Crystal Structure of MtaN, a Global Multidrug Transporter Gene Activator

Michael H Godsey,Natalya N Baranova,Alexander A Neyfakh,Richard G Brennan

doi:10.1074/jbc.m105819200

Abstract

MtaN (Multidrug Transporter Activation, N terminus) is a constitutive, transcriptionally active 109-residue truncation mutant, which contains only the N-terminal DNA-binding and dimerization domains of MerR family member Mta. The 2.75 A resolution crystal structure of apo-MtaN reveals a winged helix-turn-helix protein with a protruding 8-turn helix (alpha5) that is involved in dimerization by the formation of an antiparallel coiled-coil. The hydrophobic core and helices alpha1 through alpha4 are structurally homologous to MerR family member BmrR bound to DNA, whereas one wing (Wing 1) is shifted. Differences between the orientation of alpha5 with respect to the core and the revolution of the antiparallel coiled-coil lead to significantly altered conformations of MtaN and BmrR dimers. These shifts result in a conformation of MtaN that appears to be incompatible with the transcription activation mechanism of BmrR and suggest that additional DNA-induced structural changes are necessary.

Highlights

Bacterial multidrug resistance (MDR)1 is a growing threat to human health
The hydrophobic core and helices ␣1 through ␣4 are structurally homologous to MerR family member BmrR bound to DNA, whereas one wing (Wing 1) is shifted
Often regulated by global regulators [3, 4] such as MarA [5], which activates over a dozen genes [6, 7], many MDR genes are regulated such as qacA by QacR [8] and emrAB by EmrR [9]

Summary

TABLE I Data and refinement statistics

This unusual promoter structure has led to a model of transcription regulation by these proteins in which activation is achieved by DNA distortion and untwisting [12, 26]. The recent crystal structure of BmrR bound to DNA and coactivator has delineated a significant portion of the activation mechanism [27]. The BmrR-drug-DNA complex provides insight into one facet of transcription regulation by the MerR family. The extent of the conformational changes of these proteins needed to effect DNA binding and transcription activation, if any, are unknown. To address this aspect of the mechanism of MerR family transcription activation, we solved the crystal structure of MtaN to 2.75 Å resolution. Comparison of the structures of MtaN and DNA/drug-bound BmrR reveals their overall structural similarity, as well as significant tertiary and quaternary differences

MATERIALS AND METHODS

RESULTS AND DISCUSSION

Crystal Structure of MtaN