SpoIISA-SpoIISB proteins as suicide components of Bacillus subtilis

J. Melničáková¹, P. Florek¹, S. Rešetárová¹, A. J. Wilkinson² and I. Barák¹

¹Institute of Molecular Biology, Slovak Academy of Sciences, Dúbravská cesta 21, 845 51 Bratislava, Slovakia

²Department of Chemistry, University of York, Heslington, York YO1 5OD, UK

The bacterial cell is capable of suicide in response to a several environmental stresses or developmental processes. This response is called programmed cell death (PCD). We study one of such PCD system present in bacilli, in which the process is mediated by a pair of toxin/antitoxin (SpoIISA/SpoIISB).

This newly indentified system is coded by spoIIS locus consisting of two genes. The gene spoIISA is coding a toxin, while spoIISB is a gene for an antitoxin. It has been shown, that inactivation of toxin gene has no effect on sporulation. Inactivation of antitoxin gene decreases sporulation efficiency. SpoIISB antitoxin is required for sporulation only if SpoIISA is present in the cell [1]. Morphological effect of forced toxin production is presented by formation of the holes in cytoplasmic membrane leading to cell death. SpoIISA toxin is a membrane protein with putative three transmembrane domains and cytoplasmic domain. When the domains are separated, the toxic effect is eliminated [2]. Antitoxin SpoIISB is a small cytoplasmic protein. Despite that the maximum level of SpoIISA production was observed in B. subtilis during the entry into stationary phase, it has been shown, that its expression is not dependent on the key early sporulation-specific transcription factor, Spo0A [3]. Mechanism of inactivation of SpoIISA occurs via formation of tight heterocomplex with SpoIISB. Resolved crystal structure of cytoplasmic domain SpoIISA with SpoIISB revealed that both proteins form heterotetrameric complex in the stoichiometry C-SpoIISA₂:SpoIISB₂. The toxin contains a putative GAF domain which is present in many proteins involved in the cell signaling [4].

TA systems are wide distributed in-between many bacterial species. However, homologues of SpoIISA and SpoIISB proteins were identified only among different bacilli species [2]. These proteins often display a low level of homology. SpoIISA of B. subtilis as well as of B. cereus has inhibitory effect on E. coli growth and antitoxin SpoIISB of these Bacilli is able to neutralize the SpoIISA toxicity in E. coli [2].

This work was supported by Grants 2/0016/10 from the Slovak Academy of Sciences, by the Grant from the Slovak Research and Development Agency under the contract APVV-00335-10 and by the Welcome Trust Project Grant 082829.

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