SpoIIE links asymmetric cell division to compartmentalization of gene expression during sporulation of Bacillus subtilis
Z. Chromiková1, K. Muchová1, A. J. Wilkinson2,
1Institute of molecular biology, Slovak academy of sciences, Dúbravská cesta 21, 845 51 Bratislava 45, Slovakia
2York Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Sporulation in B. subtilis serves as an example of primary cellular differentiation. Daughter cells arisen from sporulation division follow the different fate, although they share identical genetic information. The process of sporulation is characteristic by an asymmetric cell division, which results in formation of two disimilarly sized compartments, the smaller prespore, and the bigger mother cell. The proper positioning of an asymmetric septum is partially triggered by sporulation specific SpoIIE, an exclusive component of the sporulation septum. During formation of sporulation septum, SpoIIE closely cooperates with tubuline-like protein FtsZ. FtsZ forms higher organized structures, so called Z-rings, which constitute a scaffold for assembly of division machinery. During vegetative division, there is only one Z ring formed and it is positioned at the midcell. However, during sporulation, FtsZ is over-expressed and forms a helical structures emanating from midcell Z ring towards cell poles. SpoIIE, which also forms ring-like structures, most likely stabilizes FtsZ helices at polar positions [1]. Consequently, two Z-rings are formed at the sites near cell poles, but only one is chosen to become the sporulation septum.
SpoIIE is a multi-domain 827-residue protein. It is thought to consist of three domains. Its N-terminal domain, which consists of 10 transmembrane helices, anchors the protein in sporulation septum [2, 3]. The central domain of SpoIIE, which is conserved only among SpoIIE orthologues, is thought to play a role in self-oligomerization of the protein and it is also responsible for interaction with FtsZ [4]. SpoIIE C-terminal domain is closely related to PP2C domains of eukaryotic Ser/Thr phosphatases, which regulate the stress response [5].
The key regulator SpoIIE has probably three roles in the process of sporulation. The first role of SpoIIE is to stabilize FtsZ spiral intermediates at polar positions. The second role of SpoIIE lies in contribution to prespore-specific σF activation. σF is activated as the first of sporulation compartment-specific sigma factors and its activation is linked to the completion of sporulation septum [6]. Consequent compartment specific gene expression is arranged by the activity of cell-type specific σ factors, which sequentially activate each other in a cascade-like manner. The third role of SpoIIE is unclear, but according to the dynamic localization studies using SpoIIE-GFP fusions following cytokinesis, SpoIIE might play a role in the process of engulfment [7].
Although σF becomes active only in the prespore, it is present in the predivisional cell, and after asymmetric division also in both compartments. The activity of σF is regulated through interactions between an anti-sigma factor SpoIIAB, an anti-anti-sigma factor SpoIIAA, and SpoIIE phosphatase, resident in sporulation septum. In the predivisional cell and in the mother cell, SpoIIAA is bound by SpoIIAB kinase, which results in phosphorylation of SpoIIAA and SpoIIAB free to form a SpoIIAB2:σF complex. In this complex, σF is inactive. In the prespore, SpoIIE dephosphorylates SpoIIAA-P, making it able to attack SpoIIAB2:σF complex. By binding of dephosphorylated SpoIIAA to SpoIIAB, the release of active σF is induced. σF is then free to bind to the core of RNA-polymerase and direct transcription of prespore-specific genes [8,9]. The mechanism that delays σF activation until sporulation septum is formed and confines σF activity specifically into prespore is not yet fully understood. To address these issues, several models were suggested, taking into consideration the volume difference between the compartments, the preferential localization of SpoIIE to the forespore face of the septum, and/or transient genetic asymmetry [10-13].
The structures of the SpoIIA proteins have been determined, revealing the phosphorylated and dephosphorylated forms of SpoIIAA [14,15], as well as the interactions of SpoIIAB with σF and SpoIIAA [9,16]. However, the structure of SpoIIE and the character of its interactions with SpoIIAA and FtsZ are still unknown, due to the problematic solubility of SpoIIE. Previously, using a random truncation library approach [17], a set of soluble SpoIIE fragments was identified. Out of these fragments, SpoIIE fragment (590-827) encompassing the PP2C phoshatase domain was over-expressed, characterized, and its crystal structure was determined [18]. Since SpoIIE has been a subject of intensive studies, a large number of spoIIE mutants have been characterized. Some of these mutations, like spoIIE64, spoIIE71 and spoIIE20, are located within the phosphatase domain [19] All of them prevent activation of σF, resulting in the inhibition of sporulation in cells harbouring these alleles. Cells of spoIIE64 and spoIIE 71 strains form normal asymmetric septa, but are defective in dephosphorylating SpoIIAA-P and hence activation of σF. Interestingly, spoIIE 20, although it is mapped in the phosphatase domain, is impaired not only in activation of σF, but also in formation of sporulation septa [19]. This mutation shows how the defect of phosphatase domain, which is responsible for σF activation, can impair the central domain, responsible for morphological function of SpoIIE. This possibly happens by the means of interactions between the central and phosphatase domain of the protein. The structure of the phosphatase domain of SpoIIE provides a useful frame for interpreting genetic data available for this protein and for formulating ideas about its mechanism of action.
This work was supported by the grants APVV-51-027804,
and VEGA grant 2/0016/10 from the
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