Protein Structure and Folding
Structural Basis for Microcin C7 Inactivation by the MccE Acetyltransferase*

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The antibiotic microcin C7 (McC) acts as a bacteriocide by inhibiting aspartyl-tRNA synthetase and stalling the protein translation machinery. McC is synthesized as a heptapeptide-nucleotide conjugate, which is processed by cellular peptidases within target strains to yield the biologically active compound. As unwanted processing of intact McC can result in self-toxicity, producing strains utilize multiple mechanisms for autoimmunity against processed McC. We have shown previously that the mccE gene within the biosynthetic cluster can inactivate processed McC by acetylating the antibiotic. Here, we present the characterization of this acetylation mechanism through biochemical and structural biological studies of the MccE acetyltransferase domain (MccEAcTase). We have also determined five crystal structures of the MccE-acetyl-CoA complex with bound substrates, inhibitor, and reaction product. The structural data reveal an unexpected mode of substrate recognition through π-stacking interactions similar to those found in cap-binding proteins and nucleotidyltransferases. These studies provide a rationale for the observation that MccEAcTase can detoxify a range of aminoacylnucleotides, including those that are structurally distinct from microcin C7.

Aminoacyl-tRNA Synthetase
Antibiotics
Drug Resistance
Enzyme Structure
X-ray Crystallography

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The atomic coordinates and structure factors (codes 3R95, 3R96, 3R9E, 3R9F, and 3R9G) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

*

This work was supported by a Technology Commercialization Fund grant from Rutgers University (to K. S.) and a Molecular and Cell Biology Program grant from the Russian Academy of Sciences. A. M. was supported by Grant 06-04-48865 from the Russian Foundation for Basic Research.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2.