Protein Structure and Folding
Structure and Proposed Activity of a Member of the VapBC Family of Toxin-Antitoxin Systems: VapBC-5 FROM MYCOBACTERIUM TUBERCULOSIS*

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In prokaryotes, cognate toxin-antitoxin pairs have long been known, but no three-dimensional structure has been available for any given complex from Mycobacterium tuberculosis. Here we report the crystal structure and activity of a member of the VapBC family of complexes from M. tuberculosis. The toxin VapC-5 is a compact, 150 residues, two domain α/β protein. Bent around the toxin is the VapB-5 antitoxin, a 33-residue α-helix. Assays suggest that the toxin is an Mg-enabled endoribonuclease, inhibited by the antitoxin. The lack of DNase activity is consistent with earlier suggestions that the complex represses its own operon. Furthermore, analysis of the interactions in the binding of the antitoxin to the toxin suggest that exquisite control is required to protect the bacteria cell from toxic VapC-5.

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The atomic coordinates and structure factors (code 3DBO) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

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This work was supported, in whole or in part, by National Institutes of Health Grants 23616-002-06 F3:02 and TBSGC R01. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.