Structure of SpoIISA-SpoIISB toxin-antitoxin complex from Bacillus subtilis

P. Florek¹, P. Pavelčíková¹, K. Muchová¹, V. Levdikov², A. Lebedev², A. J. Wilkinson², I. Barák¹

¹Institute of Molecular Biology, Slovak Academy of Sciences, Dúbravská cesta 21, 845 51, Bratislava 45, Slovakia

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

SpoIISA proteic toxin from Bacillus subtilis was shown to affect cell mebrane of either mother cell during sporulation process or, when artificially over-expressed, vegetatively growing cells. The toxin was predicted to be composed of two domains – a membrane spanning domain consisting of three helices and a negatively charged cytosolic domain (C-SpoIISA) [1]. Our previous results indicate that neither of these two domains alone is sufficient for toxic effect [2]. In the normal conditions, toxicity of SpoIISA is abolished by tight binding of positively charged SpoIISB antitoxin to the toxin's cytosolic domain [1].

Crystals of C-SpoIISA-SpoIISB complex we obtained in three different conditions, however unit cell content and parameters are strikingly similar for all of these. Although we first measured native datasets, the structure was solved using data collected to 1.3Å from Se-Met protein crystal, which was used as well for phase problem solution by MAD. The solved molecular structure reveals C-SpoIISA dimer binding of 2 molecules of SpoIISB and appears to shed some more light into mechanism of SpoIISA toxicity and its neutralization by SpoIISB binding. Residues, which were previously identified by genetics experiments to unleash SpoIISA toxicity [1] are observed to be directly involved in the contacts beween C-SpoIISA and SpoIISB. On the other hand, dimerization of C-SpoIISA might be responsible for bringing membrane spanning parts of SpoIISA into each other proximity allowing it to exhibit its lethal properties.

1. E. Adler, I. Barák, P. Stragier, J. Bacteriol., 183, (2001), 3574.

2. P. Florek, K. Muchová, P. Pavelčíková, I. Barák, FEMS Microbiol. Lett., 278, (2008), 177.

Acknowledgements

This work was supported by the grant APVT-51-027804, No. ESF-EC-0106, LPP-0218-06 and VEGA grant 2/7007/27 from the Slovak Academy of Sciences.