulation and at low frequency give rise to aberrantly thick asymmetric septa [1]. Accompanying these morphological changes is a coordinated programme of differential gene expression, involving intercellular signalling processes, that leads to the activation of the RNA polymerase sigma factors, sF and sG in the forespore and sE and sK in the mother cell [2].
SpoIIE from B. subtilis is an 827 residue protein that consists of three regions. It has 10 putative membrane-spanning segments (region I) at its amino terminus and a PP2C-type phosphatase domain (region III) at its C-terminus. The central region II is required for localisation of SpoIIE to the divisome and its reported interaction with FtsZ. The structure of the PP2C phosphatase domain of SpoIIE was already solved [3]. In contrast, the structure of N-terminal two-thirds of SpoIIE and the character of its interactions with partner proteins are unknown. We recently identified a new partner of SpoIIE, the cytoskeletal protein, RodZ, which is essential for cell shape determination. This interaction is additionally required for asymmetric septum formation and sporulation. Presently we try to employ a new method of "slimfield" microscopy to study the oligomeric state of SpoIIE in live cells and during its different roles in asymmetric septum site recognition and its formation, activation of sF and forespore engulfment.
Although, SpoIIE has a critical function in determining the site of formation of the sporulation septum, it is not understood (i) how it localises to the polar septum (ii) how it causes FtsZ to relocalise from mid-cell to the polar site (iii) what role SpoIIE plays in septal thinning, (iv) how its SpoIIAA~P phosphatase activity is controlled so that sF acitivation is delayed until the septum is completed (v) what role SpoIIE playes in SpoIIQ-SpoIIIAH channel formation. How, SpoIIE brings about activation of sF in the forespore but not in the mother cell has been the subject of great interest. Plausible mechanisms have been invoked based on (i) preferential SpoIIE localisation on the forespore face of the septum, (ii) transient gene asymmetry leading to accumulation of a SpoIIE inhibitor in the mother cell and (iii) the volume difference between the compartments leading to higher specific activity of equipartitioned SpoIIE, but this question is not fully resolved [4].
Acknowledgements
This work was supported by Grant 2/0009/13 from the Slovak Academy of Sciences and by a Grant from the Slovak Research and Development Agency under contract APVV-14-018.
References